[apple/libc.git] / gen / magazine_malloc.c

/*
 * Copyright (c) 1999, 2006, 2008 Apple Inc. All rights reserved.
 *
 * @APPLE_LICENSE_HEADER_START@
 * 
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this
 * file.
 * 
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 * 
 * @APPLE_LICENSE_HEADER_END@
 */

/* Author: Bertrand Serlet, August 1999 */

/*
    Multithread enhancements for "tiny" allocations introduced February 2008.
    These are in the spirit of "Hoard". See:
        Berger, E.D.; McKinley, K.S.; Blumofe, R.D.; Wilson, P.R. (2000).
        "Hoard: a scalable memory allocator for multithreaded applications".
        ACM SIGPLAN Notices 35 (11): 117-128. Berger2000. 
        <http://portal.acm.org/citation.cfm?id=356989.357000>
        Retrieved on 2008-02-22.
*/

/* gcc -g -O3 magazine_malloc.c malloc.c -o libmagmalloc.dylib -I. \
 -I/System/Library/Frameworks/System.framework/PrivateHeaders/ -funit-at-a-time \
 -dynamiclib -Wall -arch x86_64 -arch i386 -arch ppc */
        
#include "scalable_malloc.h"
#include "malloc_printf.h"
#include "_simple.h"
#include "magmallocProvider.h"

#include <pthread_internals.h>  /* for pthread_lock_t SPI */
#include <pthread.h>            /* for pthread API */

#include <stdint.h>
#include <unistd.h>
#include <mach/vm_statistics.h>
#include <mach/mach_init.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/sysctl.h>
#include <libkern/OSAtomic.h>
#include <mach-o/dyld.h> /* for NSVersionOfLinkTimeLibrary() */

/*********************  DEFINITIONS     ************************/

#define DEBUG_MALLOC                    0 // set to one to debug malloc itself

#define DEBUG_CLIENT                    0 // set to one to debug malloc client

#if DEBUG_MALLOC
#warning DEBUG_MALLOC ENABLED
# define INLINE
# define ALWAYSINLINE
# define CHECK_MAGAZINE_PTR_LOCKED(szone, mag_ptr, fun)                         \
do {                                                                            \
    if (__is_threaded && TRY_LOCK(mag_ptr->magazine_lock)) {                    \
        malloc_printf("*** magazine_lock was not set %p in %s\n",               \
        mag_ptr->magazine_lock, fun);                                           \
    }                                                                           \
} while (0)
#else
# define INLINE __inline__
# define ALWAYSINLINE __attribute__((always_inline))
# define CHECK_MAGAZINE_PTR_LOCKED(szone, mag_ptr, fun) {}
#endif

# define NOINLINE __attribute__((noinline))

#if defined(__i386__) || defined(__x86_64__)
#define CACHE_ALIGN __attribute__ ((aligned (128) )) /* Future-proofing at 128B */
#elif defined(__ppc__) || defined(__ppc64__)
#define CACHE_ALIGN __attribute__ ((aligned (128) ))
#else
#define CACHE_ALIGN
#endif

/*
 * Access to global variables is slow, so optimise our handling of vm_page_size
 * and vm_page_shift.
 */
#define _vm_page_size                   vm_page_size    /* to get to the originals */
#define _vm_page_shift                  vm_page_shift
#define vm_page_size                    4096            /* our normal working sizes */
#define vm_page_shift                   12

/*
 * msize - a type to refer to the number of quanta of a tiny or small
 * allocation.  A tiny block with an msize of 3 would be 3 << SHIFT_TINY_QUANTUM 
 * bytes in size.
 */
typedef unsigned short msize_t;

typedef union {
    void        *p;
    uintptr_t   u;
} ptr_union;

typedef struct {
    ptr_union   previous;
    ptr_union   next;
} free_list_t;

typedef unsigned int grain_t; // N.B. wide enough to index all free slots

typedef int mag_index_t;

#define CHECK_REGIONS                   (1 << 31)

#define MAX_RECORDER_BUFFER             256

/*********************  DEFINITIONS for tiny    ************************/

/*
 * Memory in the Tiny range is allocated from regions (heaps) pointed to by the
 * szone's hashed_regions pointer.
 *
 * Each region is laid out as a heap, followed by a header block, all within
 * a 1MB (2^20) block.  This means there are 64520 16-byte blocks and the header
 * is 16138 bytes, making the total 1048458 bytes, leaving 118 bytes unused.
 *
 * The header block is arranged as in struct tiny_region defined just below, and
 * consists of two bitfields (or bit arrays) interleaved 32 bits by 32 bits.
 *
 * Each bitfield comprises NUM_TINY_BLOCKS bits, and refers to the corresponding
 * TINY_QUANTUM block within the heap.
 *
 * The bitfields are used to encode the state of memory within the heap.  The header bit indicates
 * that the corresponding quantum is the first quantum in a block (either in use or free).  The
 * in-use bit is set for the header if the block has been handed out (allocated).  If the header
 * bit is not set, the in-use bit is invalid.
 *
 * The szone maintains an array of NUM_TINY_SLOTS freelists, each of which is used to hold 
 * free objects of the corresponding quantum size.
 *
 * A free block is laid out depending on its size, in order to fit all free
 * blocks in 16 bytes, on both 32 and 64 bit platforms.  One quantum blocks do
 * not store their size in the block, instead relying on the header information
 * to determine their size.  Blocks of two or more quanta have room to store
 * their size in the block, and store it both after the 'next' pointer, and in
 * the last 2 bytes of the block.
 *
 * 1-quantum block
 * Offset (32-bit mode) (64-bit mode)
 * 0x0          0x0      : previous
 * 0x4          0x08     : next
 * end          end
 *
 * >1-quantum block
 * Offset (32-bit mode) (64-bit mode)
 * 0x0          0x0      : previous
 * 0x4          0x08     : next
 * 0x8          0x10     : size (in quantum counts)
 * end - 2      end - 2  : size (in quantum counts)
 * end          end
 *
 * All fields are pointer-sized, except for the size which is an unsigned short.
 *
 */

#define SHIFT_TINY_QUANTUM              4       // Required for AltiVec
#define TINY_QUANTUM                    (1 << SHIFT_TINY_QUANTUM)

#define FOLLOWING_TINY_PTR(ptr,msize)   (((unsigned char *)(ptr)) + ((msize) << SHIFT_TINY_QUANTUM))

#ifdef __LP64__
#define NUM_TINY_SLOTS                  64      // number of slots for free-lists
#else
#define NUM_TINY_SLOTS                  32      // number of slots for free-lists
#endif

#define NUM_TINY_BLOCKS                 64520
#define SHIFT_TINY_CEIL_BLOCKS          16 // ceil(log2(NUM_TINY_BLOCKS))
#define NUM_TINY_CEIL_BLOCKS            (1 << SHIFT_TINY_CEIL_BLOCKS)
#define TINY_BLOCKS_ALIGN               (SHIFT_TINY_CEIL_BLOCKS + SHIFT_TINY_QUANTUM) // 20

/*
 * Enough room for the data, followed by the bit arrays (2-bits per block)
 * plus rounding to the nearest page.
 */
#define CEIL_NUM_TINY_BLOCKS_WORDS      (((NUM_TINY_BLOCKS + 31) & ~31) >> 5)
#define TINY_METADATA_SIZE              (sizeof(region_trailer_t) + sizeof(tiny_header_inuse_pair_t) * CEIL_NUM_TINY_BLOCKS_WORDS)
#define TINY_REGION_SIZE                                                        \
    ((NUM_TINY_BLOCKS * TINY_QUANTUM + TINY_METADATA_SIZE + vm_page_size - 1) & ~ (vm_page_size - 1))

#define TINY_METADATA_START             (NUM_TINY_BLOCKS * TINY_QUANTUM)

/*
 * Beginning and end pointers for a region's heap.
 */
#define TINY_REGION_ADDRESS(region)     ((void *)(region))
#define TINY_REGION_END(region)         ((void *)(((uintptr_t)(region)) + (NUM_TINY_BLOCKS * TINY_QUANTUM)))

/*
 * Locate the heap base for a pointer known to be within a tiny region.
 */
#define TINY_REGION_FOR_PTR(_p)         ((void *)((uintptr_t)(_p) & ~((1 << TINY_BLOCKS_ALIGN) - 1)))

/*
 * Convert between byte and msize units.
 */
#define TINY_BYTES_FOR_MSIZE(_m)        ((_m) << SHIFT_TINY_QUANTUM)
#define TINY_MSIZE_FOR_BYTES(_b)        ((_b) >> SHIFT_TINY_QUANTUM)

#ifdef __LP64__
# define TINY_FREE_SIZE(ptr)            (((msize_t *)(ptr))[8])
#else
# define TINY_FREE_SIZE(ptr)            (((msize_t *)(ptr))[4])
#endif
#define TINY_PREVIOUS_MSIZE(ptr)        ((msize_t *)(ptr))[-1]

/*
 * Layout of a tiny region
 */
typedef uint32_t tiny_block_t[4]; // assert(TINY_QUANTUM == sizeof(tiny_block_t))

typedef struct tiny_header_inuse_pair
{
    uint32_t    header;
    uint32_t    inuse;
} tiny_header_inuse_pair_t;

typedef struct region_trailer
{
    struct region_trailer       *prev;
    struct region_trailer       *next;
    boolean_t                   recirc_suitable;
    unsigned                    bytes_used;
    mag_index_t                 mag_index;
} region_trailer_t;

typedef struct tiny_region
{
    tiny_block_t blocks[NUM_TINY_BLOCKS];

    region_trailer_t trailer;
    
    // The interleaved bit arrays comprising the header and inuse bitfields.
    // The unused bits of each component in the last pair will be initialized to sentinel values.
    tiny_header_inuse_pair_t pairs[CEIL_NUM_TINY_BLOCKS_WORDS];
    
    uint8_t pad[TINY_REGION_SIZE - (NUM_TINY_BLOCKS * sizeof(tiny_block_t)) - TINY_METADATA_SIZE];
} *tiny_region_t;

/*
 * Per-region meta data for tiny allocator
 */
#define REGION_TRAILER_FOR_TINY_REGION(r)       (&(((tiny_region_t)(r))->trailer))
#define MAGAZINE_INDEX_FOR_TINY_REGION(r)       (REGION_TRAILER_FOR_TINY_REGION(r)->mag_index)
#define BYTES_USED_FOR_TINY_REGION(r)           (REGION_TRAILER_FOR_TINY_REGION(r)->bytes_used)

/*
 * Locate the block header for a pointer known to be within a tiny region.
 */
#define TINY_BLOCK_HEADER_FOR_PTR(_p)   ((void *)&(((tiny_region_t)TINY_REGION_FOR_PTR(_p))->pairs))

/*
 * Locate the inuse map for a given block header pointer.
 */ 
#define TINY_INUSE_FOR_HEADER(_h)       ((void *)&(((tiny_header_inuse_pair_t *)(_h))->inuse))

/*
 * Compute the bitmap index for a pointer known to be within a tiny region.
 */
#define TINY_INDEX_FOR_PTR(_p)          (((uintptr_t)(_p) >> SHIFT_TINY_QUANTUM) & (NUM_TINY_CEIL_BLOCKS - 1))

#define TINY_CACHE                      1       // This governs a last-free cache of 1 that bypasses the free-list

#if ! TINY_CACHE
#warning TINY_CACHE turned off
#endif

#define TINY_REGION_PAYLOAD_BYTES       (NUM_TINY_BLOCKS * TINY_QUANTUM)

/*********************  DEFINITIONS for small   ************************/

/*
 * Memory in the Small range is allocated from regions (heaps) pointed to by the szone's hashed_regions
 * pointer.
 *
 * Each region is laid out as a heap, followed by the metadata array, all within an 8MB (2^23) block.
 * The array is arranged as an array of shorts, one for each SMALL_QUANTUM in the heap.
 * This means there are 16320 512-blocks and the array is 16320*2 bytes, which totals 8388480, leaving
 * 128 bytes unused.
 *
 * The MSB of each short is set for the first quantum in a free block.  The low 15 bits encode the
 * block size (in SMALL_QUANTUM units), or are zero if the quantum is not the first in a block.
 *
 * The szone maintains an array of 32 freelists, each of which is used to hold free objects
 * of the corresponding quantum size.
 *
 * A free block is laid out as:
 *
 * Offset (32-bit mode) (64-bit mode)
 * 0x0          0x0      : previous
 * 0x4          0x08     : next
 * 0x8          0x10     : size (in quantum counts)
 * end - 2      end - 2  : size (in quantum counts)
 * end          end
 *
 * All fields are pointer-sized, except for the size which is an unsigned short.
 *
 */

#define SMALL_IS_FREE                   (1 << 15)

#define SHIFT_SMALL_QUANTUM             (SHIFT_TINY_QUANTUM + 5)        // 9
#define SMALL_QUANTUM                   (1 << SHIFT_SMALL_QUANTUM)      // 512 bytes

#define FOLLOWING_SMALL_PTR(ptr,msize)  (((unsigned char *)(ptr)) + ((msize) << SHIFT_SMALL_QUANTUM))

/*
 * The number of slots in the free-list for small blocks.  To avoid going to 
 * vm system as often on large memory machines, increase the number of free list
 * spots above some amount of RAM installed in the system.
 */
#define NUM_SMALL_SLOTS                 32
#define NUM_SMALL_SLOTS_LARGEMEM        256
#define SMALL_BITMAP_WORDS              8

/*
 * We can only represent up to 1<<15 for msize; but we choose to stay even below that to avoid the
 * convention msize=0 => msize = (1<<15)
 */
#define NUM_SMALL_BLOCKS                16320
#define SHIFT_SMALL_CEIL_BLOCKS         14 // ceil(log2(NUM_SMALL_BLOCKs))
#define NUM_SMALL_CEIL_BLOCKS           (1 << SHIFT_SMALL_CEIL_BLOCKS)
#define SMALL_BLOCKS_ALIGN              (SHIFT_SMALL_CEIL_BLOCKS + SHIFT_SMALL_QUANTUM) // 23

#define SMALL_METADATA_SIZE             (sizeof(region_trailer_t) + NUM_SMALL_BLOCKS * sizeof(msize_t))
#define SMALL_REGION_SIZE                                               \
    ((NUM_SMALL_BLOCKS * SMALL_QUANTUM + SMALL_METADATA_SIZE + vm_page_size - 1) & ~ (vm_page_size - 1))

#define SMALL_METADATA_START            (NUM_SMALL_BLOCKS * SMALL_QUANTUM)

/*
 * Beginning and end pointers for a region's heap.
 */
#define SMALL_REGION_ADDRESS(region)    ((unsigned char *)region)
#define SMALL_REGION_END(region)        (SMALL_REGION_ADDRESS(region) + (NUM_SMALL_BLOCKS * SMALL_QUANTUM))

/*
 * Locate the heap base for a pointer known to be within a small region.
 */
#define SMALL_REGION_FOR_PTR(_p)        ((void *)((uintptr_t)(_p) & ~((1 << SMALL_BLOCKS_ALIGN) - 1)))

/*
 * Convert between byte and msize units.
 */
#define SMALL_BYTES_FOR_MSIZE(_m)       ((_m) << SHIFT_SMALL_QUANTUM)
#define SMALL_MSIZE_FOR_BYTES(_b)       ((_b) >> SHIFT_SMALL_QUANTUM)

#define SMALL_PREVIOUS_MSIZE(ptr)       ((msize_t *)(ptr))[-1]

/*
 * Layout of a small region
 */
typedef uint32_t small_block_t[SMALL_QUANTUM/sizeof(uint32_t)];

typedef struct small_region
{
    small_block_t blocks[NUM_SMALL_BLOCKS];

    region_trailer_t trailer;
    
    msize_t small_meta_words[NUM_SMALL_BLOCKS];
    
    uint8_t pad[SMALL_REGION_SIZE - (NUM_SMALL_BLOCKS * sizeof(small_block_t)) - SMALL_METADATA_SIZE];
} *small_region_t;

/*
 * Per-region meta data for small allocator
 */
#define REGION_TRAILER_FOR_SMALL_REGION(r)      (&(((small_region_t)(r))->trailer))
#define MAGAZINE_INDEX_FOR_SMALL_REGION(r)      (REGION_TRAILER_FOR_SMALL_REGION(r)->mag_index)
#define BYTES_USED_FOR_SMALL_REGION(r)          (REGION_TRAILER_FOR_SMALL_REGION(r)->bytes_used)

/*
 * Locate the metadata base for a pointer known to be within a small region.
 */
#define SMALL_META_HEADER_FOR_PTR(_p)   (((small_region_t)SMALL_REGION_FOR_PTR(_p))->small_meta_words)

/*
 * Compute the metadata index for a pointer known to be within a small region.
 */
#define SMALL_META_INDEX_FOR_PTR(_p)    (((uintptr_t)(_p) >> SHIFT_SMALL_QUANTUM) & (NUM_SMALL_CEIL_BLOCKS - 1))

/*
 * Find the metadata word for a pointer known to be within a small region.
 */
#define SMALL_METADATA_FOR_PTR(_p)      (SMALL_META_HEADER_FOR_PTR(_p) + SMALL_META_INDEX_FOR_PTR(_p))

/*
 * Determine whether a pointer known to be within a small region points to memory which is free.
 */
#define SMALL_PTR_IS_FREE(_p)           (*SMALL_METADATA_FOR_PTR(_p) & SMALL_IS_FREE)

/*
 * Extract the msize value for a pointer known to be within a small region.
 */
#define SMALL_PTR_SIZE(_p)              (*SMALL_METADATA_FOR_PTR(_p) & ~SMALL_IS_FREE)

#define PROTECT_SMALL                   0       // Should be 0: 1 is too slow for normal use

#define SMALL_CACHE                     1
#if !SMALL_CACHE
#warning SMALL_CACHE turned off
#endif

#define SMALL_REGION_PAYLOAD_BYTES      (NUM_SMALL_BLOCKS * SMALL_QUANTUM)

/*************************  DEFINITIONS for large  ****************************/

#define LARGE_THRESHOLD                 (15 * 1024) // strictly above this use "large"
#define LARGE_THRESHOLD_LARGEMEM        (127 * 1024)

#if (LARGE_THRESHOLD > NUM_SMALL_SLOTS * SMALL_QUANTUM)
#error LARGE_THRESHOLD should always be less than NUM_SMALL_SLOTS * SMALL_QUANTUM
#endif

#if (LARGE_THRESHOLD_LARGEMEM > NUM_SMALL_SLOTS_LARGEMEM * SMALL_QUANTUM)
#error LARGE_THRESHOLD_LARGEMEM should always be less than NUM_SMALL_SLOTS * SMALL_QUANTUM
#endif

/*
 * When all memory is touched after a copy, vm_copy() is always a lose
 * But if the memory is only read, vm_copy() wins over memmove() at 3 or 4 pages
 * (on a G3/300MHz)
 *
 * This must be larger than LARGE_THRESHOLD
 */
#define VM_COPY_THRESHOLD               (40 * 1024)
#define VM_COPY_THRESHOLD_LARGEMEM      (128 * 1024)

typedef struct {
    vm_address_t address;
    vm_size_t size;
    boolean_t did_madvise_reusable;
} large_entry_t;

#define LARGE_CACHE                     1
#if !LARGE_CACHE
#warning LARGE_CACHE turned off
#endif
#if defined(__LP64__)
#define LARGE_ENTRY_CACHE_SIZE 16
#define LARGE_CACHE_SIZE_LIMIT ((vm_size_t)0x80000000) /* 2Gb */
#else
#define LARGE_ENTRY_CACHE_SIZE 8
#define LARGE_CACHE_SIZE_LIMIT ((vm_size_t)0x02000000) /* 32Mb */
#endif
#define LARGE_CACHE_SIZE_ENTRY_LIMIT (LARGE_CACHE_SIZE_LIMIT/LARGE_ENTRY_CACHE_SIZE)

/*******************************************************************************
 * Definitions for region hash
 ******************************************************************************/

typedef void * region_t;
typedef region_t * rgnhdl_t; /* A pointer into hashed_regions array. */

#define INITIAL_NUM_REGIONS_SHIFT       6 // log2(INITIAL_NUM_REGIONS)
#define INITIAL_NUM_REGIONS             (1 << INITIAL_NUM_REGIONS_SHIFT) // Must be a power of 2!
#define HASHRING_OPEN_ENTRY             ((region_t) 0) // Initial value and sentinel marking end of collision chain
#define HASHRING_REGION_DEALLOCATED     ((region_t)-1) // Region at this slot reclaimed by OS
#define HASH_BLOCKS_ALIGN               TINY_BLOCKS_ALIGN // MIN( TINY_BLOCKS_ALIGN, SMALL_BLOCKS_ALIGN, ... )

typedef struct region_hash_generation {
    size_t              num_regions_allocated;
    size_t              num_regions_allocated_shift; // log2(num_regions_allocated)
    region_t            *hashed_regions;  // hashed by location
    struct              region_hash_generation *nextgen;
} region_hash_generation_t;

/*******************************************************************************
 * Per-processor magazine for tiny and small allocators
 ******************************************************************************/

typedef struct {                        // vm_allocate()'d, so the array of magazines is page-aligned to begin with.
    // Take magazine_lock first,  Depot lock when needed for recirc, then szone->{tiny,small}_regions_lock when needed for alloc
    pthread_lock_t      magazine_lock CACHE_ALIGN;
    
    // One element deep "death row", optimizes malloc/free/malloc for identical size.
    void                *mag_last_free; // low SHIFT_{TINY,SMALL}_QUANTUM bits indicate the msize
    region_t            mag_last_free_rgn; // holds the region for mag_last_free

    free_list_t         *mag_free_list[256]; // assert( 256 >= MAX( NUM_TINY_SLOTS, NUM_SMALL_SLOTS_LARGEMEM ))
    unsigned            mag_bitmap[8]; // assert( sizeof(mag_bitmap) << 3 >= sizeof(mag_free_list)/sizeof(free_list_t) )
    
    // the last free region in the last block is treated as a big block in use that is not accounted for
    size_t              mag_bytes_free_at_end;
    region_t            mag_last_region; // Valid iff mag_bytes_free_at_end > 0
    
    // bean counting ...
    unsigned            mag_num_objects;
    size_t              mag_num_bytes_in_objects;
    size_t              num_bytes_in_magazine;
    
    // recirculation list -- invariant: all regions owned by this magazine that meet the emptiness criteria 
    // are located nearer to the head of the list than any region that doesn't satisfy that criteria.
    // Doubly linked list for efficient extraction.
    unsigned            recirculation_entries;
    region_trailer_t    *firstNode;
    region_trailer_t    *lastNode;
    
#if __LP64__
    uint64_t            pad[49]; // So sizeof(magazine_t) is 2560 bytes. FIXME: assert this at compile time
#else
    uint32_t            pad[45]; // So sizeof(magazine_t) is 1280 bytes. FIXME: assert this at compile time
#endif
} magazine_t;

#define TINY_MAX_MAGAZINES              16 /* MUST BE A POWER OF 2! */    
#define TINY_MAGAZINE_PAGED_SIZE                                                \
    (((sizeof(magazine_t) * (TINY_MAX_MAGAZINES + 1)) + vm_page_size - 1) &\
    ~ (vm_page_size - 1)) /* + 1 for the Depot */
                                                
#define SMALL_MAX_MAGAZINES             16 /* MUST BE A POWER OF 2! */    
#define SMALL_MAGAZINE_PAGED_SIZE                                               \
    (((sizeof(magazine_t) * (SMALL_MAX_MAGAZINES + 1)) + vm_page_size - 1) &\
    ~ (vm_page_size - 1)) /* + 1 for the Depot */

#define DEPOT_MAGAZINE_INDEX            -1

/****************************** zone itself ***********************************/

/*
 * Note that objects whose adddress are held in pointers here must be pursued 
 * individually in the {tiny,small}_in_use_enumeration() routines. See for
 * example the treatment of region_hash_generation and tiny_magazines below.
 */

typedef struct szone_s {                                // vm_allocate()'d, so page-aligned to begin with.
    malloc_zone_t               basic_zone;
    pthread_key_t               cpu_id_key;
    unsigned                    debug_flags;
    void                        *log_address;

    /* Regions for tiny objects */
    pthread_lock_t              tiny_regions_lock CACHE_ALIGN;
    size_t                      num_tiny_regions;
    size_t                      num_tiny_regions_dealloc;
    region_hash_generation_t    *tiny_region_generation;
    region_hash_generation_t    trg[2];
    
    int                         num_tiny_magazines;
    unsigned                    num_tiny_magazines_mask;
    int                         num_tiny_magazines_mask_shift;
    magazine_t                  *tiny_magazines; // array of per-processor magazines

    /* Regions for small objects */
    pthread_lock_t              small_regions_lock CACHE_ALIGN;
    size_t                      num_small_regions;
    size_t                      num_small_regions_dealloc;
    region_hash_generation_t    *small_region_generation;
    region_hash_generation_t    srg[2];
    
    unsigned                    num_small_slots; // determined by physmem size
    
    int                         num_small_magazines;
    unsigned                    num_small_magazines_mask;
    int                         num_small_magazines_mask_shift;
    magazine_t                  *small_magazines; // array of per-processor magazines

    /* large objects: all the rest */
    pthread_lock_t              large_szone_lock CACHE_ALIGN; // One customer at a time for large
    unsigned                    num_large_objects_in_use;
    unsigned                    num_large_entries;
    large_entry_t               *large_entries; // hashed by location; null entries don't count
    size_t                      num_bytes_in_large_objects;
    
#if LARGE_CACHE
    int                         large_entry_cache_oldest;
    int                         large_entry_cache_newest;
    large_entry_t               large_entry_cache[LARGE_ENTRY_CACHE_SIZE]; // "death row" for large malloc/free
    boolean_t                   large_legacy_reset_mprotect;
    size_t                      large_entry_cache_hoard_bytes;
    size_t                      large_entry_cache_hoard_lmit;
#endif

    /* flag and limits pertaining to altered malloc behavior for systems with
       large amounts of physical memory */
    unsigned  is_largemem;
    unsigned  large_threshold;
    unsigned  vm_copy_threshold;

    /* security cookie */
    uintptr_t cookie;

    /* Initial region list */
    region_t                    initial_tiny_regions[INITIAL_NUM_REGIONS];
    region_t                    initial_small_regions[INITIAL_NUM_REGIONS];
    
        /* The purgeable zone constructed by create_purgeable_zone() would like to hand off tiny and small
         * allocations to the default scalable zone. Record the latter as the "helper" zone here. */
    struct szone_s              *helper_zone; 
} szone_t;

#define SZONE_PAGED_SIZE                ((sizeof(szone_t) + vm_page_size - 1) & ~ (vm_page_size - 1))

#if DEBUG_MALLOC || DEBUG_CLIENT
static void             szone_sleep(void);
#endif
__private_extern__ void malloc_error_break(void);

// msg prints after fmt, ...
static NOINLINE void    szone_error(szone_t *szone, int is_corruption, const char *msg, const void *ptr, const char *fmt, ...)
                        __printflike(5, 6);

static void             protect(void *address, size_t size, unsigned protection, unsigned debug_flags);
static void             *allocate_pages(szone_t *szone, size_t size, unsigned char align, unsigned debug_flags,
                                        int vm_page_label);
static void             deallocate_pages(szone_t *szone, void *addr, size_t size, unsigned debug_flags);
static int              madvise_free_range(szone_t *szone, region_t r, uintptr_t pgLo, uintptr_t pgHi);
static kern_return_t    _szone_default_reader(task_t task, vm_address_t address, vm_size_t size, void **ptr);

static INLINE           mag_index_t mag_get_thread_index(szone_t *szone) ALWAYSINLINE;
static magazine_t       *mag_lock_zine_for_region_trailer(szone_t *szone, magazine_t *magazines, region_trailer_t *trailer, 
                                                          mag_index_t mag_index);
                        
static INLINE rgnhdl_t  hash_lookup_region_no_lock(region_t *regions, size_t num_entries, size_t shift, region_t r) 
                        ALWAYSINLINE;
static void             hash_region_insert_no_lock(region_t *regions, size_t num_entries, size_t shift, region_t r);
static region_t         *hash_regions_alloc_no_lock(szone_t *szone, size_t num_entries);
static region_t         *hash_regions_grow_no_lock(szone_t *szone, region_t *regions, size_t old_size, 
                                                   size_t *mutable_shift, size_t *new_size);

static INLINE uintptr_t free_list_gen_checksum(uintptr_t ptr) ALWAYSINLINE;
static INLINE uintptr_t free_list_checksum_ptr(szone_t *szone, void *p) ALWAYSINLINE;
static INLINE void      *free_list_unchecksum_ptr(szone_t *szone, ptr_union *ptr) ALWAYSINLINE;
static unsigned         free_list_count(szone_t *szone, free_list_t *ptr);

static INLINE void      recirc_list_extract(szone_t *szone, magazine_t *mag_ptr, region_trailer_t *node) ALWAYSINLINE;
static INLINE void      recirc_list_splice_last(szone_t *szone, magazine_t *mag_ptr, region_trailer_t *node) ALWAYSINLINE;
static INLINE void      recirc_list_splice_first(szone_t *szone, magazine_t *mag_ptr, region_trailer_t *node) ALWAYSINLINE;

static INLINE void      BITARRAY_SET(uint32_t *bits, msize_t index) ALWAYSINLINE;
static INLINE void      BITARRAY_CLR(uint32_t *bits, msize_t index) ALWAYSINLINE;
static INLINE boolean_t BITARRAY_BIT(uint32_t *bits, msize_t index) ALWAYSINLINE;

static msize_t          get_tiny_free_size(const void *ptr);
static msize_t          get_tiny_previous_free_msize(const void *ptr);
static INLINE msize_t   get_tiny_meta_header(const void *ptr, boolean_t *is_free) ALWAYSINLINE;
static INLINE void      set_tiny_meta_header_in_use(const void *ptr, msize_t msize) ALWAYSINLINE;
static INLINE void      set_tiny_meta_header_in_use_1(const void *ptr) ALWAYSINLINE;
static INLINE void      set_tiny_meta_header_middle(const void *ptr) ALWAYSINLINE;
static INLINE void      set_tiny_meta_header_free(const void *ptr, msize_t msize) ALWAYSINLINE;
static INLINE boolean_t tiny_meta_header_is_free(const void *ptr) ALWAYSINLINE;
static INLINE void      *tiny_previous_preceding_free(void *ptr, msize_t *prev_msize) ALWAYSINLINE;

static void             tiny_free_list_add_ptr(szone_t *szone, magazine_t *tiny_mag_ptr, void *ptr, msize_t msize);
static void             tiny_free_list_remove_ptr(szone_t *szone, magazine_t *tiny_mag_ptr, void *ptr, msize_t msize);
static INLINE region_t  tiny_region_for_ptr_no_lock(szone_t *szone, const void *ptr) ALWAYSINLINE;

static void             tiny_finalize_region(szone_t *szone, magazine_t *tiny_mag_ptr);
static int              tiny_free_detach_region(szone_t *szone, magazine_t *tiny_mag_ptr, region_t r);
static size_t           tiny_free_reattach_region(szone_t *szone, magazine_t *tiny_mag_ptr, region_t r);
static void             tiny_free_scan_madvise_free(szone_t *szone, magazine_t *depot_ptr, region_t r);
static void             tiny_free_try_depot_unmap_no_lock(szone_t *szone, magazine_t *depot_ptr, region_trailer_t *node);
static void             tiny_free_do_recirc_to_depot(szone_t *szone, magazine_t *tiny_mag_ptr, mag_index_t mag_index);
static boolean_t        tiny_get_region_from_depot(szone_t *szone, magazine_t *tiny_mag_ptr, mag_index_t mag_index);

static INLINE void      tiny_free_no_lock(szone_t *szone, magazine_t *tiny_mag_ptr, mag_index_t mag_index, region_t region, 
                                          void *ptr, msize_t msize) ALWAYSINLINE;
static void             *tiny_malloc_from_region_no_lock(szone_t *szone, magazine_t *tiny_mag_ptr, mag_index_t mag_index, 
                                                         msize_t msize);
static boolean_t        tiny_try_realloc_in_place(szone_t *szone, void *ptr, size_t old_size, size_t new_size);
static boolean_t        tiny_check_region(szone_t *szone, region_t region);
static kern_return_t    tiny_in_use_enumerator(task_t task, void *context, unsigned type_mask, szone_t *szone, 
                                               memory_reader_t reader, vm_range_recorder_t recorder);
static void             *tiny_malloc_from_free_list(szone_t *szone, magazine_t *tiny_mag_ptr, mag_index_t mag_index, 
                                                    msize_t msize);
static INLINE void      *tiny_malloc_should_clear(szone_t *szone, msize_t msize, boolean_t cleared_requested) ALWAYSINLINE;
static INLINE void      free_tiny(szone_t *szone, void *ptr, region_t tiny_region, size_t known_size) ALWAYSINLINE;
static void             print_tiny_free_list(szone_t *szone);
static void             print_tiny_region(boolean_t verbose, region_t region, size_t bytes_at_end);
static boolean_t        tiny_free_list_check(szone_t *szone, grain_t slot);

static INLINE void      small_meta_header_set_is_free(msize_t *meta_headers, unsigned index, msize_t msize) ALWAYSINLINE;
static INLINE void      small_meta_header_set_in_use(msize_t *meta_headers, msize_t index, msize_t msize) ALWAYSINLINE;
static INLINE void      small_meta_header_set_middle(msize_t *meta_headers, msize_t index) ALWAYSINLINE;
static void             small_free_list_add_ptr(szone_t *szone, magazine_t *small_mag_ptr, void *ptr, msize_t msize);
static void             small_free_list_remove_ptr(szone_t *szone, magazine_t *small_mag_ptr, void *ptr, msize_t msize);
static INLINE region_t  small_region_for_ptr_no_lock(szone_t *szone, const void *ptr) ALWAYSINLINE;

static void             small_finalize_region(szone_t *szone, magazine_t *small_mag_ptr);
static int              small_free_detach_region(szone_t *szone, magazine_t *small_mag_ptr, region_t r);
static size_t           small_free_reattach_region(szone_t *szone, magazine_t *small_mag_ptr, region_t r);
static void             small_free_scan_depot_madvise_free(szone_t *szone, magazine_t *depot_ptr, region_t r);
static void             small_free_try_depot_unmap_no_lock(szone_t *szone, magazine_t *depot_ptr, region_trailer_t *node);
static void             small_free_do_recirc_to_depot(szone_t *szone, magazine_t *small_mag_ptr, mag_index_t mag_index);
static boolean_t        small_get_region_from_depot(szone_t *szone, magazine_t *small_mag_ptr, mag_index_t mag_index);
static INLINE void      small_free_no_lock(szone_t *szone, magazine_t *small_mag_ptr, mag_index_t mag_index, region_t region, 
                                           void *ptr, msize_t msize) ALWAYSINLINE;
static void             *small_malloc_from_region_no_lock(szone_t *szone, magazine_t *small_mag_ptr, mag_index_t mag_index, 
                                                          msize_t msize);
static boolean_t        small_try_realloc_in_place(szone_t *szone, void *ptr, size_t old_size, size_t new_size);
static boolean_t        small_check_region(szone_t *szone, region_t region);
static kern_return_t    small_in_use_enumerator(task_t task, void *context, unsigned type_mask, szone_t *szone,
                                                memory_reader_t reader, vm_range_recorder_t recorder);
static void             *small_malloc_from_free_list(szone_t *szone, magazine_t *small_mag_ptr, mag_index_t mag_index, 
                                                     msize_t msize);
static INLINE void      *small_malloc_should_clear(szone_t *szone, msize_t msize, boolean_t cleared_requested) ALWAYSINLINE;
static INLINE void      free_small(szone_t *szone, void *ptr, region_t small_region, size_t known_size) ALWAYSINLINE;
static void             print_small_free_list(szone_t *szone);
static void             print_small_region(szone_t *szone, boolean_t verbose, region_t region, size_t bytes_at_end);
static boolean_t        small_free_list_check(szone_t *szone, grain_t grain);

#if DEBUG_MALLOC
static void             large_debug_print(szone_t *szone);
#endif
static large_entry_t    *large_entry_for_pointer_no_lock(szone_t *szone, const void *ptr);
static void             large_entry_insert_no_lock(szone_t *szone, large_entry_t range);
static INLINE void      large_entries_rehash_after_entry_no_lock(szone_t *szone, large_entry_t *entry) ALWAYSINLINE;
static INLINE large_entry_t *large_entries_alloc_no_lock(szone_t *szone, unsigned num) ALWAYSINLINE;
static void             large_entries_free_no_lock(szone_t *szone, large_entry_t *entries, unsigned num, 
                                                   vm_range_t *range_to_deallocate);
static large_entry_t    *large_entries_grow_no_lock(szone_t *szone, vm_range_t *range_to_deallocate);
static vm_range_t       large_entry_free_no_lock(szone_t *szone, large_entry_t *entry);
static NOINLINE         kern_return_t   large_in_use_enumerator(task_t task, void *context,
                                                                unsigned type_mask, vm_address_t large_entries_address,
                                                                unsigned num_entries, memory_reader_t reader, vm_range_recorder_t recorder);
static void             *large_malloc(szone_t *szone, size_t num_pages, unsigned char alignment, boolean_t cleared_requested);
static NOINLINE void    free_large(szone_t *szone, void *ptr);
static INLINE int       large_try_realloc_in_place(szone_t *szone, void *ptr, size_t old_size, size_t new_size) ALWAYSINLINE;

/*
 * Mark these NOINLINE to avoid bloating the purgeable zone call backs
 */
static NOINLINE void    szone_free(szone_t *szone, void *ptr);
static NOINLINE void    *szone_malloc_should_clear(szone_t *szone, size_t size, boolean_t cleared_requested);
static NOINLINE void    *szone_malloc(szone_t *szone, size_t size);
static NOINLINE void    *szone_calloc(szone_t *szone, size_t num_items, size_t size);
static NOINLINE void    *szone_valloc(szone_t *szone, size_t size);
static NOINLINE size_t  szone_size_try_large(szone_t *szone, const void *ptr);
static NOINLINE size_t  szone_size(szone_t *szone, const void *ptr);
static NOINLINE void    *szone_realloc(szone_t *szone, void *ptr, size_t new_size);
static NOINLINE void    *szone_memalign(szone_t *szone, size_t alignment, size_t size);
static NOINLINE void    szone_free_definite_size(szone_t *szone, void *ptr, size_t size);
static NOINLINE unsigned szone_batch_malloc(szone_t *szone, size_t size, void **results, unsigned count);
static NOINLINE void    szone_batch_free(szone_t *szone, void **to_be_freed, unsigned count);
static void             szone_destroy(szone_t *szone);
static NOINLINE size_t  szone_good_size(szone_t *szone, size_t size);

static NOINLINE boolean_t szone_check_all(szone_t *szone, const char *function);
static boolean_t        szone_check(szone_t *szone);
static kern_return_t    szone_ptr_in_use_enumerator(task_t task, void *context, 
                                                    unsigned type_mask, vm_address_t zone_address,
                                                    memory_reader_t reader, vm_range_recorder_t recorder);
static NOINLINE void    szone_print(szone_t *szone, boolean_t verbose);
static void             szone_log(malloc_zone_t *zone, void *log_address);
static void             szone_force_lock(szone_t *szone);
static void             szone_force_unlock(szone_t *szone);
static boolean_t        szone_locked(szone_t *szone);

static void             szone_statistics(szone_t *szone, malloc_statistics_t *stats);

static void             purgeable_free(szone_t *szone, void *ptr);
static void             *purgeable_malloc(szone_t *szone, size_t size);
static void             *purgeable_calloc(szone_t *szone, size_t num_items, size_t size);
static void             *purgeable_valloc(szone_t *szone, size_t size);
static size_t           purgeable_size(szone_t *szone, const void *ptr);
static void             *purgeable_realloc(szone_t *szone, void *ptr, size_t new_size);
static void             *purgeable_memalign(szone_t *szone, size_t alignment, size_t size);
static void             purgeable_free_definite_size(szone_t *szone, void *ptr, size_t size);
static unsigned         purgeable_batch_malloc(szone_t *szone, size_t size, void **results, unsigned count);
static void             purgeable_batch_free(szone_t *szone, void **to_be_freed, unsigned count);
static void             purgeable_destroy(szone_t *szone);
static size_t           purgeable_good_size(szone_t *szone, size_t size);

static boolean_t        purgeable_check(szone_t *szone);
static kern_return_t    purgeable_ptr_in_use_enumerator(task_t task, void *context, 
                                                        unsigned type_mask, vm_address_t zone_address,
                                                        memory_reader_t reader, vm_range_recorder_t recorder);
static void             purgeable_print(szone_t *szone, boolean_t verbose);
static void             purgeable_log(malloc_zone_t *zone, void *log_address);
static void             purgeable_force_lock(szone_t *szone);
static void             purgeable_force_unlock(szone_t *szone);
static boolean_t        purgeable_locked(szone_t *szone);

static void             purgeable_statistics(szone_t *szone, malloc_statistics_t *stats);

static void             *frozen_malloc(szone_t *zone, size_t new_size);
static void             *frozen_calloc(szone_t *zone, size_t num_items, size_t size);
static void             *frozen_valloc(szone_t *zone, size_t new_size);
static void             *frozen_realloc(szone_t *zone, void *ptr, size_t new_size);
static void             frozen_free(szone_t *zone, void *ptr);
static void             frozen_destroy(szone_t *zone);

#define SZONE_LOCK(szone)                                               \
    do {                                                                \
        LOCK(szone->large_szone_lock);                                  \
    } while (0)
        
#define SZONE_UNLOCK(szone)                                             \
    do {                                                                \
        UNLOCK(szone->large_szone_lock);                                \
    } while (0)

#define SZONE_TRY_LOCK(szone)                                           \
  TRY_LOCK(szone->large_szone_lock);

#define SZONE_MAGAZINE_PTR_LOCK(szone, mag_ptr)                         \
    do {                                                                \
        LOCK(mag_ptr->magazine_lock);                                   \
    } while(0)

#define SZONE_MAGAZINE_PTR_UNLOCK(szone, mag_ptr)                       \
    do {                                                                \
        UNLOCK(mag_ptr->magazine_lock);                                 \
    } while(0)

#define SZONE_MAGAZINE_PTR_TRY_LOCK(szone, mag_ptr)                     \
  TRY_LOCK(mag_ptr->magazine_lock);

#if DEBUG_MALLOC
# define LOG(szone,ptr)                                                 \
    (szone->log_address && (((uintptr_t)szone->log_address == -1) ||    \
    (szone->log_address == (void *)(ptr))))
#else
# define LOG(szone,ptr)         0
#endif

#if DEBUG_MALLOC || DEBUG_CLIENT
# define CHECK(szone,fun)                                               \
    if ((szone)->debug_flags & CHECK_REGIONS)                           \
        szone_check_all(szone, fun)
#else
# define CHECK(szone,fun)                                               \
    do {} while (0)
#endif

/*********************  VERY LOW LEVEL UTILITIES  ************************/

#if DEBUG_MALLOC || DEBUG_CLIENT
static void
szone_sleep(void)
{

    if (getenv("MallocErrorSleep")) {
        _malloc_printf(ASL_LEVEL_NOTICE, "*** sleeping to help debug\n");
        sleep(3600); // to help debug
    }
}
#endif

extern const char *__crashreporter_info__;

// msg prints after fmt, ...
static NOINLINE void
szone_error(szone_t *szone, int is_corruption, const char *msg, const void *ptr, const char *fmt, ...)
{
    va_list ap;
    _SIMPLE_STRING b = _simple_salloc();

    if (szone) SZONE_UNLOCK(szone); // FIXME: unlock magazine and region locks?
    if (b) {
        if (fmt) {
            va_start(ap, fmt);
            _simple_vsprintf(b, fmt, ap);
            va_end(ap);
        }
        if (ptr) {
            _simple_sprintf(b, "*** error for object %p: %s\n", ptr, msg);
        } else {
            _simple_sprintf(b, "*** error: %s\n", msg);
        }
        malloc_printf("%s*** set a breakpoint in malloc_error_break to debug\n", _simple_string(b));
    } else {
        /*
         * Should only get here if vm_allocate() can't get a single page of
         * memory, implying _simple_asl_log() would also fail.  So we just
         * print to the file descriptor.
         */
        if (fmt) {
            va_start(ap, fmt);
            _malloc_vprintf(MALLOC_PRINTF_NOLOG, fmt, ap);
            va_end(ap);
        }
        if (ptr) {
            _malloc_printf(MALLOC_PRINTF_NOLOG, "*** error for object %p: %s\n", ptr, msg);
        } else {
            _malloc_printf(MALLOC_PRINTF_NOLOG, "*** error: %s\n", msg);
        }
        _malloc_printf(MALLOC_PRINTF_NOLOG, "*** set a breakpoint in malloc_error_break to debug\n");
    }
    malloc_error_break();
#if DEBUG_MALLOC
    szone_print(szone, 1);
    szone_sleep();
#endif
#if DEBUG_CLIENT
    szone_sleep();
#endif
    // Call abort() if this is a memory corruption error and the abort on 
    // corruption flag is set, or if any error should abort.
    if ((is_corruption && (szone->debug_flags & SCALABLE_MALLOC_ABORT_ON_CORRUPTION)) ||
        (szone->debug_flags & SCALABLE_MALLOC_ABORT_ON_ERROR)) {
        __crashreporter_info__ = b ? _simple_string(b) : msg;
        abort();
    } else if (b) {
        _simple_sfree(b);
    }
}

static void
protect(void *address, size_t size, unsigned protection, unsigned debug_flags)
{
    kern_return_t       err;

    if (!(debug_flags & SCALABLE_MALLOC_DONT_PROTECT_PRELUDE)) {
        err = vm_protect(mach_task_self(), (vm_address_t)(uintptr_t)address - vm_page_size, vm_page_size, 0, protection);
        if (err) {
            malloc_printf("*** can't protect(%p) region for prelude guard page at %p\n",
              protection,(uintptr_t)address - (1 << vm_page_shift));
        }
    }
    if (!(debug_flags & SCALABLE_MALLOC_DONT_PROTECT_POSTLUDE)) {
        err = vm_protect(mach_task_self(), (vm_address_t)(uintptr_t)address + size, vm_page_size, 0, protection);
        if (err) {
            malloc_printf("*** can't protect(%p) region for postlude guard page at %p\n",
              protection, (uintptr_t)address + size);
        }
    }
}

static void *
allocate_pages(szone_t *szone, size_t size, unsigned char align, unsigned debug_flags, int vm_page_label)
{
    // align specifies a desired alignment (as a log) or 0 if no alignment requested
    void        *vm_addr;
    uintptr_t   addr = 0, aligned_address;
    boolean_t   add_guard_pages = debug_flags & SCALABLE_MALLOC_ADD_GUARD_PAGES;
    boolean_t   purgeable = debug_flags & SCALABLE_MALLOC_PURGEABLE;
    size_t      allocation_size = round_page(size);
    size_t      delta;
    int         flags = VM_MAKE_TAG(vm_page_label);

    if (align) add_guard_pages = 0; // too cumbersome to deal with that
    if (!allocation_size) allocation_size = 1 << vm_page_shift;
    if (add_guard_pages) allocation_size += 2 * (1 << vm_page_shift);
    if (align) allocation_size += (size_t)1 << align;
    if (purgeable) flags |= VM_FLAGS_PURGABLE;
    
    if (allocation_size < size) // size_t arithmetic wrapped!
        return NULL;

    vm_addr = mmap(0, allocation_size, PROT_READ | PROT_WRITE, MAP_ANON | MAP_PRIVATE, flags, 0);
    if ((uintptr_t)vm_addr == -1) {
        szone_error(szone, 0, "can't allocate region", NULL, "*** mmap(size=%lu) failed (error code=%d)\n",
        allocation_size, errno);
        return NULL;
    }
    addr = (uintptr_t)vm_addr;

    if (align) {
        aligned_address = (addr + ((uintptr_t)1 << align) - 1) & ~ (((uintptr_t)1 << align) - 1);
        if (aligned_address != addr) {
            delta = aligned_address - addr;
            if (munmap((void *)addr, delta) == -1)
                malloc_printf("*** munmap unaligned header failed with %d\n", errno);
            addr = aligned_address;
            allocation_size -= delta;
        }
        if (allocation_size > size) {
            if (munmap((void *)(addr + size), allocation_size - size) == -1)
                malloc_printf("*** munmap unaligned footer failed with %d\n", errno);
        }
    }
    if (add_guard_pages) {
        addr += (uintptr_t)1 << vm_page_shift;
        protect((void *)addr, size, 0, debug_flags);
    }
    return (void *)addr;
}

static void
deallocate_pages(szone_t *szone, void *addr, size_t size, unsigned debug_flags)
{
    int         err;
    boolean_t   add_guard_pages = debug_flags & SCALABLE_MALLOC_ADD_GUARD_PAGES;

    if (add_guard_pages) {
        addr = (void *)((uintptr_t)addr - (1 << vm_page_shift));
        size += 2 * (1 << vm_page_shift);
    }
    err = munmap(addr, size);
    if ((err == -1) && szone)
        szone_error(szone, 0, "Can't deallocate_pages region", addr, NULL);
}

static int
madvise_free_range(szone_t *szone, region_t r, uintptr_t pgLo, uintptr_t pgHi)
{
    if (pgHi > pgLo) {
        size_t len = pgHi - pgLo;

#if DEBUG_MALLOC
        if (szone->debug_flags & SCALABLE_MALLOC_DO_SCRIBBLE)
            memset((void *)pgLo, 0xed, len); // Scribble on MADV_FREEd memory
#endif
        MAGMALLOC_MADVFREEREGION((void *)szone, (void *)r, (void *)pgLo, len); // DTrace USDT Probe
        if (-1 == madvise((void *)pgLo, len, MADV_FREE_REUSABLE)) {
            /* -1 return: VM map entry change makes this unfit for reuse. Something evil lurks. */
#if DEBUG_MALLOC
            szone_error(szone, 1, "madvise_free_range madvise(..., MADV_FREE_REUSABLE) failed", (void *)pgLo, NULL);
#endif
        }
    }
    return 0;
}

static kern_return_t
_szone_default_reader(task_t task, vm_address_t address, vm_size_t size, void **ptr)
{
    *ptr = (void *)address;
    return 0;
}

// Multiplicative hash where the multiplier is a prime near (ULONG_MAX / phi). [phi = 1.618033...]
// pthread_t's are page aligned, (sometimes even in ascending sequence). These hash well.
// See Knuth TAOCP, Vol. 3. 
#if __LP64__
#define HASH_SELF() \
        ((((uintptr_t)pthread_self()) >> vm_page_shift) * 11400714819323198549ULL) >> (64 - szone->num_tiny_magazines_mask_shift)
#else
#define HASH_SELF() \
        ((((uintptr_t)pthread_self()) >> vm_page_shift) * 2654435761UL) >> (32 - szone->num_tiny_magazines_mask_shift)
#endif

#if defined(__i386__) || defined(__x86_64__)
#define __APPLE_API_PRIVATE
#include <machine/cpu_capabilities.h>
#define _COMM_PAGE_VERSION_REQD 9
#undef __APPLE_API_PRIVATE

/*
 * These commpage routines provide fast access to the logical cpu number
 * of the calling processor assuming no pre-emption occurs. 
 */
#define CPU_NUMBER() (((int (*)()) _COMM_PAGE_CPU_NUMBER)()) /* Zero-based */

static INLINE mag_index_t
mag_get_thread_index(szone_t *szone)
{
    if (!__is_threaded)
        return 0;
    else
        return CPU_NUMBER() & (TINY_MAX_MAGAZINES - 1);
}

#elif defined(__arm__)

static INLINE mag_index_t
mag_get_thread_index(szone_t *szone)
{
    return 0;
}

#else
#warning deriving magazine index from pthread_self() [want processor number]

static INLINE mag_index_t
mag_get_thread_index(szone_t *szone)
{
    if (!__is_threaded)
        return 0;
    else if ((pthread_key_t) -1 == szone->cpu_id_key) { // In case pthread_key_create() failed.
        return HASH_SELF();
    } else {
        mag_index_t idx = (mag_index_t)(intptr_t)pthread_getspecific(szone->cpu_id_key);
        
        // Has this thread been hinted with a non-zero value [i.e. 1 + cpuid()] ?
        // If so, bump down the hint to a zero-based magazine index and return it.
        if (idx) {
            return idx - 1;
        } else {
            // No hint available. Contruct a magazine index for this thread ...
            idx = HASH_SELF();

            // bump up the hint to exclude zero and try to memorize it ...
            pthread_setspecific(szone->cpu_id_key, (const void *)((uintptr_t)idx + 1));
            
            // and return the (zero-based) magazine index.
            return idx;
        }
    }
}
#endif

static magazine_t *
mag_lock_zine_for_region_trailer(szone_t *szone, magazine_t *magazines, region_trailer_t *trailer, mag_index_t mag_index) 
{
    mag_index_t refreshed_index;
    magazine_t *mag_ptr = &(magazines[mag_index]);

    // Take the lock  on entry.
    SZONE_MAGAZINE_PTR_LOCK(szone, mag_ptr);
    
    // Now in the time it took to acquire the lock, the region may have migrated
    // from one magazine to another. In which case the magazine lock we obtained 
    // (namely magazines[mag_index].mag_lock) is stale. If so, keep on tryin' ...
    while (mag_index != (refreshed_index = trailer->mag_index)) { // Note assignment
    
        SZONE_MAGAZINE_PTR_UNLOCK(szone, mag_ptr);
        
        mag_index = refreshed_index;
        mag_ptr = &(magazines[mag_index]);
        SZONE_MAGAZINE_PTR_LOCK(szone, mag_ptr);
    }
    
    return mag_ptr;
}

/*******************************************************************************
 * Region hash implementation
 *
 * This is essentially a duplicate of the existing Large allocator hash, minus
 * the ability to remove entries.  The two should be combined eventually.
 ******************************************************************************/
#pragma mark region hash

/*
 * hash_lookup_region_no_lock - Scan a hash ring looking for an entry for a 
 * given region.
 *
 * FIXME: If consecutive queries of the same region are likely, a one-entry
 * cache would likely be a significant performance win here.
 */
static INLINE rgnhdl_t
hash_lookup_region_no_lock(region_t *regions, size_t num_entries, size_t shift, region_t r) {
    size_t      index, hash_index;
    rgnhdl_t    entry;

    if (!num_entries)
        return 0;

    // Multiplicative hash where the multiplier is a prime near (ULONG_MAX / phi). [phi = 1.618033...]
    // Since the values of (((uintptr_t)r >> HASH_BLOCKS_ALIGN) are (roughly) an ascending sequence of integers,
    // this hash works really well. See Knuth TAOCP, Vol. 3. 
#if __LP64__
    index = hash_index = (((uintptr_t)r >> HASH_BLOCKS_ALIGN) * 11400714819323198549ULL) >> (64 - shift);
#else
    index = hash_index = (((uintptr_t)r >> HASH_BLOCKS_ALIGN) * 2654435761UL) >> (32 - shift);
#endif
    do {
        entry = regions + index;
        if (*entry == 0)
            return 0;
        if (*entry == r)
            return entry;
        if (++index == num_entries)
            index = 0;
    } while (index != hash_index);
    return 0;
}

/*
 * hash_region_insert_no_lock - Insert a region into the hash ring.
 */
static void
hash_region_insert_no_lock(region_t *regions, size_t num_entries, size_t shift, region_t r) {
    size_t      index, hash_index;
    rgnhdl_t    entry;

    // Multiplicative hash where the multiplier is a prime near (ULONG_MAX / phi). [phi = 1.618033...]
    // Since the values of (((uintptr_t)r >> HASH_BLOCKS_ALIGN) are (roughly) an ascending sequence of integers,
    // this hash works really well. See Knuth TAOCP, Vol. 3. 
#if __LP64__
    index = hash_index = (((uintptr_t)r >> HASH_BLOCKS_ALIGN) * 11400714819323198549ULL) >> (64 - shift);
#else
    index = hash_index = (((uintptr_t)r >> HASH_BLOCKS_ALIGN) * 2654435761UL) >> (32 - shift);
#endif
    do {
        entry = regions + index;
        if (*entry == HASHRING_OPEN_ENTRY || *entry == HASHRING_REGION_DEALLOCATED) {
            *entry = r;
            return;
        }
        if (++index == num_entries)
            index = 0;
    } while (index != hash_index);
}

/*
 * hash_regions_alloc_no_lock - Allocate space for a number of entries.  This
 * must be a VM allocation as to avoid recursing between allocating a new small
 * region, and asking the small region to allocate space for the new list of
 * regions.
 */
static region_t *
hash_regions_alloc_no_lock(szone_t *szone, size_t num_entries)
{
    size_t      size = num_entries * sizeof(region_t);
    
    return allocate_pages(szone, round_page(size), 0, 0, VM_MEMORY_MALLOC);
}

/*
 * hash_regions_grow_no_lock - Grow the hash ring, and rehash the entries.
 * Return the new region and new size to update the szone.  Do not deallocate
 * the old entries since someone may still be allocating them.
 */
static region_t *
hash_regions_grow_no_lock(szone_t *szone, region_t *regions, size_t old_size, size_t *mutable_shift,
                          size_t *new_size)
{
    // double in size and allocate memory for the regions
    *new_size = old_size + old_size;
    *mutable_shift = *mutable_shift + 1;
    region_t *new_regions = hash_regions_alloc_no_lock(szone, *new_size);

    // rehash the entries into the new list
    size_t index;
    for (index = 0; index < old_size; ++index) {
        region_t r = regions[index];
        if (r != HASHRING_OPEN_ENTRY && r != HASHRING_REGION_DEALLOCATED)
            hash_region_insert_no_lock(new_regions, *new_size, *mutable_shift, r);
    }
    return new_regions;
}

/*********************  FREE LIST UTILITIES  ************************/

// A free list entry is comprised of a pair of pointers, previous and next.
// These are used to implement a doubly-linked list, which permits efficient
// extraction.
//
// Because the free list entries are previously freed objects, a misbehaved 
// program may write to a pointer after it has called free() on that pointer,
// either by dereference or buffer overflow from an adjacent pointer. This write
// would then corrupt the free list's previous and next pointers, leading to a
// crash.  In order to detect this case, we take advantage of the fact that 
// malloc'd pointers are known to be at least 16 byte aligned, and thus have
// at least 4 trailing zero bits.
//
// When an entry is added to the free list, a checksum of the previous and next
// pointers is calculated and written to the low four bits of the respective
// pointers.  Upon detection of an invalid checksum, an error is logged and NULL
// is returned.  Since all code which un-checksums pointers checks for a NULL
// return, a potentially crashing or malicious dereference is avoided at the
// cost of leaking the corrupted block, and any subsequent blocks on the free
// list of that size.

static NOINLINE void
free_list_checksum_botch(szone_t *szone, free_list_t *ptr)
{
    szone_error(szone, 1, "incorrect checksum for freed object "
    "- object was probably modified after being freed.", ptr, NULL);
}

static INLINE uintptr_t free_list_gen_checksum(uintptr_t ptr)
{
    uint8_t chk;

    chk  = (unsigned char)(ptr >>  0);
    chk += (unsigned char)(ptr >>  8);
    chk += (unsigned char)(ptr >> 16);
    chk += (unsigned char)(ptr >> 24);
#if  __LP64__
    chk += (unsigned char)(ptr >> 32);
    chk += (unsigned char)(ptr >> 40);
    chk += (unsigned char)(ptr >> 48);
    chk += (unsigned char)(ptr >> 56);
#endif

    return chk & (uintptr_t)0xF;
}

static INLINE uintptr_t
free_list_checksum_ptr(szone_t *szone, void *ptr) 
{
    uintptr_t p = (uintptr_t)ptr;
    return p | free_list_gen_checksum(p ^ szone->cookie);
}

static INLINE void *
free_list_unchecksum_ptr(szone_t *szone, ptr_union *ptr)
{
    ptr_union p;
    p.u = (ptr->u >> 4) << 4;
    
    if ((ptr->u & (uintptr_t)0xF) != free_list_gen_checksum(p.u ^ szone->cookie))
    {
      free_list_checksum_botch(szone, (free_list_t *)ptr);
      return NULL;
    }
    return p.p;
}

static unsigned
free_list_count(szone_t *szone, free_list_t *ptr)
{
    unsigned    count = 0;

    while (ptr) {
        count++;
        ptr = free_list_unchecksum_ptr(szone, &ptr->next);
    }
    return count;
}

static INLINE void
recirc_list_extract(szone_t *szone, magazine_t *mag_ptr, region_trailer_t *node)
{
    // excise node from list
    if (NULL == node->prev)
        mag_ptr->firstNode = node->next;
    else
        node->prev->next = node->next;
    
    if (NULL == node->next)
        mag_ptr->lastNode = node->prev;
    else
        node->next->prev = node->prev;
        
    mag_ptr->recirculation_entries--;
}

static INLINE void
recirc_list_splice_last(szone_t *szone, magazine_t *mag_ptr, region_trailer_t *node)
{
    if (NULL == mag_ptr->lastNode) {
        mag_ptr->firstNode = node;
        node->prev = NULL;
    } else {
        node->prev = mag_ptr->lastNode;
        mag_ptr->lastNode->next = node;
    }
    mag_ptr->lastNode = node;
    node->next = NULL;
    node->recirc_suitable = FALSE;
    mag_ptr->recirculation_entries++;
}

static INLINE void
recirc_list_splice_first(szone_t *szone, magazine_t *mag_ptr, region_trailer_t *node)
{
    if (NULL == mag_ptr->firstNode) {
        mag_ptr->lastNode = node;
        node->next = NULL;
    } else {
        node->next = mag_ptr->firstNode;
        mag_ptr->firstNode->prev = node;
    }
    mag_ptr->firstNode = node;
    node->prev = NULL;
    node->recirc_suitable = FALSE;
    mag_ptr->recirculation_entries++;
}

/* Macros used to manipulate the uint32_t quantity mag_bitmap. */

/* BITMAPV variants are used by tiny. */
#if defined(__LP64__)
// assert(NUM_SLOTS == 64) in which case (slot >> 5) is either 0 or 1
#define BITMAPV_SET(bitmap,slot)        (bitmap[(slot) >> 5] |= 1 << ((slot) & 31))
#define BITMAPV_CLR(bitmap,slot)        (bitmap[(slot) >> 5] &= ~ (1 << ((slot) & 31)))
#define BITMAPV_BIT(bitmap,slot)        ((bitmap[(slot) >> 5] >> ((slot) & 31)) & 1)
#define BITMAPV_CTZ(bitmap)             (__builtin_ctzl(bitmap))        
#else
// assert(NUM_SLOTS == 32) in which case (slot >> 5) is always 0, so code it that way
#define BITMAPV_SET(bitmap,slot)        (bitmap[0] |= 1 << (slot))
#define BITMAPV_CLR(bitmap,slot)        (bitmap[0] &= ~ (1 << (slot)))
#define BITMAPV_BIT(bitmap,slot)        ((bitmap[0] >> (slot)) & 1)
#define BITMAPV_CTZ(bitmap)             (__builtin_ctz(bitmap))
#endif

/* BITMAPN is used by small. (slot >> 5) takes on values from 0 to 7. */
#define BITMAPN_SET(bitmap,slot)        (bitmap[(slot) >> 5] |= 1 << ((slot) & 31))
#define BITMAPN_CLR(bitmap,slot)        (bitmap[(slot) >> 5] &= ~ (1 << ((slot) & 31)))
#define BITMAPN_BIT(bitmap,slot)        ((bitmap[(slot) >> 5] >> ((slot) & 31)) & 1)

/* returns bit # of least-significant one bit, starting at 0 (undefined if !bitmap) */
#define BITMAP32_CTZ(bitmap)            (__builtin_ctz(bitmap[0]))

/*********************  TINY FREE LIST UTILITIES        ************************/

// We encode the meta-headers as follows:
// Each quantum has an associated set of 2 bits:
// block_header when 1 says this block is the beginning of a block
// in_use when 1 says this block is in use
// so a block in use of size 3 is 1-1 0-X 0-X
// for a free block TINY_FREE_SIZE(ptr) carries the size and the bits are 1-0 X-X X-X
// for a block middle the bits are 0-0

// We store the meta-header bit arrays by interleaving them 32 bits at a time.
// Initial 32 bits of block_header, followed by initial 32 bits of in_use, followed
// by next 32 bits of block_header, followed by next 32 bits of in_use, etc.
// This localizes memory references thereby reducing cache and TLB pressures.

static INLINE void
BITARRAY_SET(uint32_t *bits, msize_t index)
{
    // index >> 5 identifies the uint32_t to manipulate in the conceptually contiguous bits array
    // (index >> 5) << 1 identifies the uint32_t allowing for the actual interleaving
    bits[(index >> 5) << 1] |= (1 << (index & 31));
}

static INLINE void
BITARRAY_CLR(uint32_t *bits, msize_t index)
{
    bits[(index >> 5) << 1] &= ~(1 << (index & 31));
}

static INLINE boolean_t
BITARRAY_BIT(uint32_t *bits, msize_t index)
{
    return ((bits[(index >> 5) << 1]) >> (index & 31)) & 1;
}

#if 0
static INLINE void      bitarray_mclr(uint32_t *bits, unsigned start, unsigned end) ALWAYSINLINE;

static INLINE void
bitarray_mclr(uint32_t *bits, unsigned start, unsigned end) 
{
    // start >> 5 identifies the uint32_t to manipulate in the conceptually contiguous bits array
    // (start >> 5) << 1 identifies the uint32_t allowing for the actual interleaving
    uint32_t    *addr = bits + ((start >> 5) << 1);
    
    uint32_t    span = end - start;
    start = start & 31;
    end = start + span;
    
    if (end > 31) {
        addr[0] &= (0xFFFFFFFFU >> (31 - start)) >> 1;
        addr[2] &= (0xFFFFFFFFU << (end - 32));
    } else {
        unsigned mask = (0xFFFFFFFFU >> (31 - start)) >> 1;
        mask |= (0xFFFFFFFFU << end);
        addr[0] &= mask;
    }
}
#endif

/*
 * Obtain the size of a free tiny block (in msize_t units).
 */
static msize_t
get_tiny_free_size(const void *ptr)
{
    void        *next_block = (void *)((uintptr_t)ptr + TINY_QUANTUM);
    void        *region_end = TINY_REGION_END(TINY_REGION_FOR_PTR(ptr));

    // check whether the next block is outside the tiny region or a block header
    // if so, then the size of this block is one, and there is no stored size.
    if (next_block < region_end)
    {
        uint32_t        *next_header = TINY_BLOCK_HEADER_FOR_PTR(next_block);
        msize_t         next_index  = TINY_INDEX_FOR_PTR(next_block);
        
        if (!BITARRAY_BIT(next_header, next_index))
            return TINY_FREE_SIZE(ptr);
    }
    return 1;
}

/*
 * Get the size of the previous free block, which is stored in the last two
 * bytes of the block.  If the previous block is not free, then the result is
 * undefined.
 */
static msize_t
get_tiny_previous_free_msize(const void *ptr)
{
    // check whether the previous block is in the tiny region and a block header
    // if so, then the size of the previous block is one, and there is no stored
    // size.
    if (ptr != TINY_REGION_FOR_PTR(ptr))
    {
        void            *prev_block = (void *)((uintptr_t)ptr - TINY_QUANTUM);
        uint32_t        *prev_header = TINY_BLOCK_HEADER_FOR_PTR(prev_block);
        msize_t         prev_index  = TINY_INDEX_FOR_PTR(prev_block);
        if (BITARRAY_BIT(prev_header, prev_index))
            return 1;
        return TINY_PREVIOUS_MSIZE(ptr);
    }
    // don't read possibly unmapped memory before the beginning of the region
    return 0;
}

static INLINE msize_t
get_tiny_meta_header(const void *ptr, boolean_t *is_free)
{
    // returns msize and is_free
    // may return 0 for the msize component (meaning 65536)
    uint32_t    *block_header;
    msize_t     index;

    block_header = TINY_BLOCK_HEADER_FOR_PTR(ptr);
    index = TINY_INDEX_FOR_PTR(ptr);

    msize_t midx = (index >> 5) << 1;
    uint32_t mask = 1 << (index & 31);
    *is_free = 0;
    if (0 == (block_header[midx] & mask)) // if (!BITARRAY_BIT(block_header, index))
        return 0;
    if (0 == (block_header[midx + 1] & mask)) { // if (!BITARRAY_BIT(in_use, index))
        *is_free = 1;
        return get_tiny_free_size(ptr);
    }
    
    // index >> 5 identifies the uint32_t to manipulate in the conceptually contiguous bits array
    // (index >> 5) << 1 identifies the uint32_t allowing for the actual interleaving
#if defined(__LP64__)
    // The return value, msize, is computed as the distance to the next 1 bit in block_header.
    // That's guaranteed to be somewhwere in the next 64 bits. And those bits could span three
    // uint32_t block_header elements. Collect the bits into a single uint64_t and measure up with ffsl.
    uint32_t    *addr = ((uint32_t *)block_header) + ((index >> 5) << 1);
    uint32_t    bitidx = index & 31;
    uint64_t    word_lo = addr[0];
    uint64_t    word_mid = addr[2];
    uint64_t    word_hi = addr[4];
    uint64_t    word_lomid = (word_lo >> bitidx) | (word_mid << (32 - bitidx));
    uint64_t    word = bitidx ? word_lomid | (word_hi << (64 - bitidx)) : word_lomid;
    uint32_t    result = __builtin_ffsl(word >> 1);
#else
    // The return value, msize, is computed as the distance to the next 1 bit in block_header.
    // That's guaranteed to be somwhwere in the next 32 bits. And those bits could span two
    // uint32_t block_header elements. Collect the bits into a single uint32_t and measure up with ffs.
    uint32_t    *addr = ((uint32_t *)block_header) + ((index >> 5) << 1);
    uint32_t    bitidx = index & 31;
    uint32_t    word = bitidx ? (addr[0] >> bitidx) | (addr[2] << (32 - bitidx)) : addr[0];
    uint32_t    result = __builtin_ffs(word >> 1);
#endif
    return result;
}

static INLINE void
set_tiny_meta_header_in_use(const void *ptr, msize_t msize)
{
    uint32_t    *block_header = TINY_BLOCK_HEADER_FOR_PTR(ptr);
    msize_t     index = TINY_INDEX_FOR_PTR(ptr);
    msize_t     clr_msize = msize - 1;
    msize_t     midx = (index >> 5) << 1;
    uint32_t    val = (1 << (index & 31));
    
#if DEBUG_MALLOC
    if (msize >= NUM_TINY_SLOTS)
        malloc_printf("set_tiny_meta_header_in_use() invariant broken %p %d\n", ptr, msize);
    if ((unsigned)index + (unsigned)msize > 0x10000)
        malloc_printf("set_tiny_meta_header_in_use() invariant broken (2) %p %d\n", ptr, msize);
#endif

    block_header[midx] |= val; // BITARRAY_SET(block_header, index);
    block_header[midx + 1] |= val; // BITARRAY_SET(in_use, index);

    // bitarray_mclr(block_header, index, end_bit);
    // bitarray_mclr(in_use, index, end_bit);

    index++;
    midx = (index >> 5) << 1;
    
    unsigned    start = index & 31;
    unsigned    end = start + clr_msize;

#if defined(__LP64__)
    if (end > 63) {
        unsigned mask0 = (0xFFFFFFFFU >> (31 - start)) >> 1;
        unsigned mask1 = (0xFFFFFFFFU << (end - 64));
        block_header[midx + 0] &= mask0; // clear header
        block_header[midx + 1] &= mask0; // clear in_use
        block_header[midx + 2] = 0; // clear header
        block_header[midx + 3] = 0; // clear in_use
        block_header[midx + 4] &= mask1; // clear header
        block_header[midx + 5] &= mask1; // clear in_use
    } else
#endif
    if (end > 31) {
        unsigned mask0 = (0xFFFFFFFFU >> (31 - start)) >> 1;
        unsigned mask1 = (0xFFFFFFFFU << (end - 32));
        block_header[midx + 0] &= mask0;
        block_header[midx + 1] &= mask0;
        block_header[midx + 2] &= mask1;
        block_header[midx + 3] &= mask1;
    } else {
        unsigned mask = (0xFFFFFFFFU >> (31 - start)) >> 1;
        mask |= (0xFFFFFFFFU << end);
        block_header[midx + 0] &= mask;
        block_header[midx + 1] &= mask;
    }

    // we set the block_header bit for the following block to reaffirm next block is a block
    index += clr_msize;
    midx = (index >> 5) << 1;
    val = (1 << (index & 31));
    block_header[midx] |= val; // BITARRAY_SET(block_header, (index+clr_msize));
#if DEBUG_MALLOC
    {
        boolean_t ff;
        msize_t mf;
        
        mf = get_tiny_meta_header(ptr, &ff);
        if (msize != mf) {
            malloc_printf("setting header for tiny in_use %p : %d\n", ptr, msize);
            malloc_printf("reading header for tiny %p : %d %d\n", ptr, mf, ff);
        }
    }
#endif
}

static INLINE void
set_tiny_meta_header_in_use_1(const void *ptr) // As above with msize == 1
{
    uint32_t    *block_header = TINY_BLOCK_HEADER_FOR_PTR(ptr);
    msize_t     index = TINY_INDEX_FOR_PTR(ptr);
    msize_t     midx = (index >> 5) << 1;
    uint32_t    val = (1 << (index & 31));
    
    block_header[midx] |= val; // BITARRAY_SET(block_header, index);
    block_header[midx + 1] |= val; // BITARRAY_SET(in_use, index);

    index++;
    midx = (index >> 5) << 1;
    val = (1 << (index & 31));
       
    block_header[midx] |= val; // BITARRAY_SET(block_header, (index+clr_msize))
}

static INLINE void
set_tiny_meta_header_middle(const void *ptr)
{
    // indicates this block is in the middle of an in use block
    uint32_t    *block_header;
    uint32_t    *in_use;
    msize_t     index;

    block_header = TINY_BLOCK_HEADER_FOR_PTR(ptr);
    in_use = TINY_INUSE_FOR_HEADER(block_header);
    index = TINY_INDEX_FOR_PTR(ptr);

    BITARRAY_CLR(block_header, index);
    BITARRAY_CLR(in_use, index); 
}

static INLINE void
set_tiny_meta_header_free(const void *ptr, msize_t msize)
{
    // !msize is acceptable and means 65536
    uint32_t    *block_header = TINY_BLOCK_HEADER_FOR_PTR(ptr);
    msize_t     index = TINY_INDEX_FOR_PTR(ptr);
    msize_t     midx = (index >> 5) << 1;
    uint32_t    val = (1 << (index & 31));

#if DEBUG_MALLOC
    if ((unsigned)index + (unsigned)msize > 0x10000) {
        malloc_printf("setting header for tiny free %p msize too large: %d\n", ptr, msize);
    }
#endif

    block_header[midx] |= val; // BITARRAY_SET(block_header, index);
    block_header[midx + 1] &= ~val; // BITARRAY_CLR(in_use, index); 
    
    // mark the end of this block if msize is > 1.  For msize == 0, the whole
    // region is free, so there is no following block. For msize == 1, there is
    // no space to write the size on 64 bit systems.  The size for 1 quantum
    // blocks is computed from the metadata bitmaps.
    if (msize > 1) {
        void    *follower = FOLLOWING_TINY_PTR(ptr, msize);
        TINY_PREVIOUS_MSIZE(follower) = msize;
        TINY_FREE_SIZE(ptr) = msize;
    }
    if (msize == 0) {
        TINY_FREE_SIZE(ptr) = msize;
    }
#if DEBUG_MALLOC
    boolean_t   ff;
    msize_t     mf = get_tiny_meta_header(ptr, &ff);
    if ((msize != mf) || !ff) {
        malloc_printf("setting header for tiny free %p : %u\n", ptr, msize);
        malloc_printf("reading header for tiny %p : %u %u\n", ptr, mf, ff);
    }
#endif
}

static INLINE boolean_t
tiny_meta_header_is_free(const void *ptr)
{
    uint32_t    *block_header;
    uint32_t    *in_use;
    msize_t     index;

    block_header = TINY_BLOCK_HEADER_FOR_PTR(ptr);
    in_use = TINY_INUSE_FOR_HEADER(block_header);
    index = TINY_INDEX_FOR_PTR(ptr);
    if (!BITARRAY_BIT(block_header, index))
        return 0;
    return !BITARRAY_BIT(in_use, index);
}

static INLINE void *
tiny_previous_preceding_free(void *ptr, msize_t *prev_msize)
{
    // returns the previous block, assuming and verifying it's free
    uint32_t    *block_header;
    uint32_t    *in_use;
    msize_t     index;
    msize_t     previous_msize;
    msize_t     previous_index;
    void        *previous_ptr;

    block_header = TINY_BLOCK_HEADER_FOR_PTR(ptr);
    in_use = TINY_INUSE_FOR_HEADER(block_header);
    index = TINY_INDEX_FOR_PTR(ptr);
    
    if (!index)
        return NULL;
    if ((previous_msize = get_tiny_previous_free_msize(ptr)) > index)
        return NULL;
    
    previous_index = index - previous_msize;
    previous_ptr = (void *)((uintptr_t)TINY_REGION_FOR_PTR(ptr) + TINY_BYTES_FOR_MSIZE(previous_index));
    if (!BITARRAY_BIT(block_header, previous_index))
        return NULL;
    if (BITARRAY_BIT(in_use, previous_index))
        return NULL;
    if (get_tiny_free_size(previous_ptr) != previous_msize)
        return NULL;
    
    // conservative check did match true check
    *prev_msize = previous_msize;
    return previous_ptr;
}

/* 
 * Adds an item to the proper free list, and also marks the meta-header of the 
 * block properly.
 * Assumes szone has been locked
 */ 
static void
tiny_free_list_add_ptr(szone_t *szone, magazine_t *tiny_mag_ptr, void *ptr, msize_t msize)
{
    grain_t     slot = (!msize || (msize >= NUM_TINY_SLOTS)) ? NUM_TINY_SLOTS - 1 : msize - 1;
    free_list_t *free_ptr = ptr;
    free_list_t *free_head = tiny_mag_ptr->mag_free_list[slot];

#if DEBUG_MALLOC
    if (LOG(szone,ptr)) {
        malloc_printf("in %s, ptr=%p, msize=%d\n", __FUNCTION__, ptr, msize);
    }
    if (((uintptr_t)ptr) & (TINY_QUANTUM - 1)) {
        szone_error(szone, 1, "tiny_free_list_add_ptr: Unaligned ptr", ptr, NULL);
    }
#endif
    set_tiny_meta_header_free(ptr, msize);
    if (free_head) {
#if DEBUG_MALLOC
        if (free_list_unchecksum_ptr(szone, &free_head->previous)) {
            szone_error(szone, 1, "tiny_free_list_add_ptr: Internal invariant broken (free_head->previous)", ptr,
            "ptr=%p slot=%d free_head=%p previous=%p\n", ptr, slot, (void *)free_head, free_head->previous.p);
        }
        if (! tiny_meta_header_is_free(free_head)) {
            szone_error(szone, 1, "tiny_free_list_add_ptr: Internal invariant broken (free_head is not a free pointer)", ptr,
            "ptr=%p slot=%d free_head=%p\n", ptr, slot, (void *)free_head);
        }
#endif
        free_head->previous.u = free_list_checksum_ptr(szone, free_ptr);
    } else {
        BITMAPV_SET(tiny_mag_ptr->mag_bitmap, slot);
    }
    free_ptr->previous.u = free_list_checksum_ptr(szone, NULL);
    free_ptr->next.u = free_list_checksum_ptr(szone, free_head);

    tiny_mag_ptr->mag_free_list[slot] = free_ptr;
}

/* 
 * Removes the item pointed to by ptr in the proper free list. 
 * Assumes szone has been locked
 */ 
static void
tiny_free_list_remove_ptr(szone_t *szone, magazine_t *tiny_mag_ptr, void *ptr, msize_t msize)
{
    grain_t     slot = (!msize || (msize >= NUM_TINY_SLOTS)) ? NUM_TINY_SLOTS - 1 : msize - 1;
    free_list_t *free_ptr = ptr, *next, *previous;
    
    next = free_list_unchecksum_ptr(szone, &free_ptr->next);
    previous = free_list_unchecksum_ptr(szone, &free_ptr->previous);

#if DEBUG_MALLOC
    if (LOG(szone,ptr)) {
        malloc_printf("In %s, ptr=%p, msize=%d\n", __FUNCTION__, ptr, msize);
    }
#endif
    if (!previous) { 
    // The block to remove is the head of the free list
#if DEBUG_MALLOC
        if (tiny_mag_ptr->mag_free_list[slot] != ptr) {
            szone_error(szone, 1, "tiny_free_list_remove_ptr: Internal invariant broken (tiny_mag_ptr->mag_free_list[slot])", ptr,
              "ptr=%p slot=%d msize=%d tiny_mag_ptr->mag_free_list[slot]=%p\n",
              ptr, slot, msize, (void *)tiny_mag_ptr->mag_free_list[slot]);
            return;
        }
#endif
        tiny_mag_ptr->mag_free_list[slot] = next;
        if (!next) BITMAPV_CLR(tiny_mag_ptr->mag_bitmap, slot);
    } else {
        // We know free_ptr is already checksummed, so we don't need to do it
        // again.
        previous->next = free_ptr->next;
    }
    if (next) {
        // We know free_ptr is already checksummed, so we don't need to do it
        // again.
        next->previous = free_ptr->previous;
    }
}

/*
 * tiny_region_for_ptr_no_lock - Returns the tiny region containing the pointer,
 * or NULL if not found.
 */
static INLINE region_t
tiny_region_for_ptr_no_lock(szone_t *szone, const void *ptr)
{
    rgnhdl_t r = hash_lookup_region_no_lock(szone->tiny_region_generation->hashed_regions,
        szone->tiny_region_generation->num_regions_allocated,
        szone->tiny_region_generation->num_regions_allocated_shift,
        TINY_REGION_FOR_PTR(ptr));
    return r ? *r : r;
}

static void
tiny_finalize_region(szone_t *szone, magazine_t *tiny_mag_ptr) {
    void      *last_block, *previous_block;
    uint32_t  *last_header;
    msize_t   last_msize, previous_msize, last_index;
       
    last_block = (void *)
        ((uintptr_t)TINY_REGION_END(tiny_mag_ptr->mag_last_region) - tiny_mag_ptr->mag_bytes_free_at_end);
    last_msize = TINY_MSIZE_FOR_BYTES(tiny_mag_ptr->mag_bytes_free_at_end);
    last_header = TINY_BLOCK_HEADER_FOR_PTR(last_block);
    last_index  = TINY_INDEX_FOR_PTR(last_block);

    // Before anything we transform any remaining mag_bytes_free_at_end into a
    // regular free block.  We take special care here to update the bitfield
    // information, since we are bypassing the normal free codepath.  If there
    // is more than one quanta worth of memory in mag_bytes_free_at_end, then
    // there will be two block headers:
    // 1) header for the free space at end, msize = 1
    // 2) header inserted by set_tiny_meta_header_in_use after block
    // We must clear the second one so that when the free block's size is
    // queried, we do not think the block is only 1 quantum in size because
    // of the second set header bit.
    if (last_index != (NUM_TINY_BLOCKS - 1))
        BITARRAY_CLR(last_header, (last_index + 1));

    // It is possible that the block prior to the last block in the region has
    // been free'd, but was not coalesced with the free bytes at the end of the
    // block, since we treat the bytes at the end of the region as "in use" in
    // the meta headers. Attempt to coalesce the last block with the previous 
    // block, so we don't violate the "no consecutive free blocks" invariant.
    //
    // FIXME: Need to investigate how much work would be required to increase
    //        'mag_bytes_free_at_end' when freeing the preceding block, rather
    //        than performing this workaround.
    //
    previous_block = tiny_previous_preceding_free(last_block, &previous_msize);
    if (previous_block) {
        set_tiny_meta_header_middle(last_block);
        tiny_free_list_remove_ptr(szone, tiny_mag_ptr, previous_block, previous_msize);
        last_block = previous_block;
        last_msize += previous_msize;
    }

    // splice last_block into the free list
    tiny_free_list_add_ptr(szone, tiny_mag_ptr, last_block, last_msize); 
    tiny_mag_ptr->mag_bytes_free_at_end = 0;
    tiny_mag_ptr->mag_last_region = NULL;
}

static int
tiny_free_detach_region(szone_t *szone, magazine_t *tiny_mag_ptr, region_t r) {
    uintptr_t   start = (uintptr_t)TINY_REGION_ADDRESS(r);
    uintptr_t   current = start;
    uintptr_t   limit =  (uintptr_t)TINY_REGION_END(r);
    boolean_t   is_free;
    msize_t     msize;
    int         total_alloc = 0;

    while (current < limit) {
        msize = get_tiny_meta_header((void *)current, &is_free);
        if (is_free && !msize && (current == start)) {
            // first block is all free
            break;
        }
        if (!msize) {
#if DEBUG_MALLOC
            malloc_printf("*** tiny_free_detach_region error with %p: msize=%d is_free =%d\n", 
            (void *)current, msize, is_free);
#endif
            break;
        }
        if (is_free) {
            tiny_free_list_remove_ptr(szone, tiny_mag_ptr, (void *)current, msize);
        } else {
            total_alloc++;
        }
        current += TINY_BYTES_FOR_MSIZE(msize);
    }
    return total_alloc;
}

static size_t
tiny_free_reattach_region(szone_t *szone, magazine_t *tiny_mag_ptr, region_t r) {
    uintptr_t   start = (uintptr_t)TINY_REGION_ADDRESS(r);
    uintptr_t   current = start;
    uintptr_t   limit =  (uintptr_t)TINY_REGION_END(r);
    boolean_t   is_free;
    msize_t     msize;
    size_t      total_alloc = 0;

    while (current < limit) {
        msize = get_tiny_meta_header((void *)current, &is_free);
        if (is_free && !msize && (current == start)) {
            // first block is all free
            break;
        }
        if (!msize) {
#if DEBUG_MALLOC
            malloc_printf("*** tiny_free_reattach_region error with %p: msize=%d is_free =%d\n", 
            (void *)current, msize, is_free);
#endif
            break;
        }
        if (is_free) {
            tiny_free_list_add_ptr(szone, tiny_mag_ptr, (void *)current, msize);
        } else {
            total_alloc += TINY_BYTES_FOR_MSIZE(msize);
        }
        current += TINY_BYTES_FOR_MSIZE(msize);
    }
    return total_alloc;
}

static void 
tiny_free_scan_madvise_free(szone_t *szone, magazine_t *depot_ptr, region_t r) {
    uintptr_t   start = (uintptr_t)TINY_REGION_ADDRESS(r);
    uintptr_t   current = start;
    uintptr_t   limit =  (uintptr_t)TINY_REGION_END(r);
    boolean_t   is_free;
    msize_t     msize;
    boolean_t   did_advise = FALSE;

    // Scan the metadata identifying blocks which span one or more pages. Mark the pages MADV_FREE taking care to preserve free list
    // management data. 
    while (current < limit) {
        msize = get_tiny_meta_header((void *)current, &is_free);
        if (is_free && !msize && (current == start)) {
            // first block is all free
#if DEBUG_MALLOC
            malloc_printf("*** tiny_free_scan_madvise_free first block is all free! %p: msize=%d is_free =%d\n", 
            (void *)current, msize, is_free);
#endif      
            uintptr_t pgLo = round_page(start + sizeof(free_list_t) + sizeof(msize_t));
            uintptr_t pgHi = trunc_page(start + TINY_REGION_SIZE - sizeof(msize_t));
            
            if (pgLo < pgHi) {
                madvise_free_range(szone, r, pgLo, pgHi);
                did_advise = TRUE;
            }
            break;
        }
        if (!msize) {
#if DEBUG_MALLOC
            malloc_printf("*** tiny_free_scan_madvise_free error with %p: msize=%d is_free =%d\n", 
            (void *)current, msize, is_free);
#endif
            break;
        }
        if (is_free) {
            uintptr_t pgLo = round_page(current + sizeof(free_list_t) + sizeof(msize_t));
            uintptr_t pgHi = trunc_page(current + TINY_BYTES_FOR_MSIZE(msize) - sizeof(msize_t));
            
            if (pgLo < pgHi) {
                madvise_free_range(szone, r, pgLo, pgHi);
                did_advise = TRUE;
            }
        }
        current += TINY_BYTES_FOR_MSIZE(msize);
    }
    
    if (did_advise) {
        /* Move the node to the tail of the Deopt's recirculation list to delay its re-use. */
        region_trailer_t *node = REGION_TRAILER_FOR_TINY_REGION(r);
        recirc_list_extract(szone, depot_ptr, node); // excise node from list
        recirc_list_splice_last(szone, depot_ptr, node); // connect to magazine as last node
    }
}

static void
tiny_free_try_depot_unmap_no_lock(szone_t *szone, magazine_t *depot_ptr, region_trailer_t *node)
{
#warning Tune Depot headroom    
    if (0 < node->bytes_used || 
        depot_ptr->recirculation_entries < (szone->num_tiny_magazines * 2)) {
        return;
    }
    
    // disconnect node from Depot
    recirc_list_extract(szone, depot_ptr, node);
    
    // Iterate the region pulling its free entries off the (locked) Depot's free list
    region_t sparse_region = TINY_REGION_FOR_PTR(node);
    int objects_in_use = tiny_free_detach_region(szone, depot_ptr, sparse_region);
    
    if (0 == objects_in_use) {
        // Invalidate the hash table entry for this region with HASHRING_REGION_DEALLOCATED.
        // Using HASHRING_REGION_DEALLOCATED preserves the collision chain, using HASHRING_OPEN_ENTRY (0) would not.
        rgnhdl_t pSlot = hash_lookup_region_no_lock(szone->tiny_region_generation->hashed_regions, 
                                     szone->tiny_region_generation->num_regions_allocated, 
                                     szone->tiny_region_generation->num_regions_allocated_shift, sparse_region); 
        *pSlot = HASHRING_REGION_DEALLOCATED;
        depot_ptr->num_bytes_in_magazine -= TINY_REGION_PAYLOAD_BYTES;
#if ((__GNUC__ > 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ >= 1))) /* GCC 4.1 and forward supports atomic builtins */
        __sync_fetch_and_add( &(szone->num_tiny_regions_dealloc), 1); // Atomically increment num_tiny_regions_dealloc
#else
#ifdef __LP64__
        OSAtomicIncrement64( (volatile int64_t *)&(szone->num_tiny_regions_dealloc) );
#else
        OSAtomicIncrement32( (volatile int32_t *)&(szone->num_tiny_regions_dealloc) );
#endif
#endif
        
        // Transfer ownership of the region back to the OS
        SZONE_MAGAZINE_PTR_UNLOCK(szone, depot_ptr); // Avoid denial of service to Depot while in kernel
        deallocate_pages(szone, sparse_region, TINY_REGION_SIZE, 0);
        SZONE_MAGAZINE_PTR_LOCK(szone, depot_ptr);

        MAGMALLOC_DEALLOCREGION((void *)szone, (void *)sparse_region); // DTrace USDT Probe
        
    } else {
        szone_error(szone, 1, "tiny_free_try_depot_unmap_no_lock objects_in_use not zero:", NULL, "%d\n", objects_in_use);
    }
}

static void
tiny_free_do_recirc_to_depot(szone_t *szone, magazine_t *tiny_mag_ptr, mag_index_t mag_index)
{       
        // The entire magazine crossed the "emptiness threshold". Transfer a region
        // from this magazine to the Depot. Choose a region that itself has crossed the emptiness threshold (i.e
        // is at least fraction "f" empty.) Such a region will be marked "suitable" on the recirculation list.
        region_trailer_t *node = tiny_mag_ptr->firstNode;
        
        while (node && !node->recirc_suitable) {
            node = node->next;
        }
        
        if (NULL == node) {
#if DEBUG_MALLOC
            malloc_printf("*** tiny_free_do_recirc_to_depot end of list\n");
#endif  
            return;
        }

        region_t sparse_region = TINY_REGION_FOR_PTR(node);

        // Deal with unclaimed memory -- mag_bytes_free_at_end
        if (sparse_region == tiny_mag_ptr->mag_last_region && tiny_mag_ptr->mag_bytes_free_at_end) {
            tiny_finalize_region(szone, tiny_mag_ptr);
        }
        
        // disconnect "suitable" node from magazine
        recirc_list_extract(szone, tiny_mag_ptr, node);

        // Iterate the region pulling its free entries off its (locked) magazine's free list
        int objects_in_use = tiny_free_detach_region(szone, tiny_mag_ptr, sparse_region);
        magazine_t *depot_ptr = &(szone->tiny_magazines[DEPOT_MAGAZINE_INDEX]);
        
        // hand over the region to the (locked) Depot
        SZONE_MAGAZINE_PTR_LOCK(szone,depot_ptr);
        // this will cause tiny_free_list_add_ptr called by tiny_free_reattach_region to use 
        // the depot as its target magazine, rather than magazine formerly associated with sparse_region
        MAGAZINE_INDEX_FOR_TINY_REGION(sparse_region) = DEPOT_MAGAZINE_INDEX;

        // Iterate the region putting its free entries on Depot's free list
        size_t bytes_inplay = tiny_free_reattach_region(szone, depot_ptr, sparse_region);
        
        tiny_mag_ptr->mag_num_bytes_in_objects -= bytes_inplay;
        tiny_mag_ptr->num_bytes_in_magazine -= TINY_REGION_PAYLOAD_BYTES;
        tiny_mag_ptr->mag_num_objects -= objects_in_use;
        
        depot_ptr->mag_num_bytes_in_objects += bytes_inplay;
        depot_ptr->num_bytes_in_magazine += TINY_REGION_PAYLOAD_BYTES;
        depot_ptr->mag_num_objects += objects_in_use;
        
        // connect to Depot as first (MRU) node
        recirc_list_splice_first(szone, depot_ptr, node);

        MAGMALLOC_RECIRCREGION((void *)szone, (int)mag_index, (int)BYTES_USED_FOR_TINY_REGION(sparse_region));  // DTrace USDT Probe

        // Mark free'd dirty pages with MADV_FREE to reduce memory pressure
        tiny_free_scan_madvise_free(szone, depot_ptr, sparse_region);
        
        // If the region is entirely empty vm_deallocate() it
        tiny_free_try_depot_unmap_no_lock(szone, depot_ptr, node);
        
        SZONE_MAGAZINE_PTR_UNLOCK(szone,depot_ptr);
}    

static boolean_t
tiny_get_region_from_depot(szone_t *szone, magazine_t *tiny_mag_ptr, mag_index_t mag_index)
{
    magazine_t *depot_ptr = &(szone->tiny_magazines[DEPOT_MAGAZINE_INDEX]);

    /* FIXME: Would Uniprocessor benefit from recirc and MADV_FREE? */
    if (szone->num_tiny_magazines == 1) // Uniprocessor, single magazine, so no recirculation necessary
        return 0;

#if DEBUG_MALLOC
    if (DEPOT_MAGAZINE_INDEX == mag_index) {
        szone_error(szone, 1, "tiny_get_region_from_depot called for magazine index -1", NULL, NULL);
        return 0;
    }
#endif
        
    SZONE_MAGAZINE_PTR_LOCK(szone,depot_ptr);

    // Appropriate one of the Depot's regions. Prefer LIFO selection for best cache utilization.
    region_trailer_t *node = depot_ptr->firstNode;
        
    if (NULL == node) { // Depot empty?
        SZONE_MAGAZINE_PTR_UNLOCK(szone,depot_ptr);
        return 0;
    }
    
    // disconnect first node from Depot
    recirc_list_extract(szone, depot_ptr, node);
    
    // Iterate the region pulling its free entries off the (locked) Depot's free list
    region_t sparse_region = TINY_REGION_FOR_PTR(node);
    int objects_in_use = tiny_free_detach_region(szone, depot_ptr, sparse_region);

    // Transfer ownership of the region
    MAGAZINE_INDEX_FOR_TINY_REGION(sparse_region) = mag_index;
    
    // Iterate the region putting its free entries on its new (locked) magazine's free list
    size_t bytes_inplay = tiny_free_reattach_region(szone, tiny_mag_ptr, sparse_region);
    
    depot_ptr->mag_num_bytes_in_objects -= bytes_inplay;
    depot_ptr->num_bytes_in_magazine -= TINY_REGION_PAYLOAD_BYTES;
    depot_ptr->mag_num_objects -= objects_in_use;
        
    tiny_mag_ptr->mag_num_bytes_in_objects += bytes_inplay;
    tiny_mag_ptr->num_bytes_in_magazine += TINY_REGION_PAYLOAD_BYTES;
    tiny_mag_ptr->mag_num_objects += objects_in_use;
    
    // connect to magazine as first node (it's maximally sparse at this moment)
    recirc_list_splice_first(szone, tiny_mag_ptr, node);
        
    SZONE_MAGAZINE_PTR_UNLOCK(szone,depot_ptr);

    MAGMALLOC_DEPOTREGION((void *)szone, (int)mag_index, (int)BYTES_USED_FOR_TINY_REGION(sparse_region)); // DTrace USDT Probe
    
    if (-1 == madvise((void *)sparse_region, TINY_REGION_PAYLOAD_BYTES, MADV_FREE_REUSE)) {
        /* -1 return: VM map entry change makes this unfit for reuse. Something evil lurks. */
#if DEBUG_MALLOC
        szone_error(szone, 1, "tiny_get_region_from_depot madvise(..., MADV_FREE_REUSE) failed", sparse_region, NULL);
#endif
        return 0;
    }

    return 1;
}

#warning Tune K and f!
#define K 1.5 // headroom measured in number of 1Mb regions
#define DENSITY_THRESHOLD(a) \
    ((a) - ((a) >> 2)) // "Emptiness" f = 0.25, so "Density" is (1 - f)*a. Generally: ((a) - ((a) >> -log2(f))) 

static INLINE void
tiny_free_no_lock(szone_t *szone, magazine_t *tiny_mag_ptr, mag_index_t mag_index, region_t region, void *ptr, 
                  msize_t msize)
{
    void        *original_ptr = ptr;
    size_t      original_size = TINY_BYTES_FOR_MSIZE(msize);
    void        *next_block = ((unsigned char *)ptr + original_size);
    msize_t     previous_msize, next_msize;
    void        *previous;
    free_list_t *big_free_block;
    free_list_t *after_next_block;
    free_list_t *before_next_block;
    boolean_t   did_prepend = FALSE;
    boolean_t   did_append = FALSE;

#if DEBUG_MALLOC
    if (LOG(szone,ptr)) {
        malloc_printf("in tiny_free_no_lock(), ptr=%p, msize=%d\n", ptr, msize);
    }
    if (! msize) {
        szone_error(szone, 1, "trying to free tiny block that is too small", ptr,
            "in tiny_free_no_lock(), ptr=%p, msize=%d\n", ptr, msize);
    }
#endif
    
    // We try to coalesce this block with the preceeding one
    previous = tiny_previous_preceding_free(ptr, &previous_msize);
    if (previous) {
#if DEBUG_MALLOC
        if (LOG(szone, ptr) || LOG(szone,previous)) { 
            malloc_printf("in tiny_free_no_lock(), coalesced backwards for %p previous=%p\n", ptr, previous);
        }
#endif
        did_prepend = TRUE;

        // clear the meta_header since this is no longer the start of a block
        set_tiny_meta_header_middle(ptr);
        tiny_free_list_remove_ptr(szone, tiny_mag_ptr, previous, previous_msize);
        ptr = previous;
        msize += previous_msize;
    }
    // We try to coalesce with the next block
    if ((next_block < TINY_REGION_END(region)) && tiny_meta_header_is_free(next_block)) {
        did_append = TRUE;
        next_msize = get_tiny_free_size(next_block);
#if DEBUG_MALLOC
        if (LOG(szone, ptr) || LOG(szone, next_block)) {
            malloc_printf("in tiny_free_no_lock(), for ptr=%p, msize=%d coalesced forward=%p next_msize=%d\n",
              ptr, msize, next_block, next_msize);
        }
#endif
        // If we are coalescing with the next block, and the next block is in
        // the last slot of the free list, then we optimize this case here to 
        // avoid removing next_block from the slot (NUM_TINY_SLOTS - 1) and then adding ptr back
        // to slot (NUM_TINY_SLOTS - 1).
        if (next_msize >= NUM_TINY_SLOTS) {
            msize += next_msize;
            
            big_free_block = (free_list_t *)next_block;
            after_next_block = free_list_unchecksum_ptr(szone, &big_free_block->next);
            before_next_block = free_list_unchecksum_ptr(szone, &big_free_block->previous);
            
            if (!before_next_block) {
                tiny_mag_ptr->mag_free_list[NUM_TINY_SLOTS-1] = ptr;
            } else {
                before_next_block->next.u = free_list_checksum_ptr(szone, ptr);
            }
            
            if (after_next_block) {
                after_next_block->previous.u = free_list_checksum_ptr(szone, ptr);
            }
            
            // we don't need to checksum these since they are already checksummed
            ((free_list_t *)ptr)->previous = big_free_block->previous;
            ((free_list_t *)ptr)->next = big_free_block->next;
            
            // clear the meta_header to enable coalescing backwards
            set_tiny_meta_header_middle(big_free_block);
            set_tiny_meta_header_free(ptr, msize);
            
            goto tiny_free_ending;
        }
        tiny_free_list_remove_ptr(szone, tiny_mag_ptr, next_block, next_msize);
        set_tiny_meta_header_middle(next_block); // clear the meta_header to enable coalescing backwards
        msize += next_msize;
    }
#if !TINY_CACHE
    // The tiny cache already scribbles free blocks as they go through the
    // cache, so we do not need to do it here.
    if ((szone->debug_flags & SCALABLE_MALLOC_DO_SCRIBBLE) && msize)
        memset(ptr, 0x55, TINY_BYTES_FOR_MSIZE(msize));

#endif
    tiny_free_list_add_ptr(szone, tiny_mag_ptr, ptr, msize);
  tiny_free_ending:
    // When in proper debug mode we write on the memory to help debug memory smashers
    
    tiny_mag_ptr->mag_num_objects--;
    // we use original_size and not msize to avoid double counting the coalesced blocks
    tiny_mag_ptr->mag_num_bytes_in_objects -= original_size;
    
    // Update this region's bytes in use count
    region_trailer_t *node = REGION_TRAILER_FOR_TINY_REGION(region);
    size_t bytes_used = node->bytes_used - original_size;
    node->bytes_used = bytes_used;
    
    /* FIXME: Would Uniprocessor benefit from recirc and MADV_FREE? */
    if (szone->num_tiny_magazines == 1) { // Uniprocessor, single magazine, so no recirculation necessary
        /* NOTHING */
    } else if (DEPOT_MAGAZINE_INDEX != mag_index) {
        // Emptiness discriminant
        if (bytes_used < DENSITY_THRESHOLD(TINY_REGION_PAYLOAD_BYTES)) {
            /* Region has crossed threshold from density to sparsity. Mark it "suitable" on the
               recirculation candidates list. */
            node->recirc_suitable = TRUE;
        } else {
            /* After this free, we've found the region is still dense, so it must have been even more so before
               the free. That implies the region is already correctly marked. Do nothing. */
        }
    
        // Has the entire magazine crossed the "emptiness threshold"? If so, transfer a region
        // from this magazine to the Depot. Choose a region that itself has crossed the emptiness threshold (i.e
        // is at least fraction "f" empty.) Such a region will be marked "suitable" on the recirculation list.
        size_t a = tiny_mag_ptr->num_bytes_in_magazine; // Total bytes allocated to this magazine
        size_t u = tiny_mag_ptr->mag_num_bytes_in_objects; // In use (malloc'd) from this magaqzine

        if (a - u > ((3 * TINY_REGION_PAYLOAD_BYTES) / 2) && u < DENSITY_THRESHOLD(a))
            tiny_free_do_recirc_to_depot(szone, tiny_mag_ptr, mag_index);
            
    } else {
        // Freed to Depot. N.B. Lock on tiny_magazines[DEPOT_MAGAZINE_INDEX] is already held
        uintptr_t safe_ptr = (uintptr_t)ptr + sizeof(free_list_t) + sizeof(msize_t);
        uintptr_t round_safe = round_page(safe_ptr);
        
        uintptr_t safe_extent = (uintptr_t)ptr + TINY_BYTES_FOR_MSIZE(msize) - sizeof(msize_t);
        uintptr_t trunc_extent = trunc_page(safe_extent);

        // The newly freed block may complete a span of bytes that cover a page. Mark it with MADV_FREE.
        if (round_safe < trunc_extent) { // Safe area covers a page
            if (did_prepend & did_append) { // Coalesced preceding with original_ptr *and* with following
                uintptr_t trunc_safe_prev = trunc_page((uintptr_t)original_ptr - sizeof(msize_t));
                uintptr_t rnd_safe_follow = 
                    round_page((uintptr_t)original_ptr + original_size + sizeof(free_list_t) + sizeof(msize_t));
                
                madvise_free_range(szone, region, MAX(round_safe, trunc_safe_prev), MIN(rnd_safe_follow, trunc_extent));
            } else if (did_prepend) { // Coalesced preceding with original_ptr
                uintptr_t trunc_safe_prev = trunc_page((uintptr_t)original_ptr - sizeof(msize_t));
                
                madvise_free_range(szone, region, MAX(round_safe, trunc_safe_prev), trunc_extent);
            } else if (did_append) { // Coalesced original_ptr with following
                uintptr_t rnd_safe_follow = 
                    round_page((uintptr_t)original_ptr + original_size + sizeof(free_list_t) + sizeof(msize_t));
                
                madvise_free_range(szone, region, round_safe, MIN(rnd_safe_follow, trunc_extent));
            } else { // Isolated free cannot exceed 496 bytes, thus round_safe == trunc_extent, and so never get here.
                /* madvise_free_range(szone, region, round_safe, trunc_extent); */
            }
        }
        
        if (0 < bytes_used) {
            /* Depot'd region is still live. Leave it in place on the Depot's recirculation list
               so as to avoid thrashing between the Depot's free list and a magazines's free list
               with detach_region/reattach_region */
        } else {
            /* Depot'd region is just now empty. Consider return to OS. */
            region_trailer_t *node = REGION_TRAILER_FOR_TINY_REGION(region);
            magazine_t *depot_ptr = &(szone->tiny_magazines[DEPOT_MAGAZINE_INDEX]);
            tiny_free_try_depot_unmap_no_lock(szone, depot_ptr, node); // FIXME: depot_ptr is simply tiny_mag_ptr?
        }
    }
}

// Allocates from the last region or a freshly allocated region
static void *
tiny_malloc_from_region_no_lock(szone_t *szone, magazine_t *tiny_mag_ptr, mag_index_t mag_index, msize_t msize)
{
    void        *ptr, *aligned_address;

    // Deal with unclaimed memory -- mag_bytes_free_at_end
    if (tiny_mag_ptr->mag_bytes_free_at_end)
        tiny_finalize_region(szone, tiny_mag_ptr);

    // time to create a new region
    aligned_address = allocate_pages(szone, TINY_REGION_SIZE, TINY_BLOCKS_ALIGN, 0, VM_MEMORY_MALLOC_TINY);
    if (!aligned_address) // out of memory!
        return NULL;

    MAGMALLOC_ALLOCREGION((void *)szone, (int)mag_index); // DTrace USDT Probe

    // We set the unused bits of the header in the last pair to be all ones, and those of the inuse to zeroes.
    ((tiny_region_t)aligned_address)->pairs[CEIL_NUM_TINY_BLOCKS_WORDS-1].header = 
        (NUM_TINY_BLOCKS & 31) ? (0xFFFFFFFFU << (NUM_TINY_BLOCKS & 31)) : 0;
    ((tiny_region_t)aligned_address)->pairs[CEIL_NUM_TINY_BLOCKS_WORDS-1].inuse = 0;
    
    // Here find the only place in tinyland that (infrequently) takes the tiny_regions_lock.
    // Only one thread at a time should be permitted to assess the density of the hash
    // ring and adjust if needed. 
    // Only one thread at a time should be permitted to insert its new region on
    // the hash ring.
    // It is safe for all other threads to read the hash ring (hashed_regions) and
    // the associated sizes (num_regions_allocated and num_tiny_regions).
    
    LOCK(szone->tiny_regions_lock);
    
    // Check to see if the hash ring of tiny regions needs to grow.  Try to
    // avoid the hash ring becoming too dense.
    if (szone->tiny_region_generation->num_regions_allocated < (2 * szone->num_tiny_regions)) {
        region_t *new_regions;
        size_t new_size;
        size_t new_shift = szone->tiny_region_generation->num_regions_allocated_shift; // In/Out parameter
        new_regions = hash_regions_grow_no_lock(szone, szone->tiny_region_generation->hashed_regions,
                                      szone->tiny_region_generation->num_regions_allocated,
                                      &new_shift,
                                      &new_size);
        // Do not deallocate the current hashed_regions allocation since someone may
        // be iterating it.  Instead, just leak it.

        // Prepare to advance to the "next generation" of the hash ring.
        szone->tiny_region_generation->nextgen->hashed_regions = new_regions;
        szone->tiny_region_generation->nextgen->num_regions_allocated = new_size;
        szone->tiny_region_generation->nextgen->num_regions_allocated_shift = new_shift;

        // Throw the switch to atomically advance to the next generation.
        szone->tiny_region_generation = szone->tiny_region_generation->nextgen;
        // Ensure everyone sees the advance.
#if ((__GNUC__ > 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ >= 1))) /* GCC 4.1 and forward supports atomic builtins */
        __sync_synchronize();
#else
        OSMemoryBarrier();
#endif
    }
    // Tag the region at "aligned_address" as belonging to us, 
    // and so put it under the protection of the magazine lock we are holding.
    // Do this before advertising "aligned_address" on the hash ring(!)
    MAGAZINE_INDEX_FOR_TINY_REGION(aligned_address) = mag_index;

    // Insert the new region into the hash ring, and update malloc statistics
    hash_region_insert_no_lock(szone->tiny_region_generation->hashed_regions, 
                               szone->tiny_region_generation->num_regions_allocated,
                               szone->tiny_region_generation->num_regions_allocated_shift,
                               aligned_address);
    
    szone->num_tiny_regions++;
    UNLOCK(szone->tiny_regions_lock);
    
    tiny_mag_ptr->mag_last_region = aligned_address;
    BYTES_USED_FOR_TINY_REGION(aligned_address) = TINY_BYTES_FOR_MSIZE(msize);
    ptr = aligned_address; 
    set_tiny_meta_header_in_use(ptr, msize);
    tiny_mag_ptr->mag_num_objects++;
    tiny_mag_ptr->mag_num_bytes_in_objects += TINY_BYTES_FOR_MSIZE(msize);
    tiny_mag_ptr->num_bytes_in_magazine += TINY_REGION_PAYLOAD_BYTES;
 
    // We put a header on the last block so that it appears in use (for coalescing, etc...)
    set_tiny_meta_header_in_use_1((void *)((uintptr_t)ptr + TINY_BYTES_FOR_MSIZE(msize)));
    tiny_mag_ptr->mag_bytes_free_at_end = TINY_BYTES_FOR_MSIZE(NUM_TINY_BLOCKS - msize);

    // connect to magazine as first node (it's maximally sparse at this moment)
    recirc_list_splice_first(szone, tiny_mag_ptr, REGION_TRAILER_FOR_TINY_REGION(aligned_address));

#if DEBUG_MALLOC
    if (LOG(szone,ptr)) {
        malloc_printf("in tiny_malloc_from_region_no_lock(), ptr=%p, msize=%d\n", ptr, msize);
    }
#endif
    return ptr;
}

static INLINE boolean_t
tiny_try_realloc_in_place(szone_t *szone, void *ptr, size_t old_size, size_t new_size)
{
    // returns 1 on success
    msize_t     index;
    msize_t     old_msize;
    unsigned    next_index;
    void        *next_block;
    boolean_t   is_free;
    msize_t     next_msize, coalesced_msize, leftover_msize;
    void        *leftover;

    index = TINY_INDEX_FOR_PTR(ptr);
    old_msize = TINY_MSIZE_FOR_BYTES(old_size);
    next_index = index + old_msize;
    
    if (next_index >= NUM_TINY_BLOCKS) {
        return 0;
    }
    next_block = (char *)ptr + old_size;
    
    magazine_t  *tiny_mag_ptr = mag_lock_zine_for_region_trailer(szone, szone->tiny_magazines,
                                    REGION_TRAILER_FOR_TINY_REGION(TINY_REGION_FOR_PTR(ptr)), 
                                    MAGAZINE_INDEX_FOR_TINY_REGION(TINY_REGION_FOR_PTR(ptr)));    
    
    /*
     * Look for a free block immediately afterwards.  If it's large enough, we can consume (part of)
     * it.
     */
    is_free = tiny_meta_header_is_free(next_block);
    if (!is_free) {
        SZONE_MAGAZINE_PTR_UNLOCK(szone,tiny_mag_ptr);
        return 0; // next_block is in use;
    }
    next_msize = get_tiny_free_size(next_block);
    if (old_size + TINY_BYTES_FOR_MSIZE(next_msize) < new_size) {
        SZONE_MAGAZINE_PTR_UNLOCK(szone,tiny_mag_ptr);
        return 0; // even with next block, not enough
    }
    /*
     * The following block is big enough; pull it from its freelist and chop off enough to satisfy
     * our needs.
     */
    tiny_free_list_remove_ptr(szone, tiny_mag_ptr, next_block, next_msize);
    set_tiny_meta_header_middle(next_block); // clear the meta_header to enable coalescing backwards
    coalesced_msize = TINY_MSIZE_FOR_BYTES(new_size - old_size + TINY_QUANTUM - 1);
    leftover_msize = next_msize - coalesced_msize;
    if (leftover_msize) {
        /* there's some left, so put the remainder back */
        leftover = (void *)((uintptr_t)next_block + TINY_BYTES_FOR_MSIZE(coalesced_msize));
        
        tiny_free_list_add_ptr(szone, tiny_mag_ptr, leftover, leftover_msize);
    }
    set_tiny_meta_header_in_use(ptr, old_msize + coalesced_msize);
#if DEBUG_MALLOC
    if (LOG(szone,ptr)) {
        malloc_printf("in tiny_try_realloc_in_place(), ptr=%p, msize=%d\n", ptr, old_msize + coalesced_msize);
    }
#endif
    tiny_mag_ptr->mag_num_bytes_in_objects += TINY_BYTES_FOR_MSIZE(coalesced_msize);

    // Update this region's bytes in use count
    region_trailer_t *node = REGION_TRAILER_FOR_TINY_REGION(TINY_REGION_FOR_PTR(ptr));
    size_t bytes_used = node->bytes_used + TINY_BYTES_FOR_MSIZE(coalesced_msize);
    node->bytes_used = bytes_used;

    // Emptiness discriminant
    if (bytes_used < DENSITY_THRESHOLD(TINY_REGION_PAYLOAD_BYTES)) {
        /* After this reallocation the region is still sparse, so it must have been even more so before
           the reallocation. That implies the region is already correctly marked. Do nothing. */
    } else {
        /* Region has crossed threshold from sparsity to density. Mark it not "suitable" on the
           recirculation candidates list. */
        node->recirc_suitable = FALSE;
   }

    SZONE_MAGAZINE_PTR_UNLOCK(szone,tiny_mag_ptr);
    CHECK(szone, __PRETTY_FUNCTION__);
    return 1;
}

static boolean_t
tiny_check_region(szone_t *szone, region_t region)
{
    uintptr_t   start, ptr, region_end;
    boolean_t   prev_free = 0;
    boolean_t   is_free;
    msize_t     msize;
    free_list_t *free_head;
    void        *follower, *previous, *next;
    mag_index_t mag_index = MAGAZINE_INDEX_FOR_TINY_REGION(region);
    magazine_t  *tiny_mag_ptr = &(szone->tiny_magazines[mag_index]);

    // Assumes locked
    CHECK_MAGAZINE_PTR_LOCKED(szone, tiny_mag_ptr, __PRETTY_FUNCTION__);
    
    /* establish region limits */
    start = (uintptr_t)TINY_REGION_ADDRESS(region);
    ptr = start;
    region_end = (uintptr_t)TINY_REGION_END(region);

    /*
     * The last region may have a trailing chunk which has not been converted into inuse/freelist
     * blocks yet.
     */
    if (region == tiny_mag_ptr->mag_last_region)
            region_end -= tiny_mag_ptr->mag_bytes_free_at_end;

    /*
     * Scan blocks within the region.
     */
    while (ptr < region_end) {
        /*
         * If the first block is free, and its size is 65536 (msize = 0) then the entire region is
         * free.
         */
        msize = get_tiny_meta_header((void *)ptr, &is_free);
        if (is_free && !msize && (ptr == start)) {
            return 1;
        }

        /*
         * If the block's size is 65536 (msize = 0) then since we're not the first entry the size is
         * corrupt.
         */
        if (!msize) {
            malloc_printf("*** invariant broken for tiny block %p this msize=%d - size is too small\n",
              ptr, msize);
            return 0;
        }

        if (!is_free) {
            /*
             * In use blocks cannot be more than (NUM_TINY_SLOTS - 1) quanta large.
             */
            prev_free = 0;
            if (msize > (NUM_TINY_SLOTS - 1)) {
                malloc_printf("*** invariant broken for %p this tiny msize=%d - size is too large\n",
                  ptr, msize);
                return 0;
            }
            /* move to next block */
            ptr += TINY_BYTES_FOR_MSIZE(msize);
        } else {
            /*
             * Free blocks must have been coalesced, we cannot have a free block following another
             * free block.
             */
            if (prev_free) {
                malloc_printf("*** invariant broken for free block %p this tiny msize=%d: two free blocks in a row\n",
                  ptr, msize);
                return 0;
            }
            prev_free = 1;
            /*
             * Check the integrity of this block's entry in its freelist.
             */
            free_head = (free_list_t *)ptr;
            previous = free_list_unchecksum_ptr(szone, &free_head->previous);
            next = free_list_unchecksum_ptr(szone, &free_head->next);
            if (previous && !tiny_meta_header_is_free(previous)) {
                malloc_printf("*** invariant broken for %p (previous %p is not a free pointer)\n",
                  ptr, previous);
                return 0;
            }
            if (next && !tiny_meta_header_is_free(next)) {
                malloc_printf("*** invariant broken for %p (next in free list %p is not a free pointer)\n",
                  ptr, next);
                return 0;
            }
            /*
             * Check the free block's trailing size value.
             */
            follower = FOLLOWING_TINY_PTR(ptr, msize);
            if (((uintptr_t)follower != region_end) && (get_tiny_previous_free_msize(follower) != msize)) {
                malloc_printf("*** invariant broken for tiny free %p followed by %p in region [%p-%p] "
                  "(end marker incorrect) should be %d; in fact %d\n",
                  ptr, follower, TINY_REGION_ADDRESS(region), region_end, msize, get_tiny_previous_free_msize(follower));
                return 0;
            }
            /* move to next block */
            ptr = (uintptr_t)follower;
        }
    }
    /*
     * Ensure that we scanned the entire region
     */
    if (ptr != region_end) {
        malloc_printf("*** invariant broken for region end %p - %p\n", ptr, region_end);
        return 0;
    }
    /*
     * Check the trailing block's integrity.
     */
    if (region == tiny_mag_ptr->mag_last_region) {
        if (tiny_mag_ptr->mag_bytes_free_at_end) {
            msize = get_tiny_meta_header((void *)ptr, &is_free);
            if (is_free || (msize != 1)) {
                malloc_printf("*** invariant broken for blocker block %p - %d %d\n", ptr, msize, is_free);
            }
        }
    }
    return 1;
}

static kern_return_t
tiny_in_use_enumerator(task_t task, void *context, unsigned type_mask, szone_t *szone,
                       memory_reader_t reader, vm_range_recorder_t recorder)
{
    size_t              num_regions;
    size_t              index;
    region_t            *regions;
    vm_range_t          buffer[MAX_RECORDER_BUFFER];
    unsigned            count = 0;
    kern_return_t       err;
    region_t            region;
    vm_range_t          range;
    vm_range_t          admin_range;
    vm_range_t          ptr_range;
    unsigned char       *mapped_region;
    uint32_t            *block_header;
    uint32_t            *in_use;
    unsigned            block_index;
    unsigned            block_limit;
    boolean_t           is_free;
    msize_t             msize;
    void                *mapped_ptr;
    unsigned            bit;
    vm_address_t        mag_last_free_ptr = 0;
    msize_t             mag_last_free_msize = 0;

    region_hash_generation_t *trg_ptr;
    err = reader(task, (vm_address_t)szone->tiny_region_generation, sizeof(region_hash_generation_t), (void **)&trg_ptr);
    if (err) return err;

    num_regions = trg_ptr->num_regions_allocated;
    err = reader(task, (vm_address_t)trg_ptr->hashed_regions, sizeof(region_t) * num_regions, (void **)&regions);
    if (err) return err;

    for (index = 0; index < num_regions; ++index) {
        region = regions[index];
        if (HASHRING_OPEN_ENTRY != region && HASHRING_REGION_DEALLOCATED != region) {
            range.address = (vm_address_t)TINY_REGION_ADDRESS(region);
            range.size = (vm_size_t)TINY_REGION_SIZE;
            if (type_mask & MALLOC_ADMIN_REGION_RANGE_TYPE) {
                admin_range.address = range.address + TINY_METADATA_START;
                admin_range.size = TINY_METADATA_SIZE;
                recorder(task, context, MALLOC_ADMIN_REGION_RANGE_TYPE, &admin_range, 1);
            }
            if (type_mask & (MALLOC_PTR_REGION_RANGE_TYPE | MALLOC_ADMIN_REGION_RANGE_TYPE)) {
                ptr_range.address = range.address;
                ptr_range.size = NUM_TINY_BLOCKS * TINY_QUANTUM;
                recorder(task, context, MALLOC_PTR_REGION_RANGE_TYPE, &ptr_range, 1);
            }
            if (type_mask & MALLOC_PTR_IN_USE_RANGE_TYPE) {
                err = reader(task, range.address, range.size, (void **)&mapped_region);
                if (err)
                    return err;

                mag_index_t mag_index = MAGAZINE_INDEX_FOR_TINY_REGION(mapped_region);
                magazine_t *tiny_mag_ptr;
                err = reader(task, (vm_address_t)&(szone->tiny_magazines[mag_index]), sizeof(magazine_t), 
                        (void **)&tiny_mag_ptr);
                if (err) return err;

                void *mag_last_free = tiny_mag_ptr->mag_last_free;
                if (mag_last_free) {
                    mag_last_free_ptr = (uintptr_t) mag_last_free & ~(TINY_QUANTUM - 1);
                    mag_last_free_msize = (uintptr_t) mag_last_free & (TINY_QUANTUM - 1);
                }

                block_header = (uint32_t *)(mapped_region + TINY_METADATA_START + sizeof(region_trailer_t));
                in_use = TINY_INUSE_FOR_HEADER(block_header);
                block_index = 0;
                block_limit = NUM_TINY_BLOCKS;
                if (region == tiny_mag_ptr->mag_last_region)
                    block_limit -= TINY_MSIZE_FOR_BYTES(tiny_mag_ptr->mag_bytes_free_at_end);

                while (block_index < block_limit) {
                    vm_size_t block_offset = TINY_BYTES_FOR_MSIZE(block_index);
                    is_free = !BITARRAY_BIT(in_use, block_index);
                    if (is_free) {
                        mapped_ptr = mapped_region + block_offset;

                        // mapped_region, the address at which 'range' in 'task' has been
                        // mapped into our process, is not necessarily aligned to 
                        // TINY_BLOCKS_ALIGN.
                        //
                        // Since the code in get_tiny_free_size() assumes the pointer came
                        // from a properly aligned tiny region, and mapped_region is not
                        // necessarily aligned, then do the size calculation directly.
                        // If the next bit is set in the header bitmap, then the size is one
                        // quantum.  Otherwise, read the size field.
                        if (!BITARRAY_BIT(block_header, (block_index+1)))
                            msize = TINY_FREE_SIZE(mapped_ptr);
                        else
                            msize = 1;

                        if (!msize)
                            break;
                    } else if (range.address + block_offset != mag_last_free_ptr) {
                        msize = 1; 
                        bit = block_index + 1;
                        while (! BITARRAY_BIT(block_header, bit)) {
                            bit++;
                            msize ++;
                        }
                        buffer[count].address = range.address + block_offset;
                        buffer[count].size = TINY_BYTES_FOR_MSIZE(msize);
                        count++;
                        if (count >= MAX_RECORDER_BUFFER) {
                            recorder(task, context, MALLOC_PTR_IN_USE_RANGE_TYPE, buffer, count);
                            count = 0;
                        }
                    } else {
                        // Block is not free but it matches mag_last_free_ptr so even
                        // though it is not marked free in the bitmap, we treat it as if
                        // it is and move on
                        msize = mag_last_free_msize;
                    }
                    block_index += msize;
                }
                if (count) {
                    recorder(task, context, MALLOC_PTR_IN_USE_RANGE_TYPE, buffer, count);
                    count = 0;
                }
            }
        }
    }
    return 0;
}

static void *
tiny_malloc_from_free_list(szone_t *szone, magazine_t *tiny_mag_ptr, mag_index_t mag_index, msize_t msize)
{
    free_list_t         *ptr;
    msize_t             this_msize;
    grain_t             slot = msize - 1;
    free_list_t         **free_list = tiny_mag_ptr->mag_free_list;
    free_list_t         **the_slot = free_list + slot;
    free_list_t         *next;
    free_list_t         **limit;
#if defined(__LP64__)
    uint64_t            bitmap;
#else
    uint32_t            bitmap;
#endif
    msize_t             leftover_msize;
    free_list_t         *leftover_ptr;

    // Assumes we've locked the region
    CHECK_MAGAZINE_PTR_LOCKED(szone, tiny_mag_ptr, __PRETTY_FUNCTION__);
    
    // Look for an exact match by checking the freelist for this msize.
    // 
    ptr = *the_slot;
    if (ptr) {
        next = free_list_unchecksum_ptr(szone, &ptr->next);
        if (next) {
            next->previous = ptr->previous;
        } else {
            BITMAPV_CLR(tiny_mag_ptr->mag_bitmap, slot);
        }
        *the_slot = next;
        this_msize = msize;
#if DEBUG_MALLOC
        if (LOG(szone, ptr)) {
            malloc_printf("in tiny_malloc_from_free_list(), exact match ptr=%p, this_msize=%d\n", ptr, this_msize);
        }
#endif
        goto return_tiny_alloc;
    }

    // Mask off the bits representing slots holding free blocks smaller than the
    // size we need.  If there are no larger free blocks, try allocating from
    // the free space at the end of the tiny region.
#if defined(__LP64__)
    bitmap = ((uint64_t *)(tiny_mag_ptr->mag_bitmap))[0] & ~ ((1ULL << slot) - 1);
#else
    bitmap = tiny_mag_ptr->mag_bitmap[0] & ~ ((1 << slot) - 1);
#endif
    if (!bitmap)
        goto try_tiny_malloc_from_end;
        
    slot = BITMAPV_CTZ(bitmap);
    limit = free_list + NUM_TINY_SLOTS - 1;
    free_list += slot;
    
    if (free_list < limit) {
        ptr = *free_list;
        if (ptr) {
            next = free_list_unchecksum_ptr(szone, &ptr->next);
            *free_list = next;
            if (next) {
                next->previous = ptr->previous;
            } else {
                BITMAPV_CLR(tiny_mag_ptr->mag_bitmap, slot);
            }
            this_msize = get_tiny_free_size(ptr);
            goto add_leftover_and_proceed;
        }
#if DEBUG_MALLOC
        malloc_printf("in tiny_malloc_from_free_list(), mag_bitmap out of sync, slot=%d\n",slot);
#endif
    }

    // We are now looking at the last slot, which contains blocks equal to, or
    // due to coalescing of free blocks, larger than (NUM_TINY_SLOTS - 1) * tiny quantum size.
    // If the last freelist is not empty, and the head contains a block that is
    // larger than our request, then the remainder is put back on the free list.
    ptr = *limit;
    if (ptr) {
        this_msize = get_tiny_free_size(ptr);
        next = free_list_unchecksum_ptr(szone, &ptr->next);
        if (this_msize - msize >= NUM_TINY_SLOTS) {
            // the leftover will go back to the free list, so we optimize by
            // modifying the free list rather than a pop and push of the head
            leftover_msize = this_msize - msize;
            leftover_ptr = (free_list_t *)((unsigned char *)ptr + TINY_BYTES_FOR_MSIZE(msize));
            *limit = leftover_ptr;
            if (next) {
                next->previous.u = free_list_checksum_ptr(szone, leftover_ptr);
            }
            leftover_ptr->previous = ptr->previous;
            leftover_ptr->next = ptr->next;
            set_tiny_meta_header_free(leftover_ptr, leftover_msize);
#if DEBUG_MALLOC
            if (LOG(szone,ptr)) {
            malloc_printf("in tiny_malloc_from_free_list(), last slot ptr=%p, msize=%d this_msize=%d\n", 
                          ptr, msize, this_msize);
            }
#endif
            this_msize = msize;
            goto return_tiny_alloc;
        }
        if (next) {
            next->previous = ptr->previous;
        }
        *limit = next;
        goto add_leftover_and_proceed;
        /* NOTREACHED */
    }

try_tiny_malloc_from_end:
    // Let's see if we can use tiny_mag_ptr->mag_bytes_free_at_end
    if (tiny_mag_ptr->mag_bytes_free_at_end >= TINY_BYTES_FOR_MSIZE(msize)) {
        ptr = (free_list_t *)((uintptr_t)TINY_REGION_END(tiny_mag_ptr->mag_last_region) - 
                              tiny_mag_ptr->mag_bytes_free_at_end);
        tiny_mag_ptr->mag_bytes_free_at_end -= TINY_BYTES_FOR_MSIZE(msize);
        if (tiny_mag_ptr->mag_bytes_free_at_end) {
            // let's add an in use block after ptr to serve as boundary
            set_tiny_meta_header_in_use_1((unsigned char *)ptr + TINY_BYTES_FOR_MSIZE(msize));
        }
        this_msize = msize;
#if DEBUG_MALLOC
        if (LOG(szone, ptr)) {
            malloc_printf("in tiny_malloc_from_free_list(), from end ptr=%p, msize=%d\n", ptr, msize);
        }
#endif
        goto return_tiny_alloc;
    }
    return NULL;

add_leftover_and_proceed:
    if (!this_msize || (this_msize > msize)) {
        leftover_msize = this_msize - msize;
        leftover_ptr = (free_list_t *)((unsigned char *)ptr + TINY_BYTES_FOR_MSIZE(msize));
#if DEBUG_MALLOC
        if (LOG(szone,ptr)) {
            malloc_printf("in tiny_malloc_from_free_list(), adding leftover ptr=%p, this_msize=%d\n", ptr, this_msize);
        }
#endif
        tiny_free_list_add_ptr(szone, tiny_mag_ptr, leftover_ptr, leftover_msize);
        this_msize = msize;
    }
    
return_tiny_alloc:
    tiny_mag_ptr->mag_num_objects++;
    tiny_mag_ptr->mag_num_bytes_in_objects += TINY_BYTES_FOR_MSIZE(this_msize);

    // Update this region's bytes in use count
    region_trailer_t *node = REGION_TRAILER_FOR_TINY_REGION(TINY_REGION_FOR_PTR(ptr));
    size_t bytes_used = node->bytes_used + TINY_BYTES_FOR_MSIZE(this_msize);
    node->bytes_used = bytes_used;

    // Emptiness discriminant
    if (bytes_used < DENSITY_THRESHOLD(TINY_REGION_PAYLOAD_BYTES)) {
        /* After this allocation the region is still sparse, so it must have been even more so before
           the allocation. That implies the region is already correctly marked. Do nothing. */
    } else {
        /* Region has crossed threshold from sparsity to density. Mark it not "suitable" on the
           recirculation candidates list. */
        node->recirc_suitable = FALSE;
   }
#if DEBUG_MALLOC
    if (LOG(szone,ptr)) {
        malloc_printf("in tiny_malloc_from_free_list(), ptr=%p, this_msize=%d, msize=%d\n", ptr, this_msize, msize);
    }
#endif
    if (this_msize > 1)
        set_tiny_meta_header_in_use(ptr, this_msize);
    else
        set_tiny_meta_header_in_use_1(ptr);
    return ptr;
}
#undef DENSITY_THRESHOLD
#undef K

static INLINE void *
tiny_malloc_should_clear(szone_t *szone, msize_t msize, boolean_t cleared_requested)
{
    void        *ptr;
    mag_index_t mag_index = mag_get_thread_index(szone);
    magazine_t  *tiny_mag_ptr = &(szone->tiny_magazines[mag_index]);
 
#if DEBUG_MALLOC
    if (DEPOT_MAGAZINE_INDEX == mag_index) {
        szone_error(szone, 1, "malloc called for magazine index -1", NULL, NULL);
        return(NULL);
    }
    
    if (!msize) {
        szone_error(szone, 1, "invariant broken (!msize) in allocation (region)", NULL, NULL);
        return(NULL);
    }
#endif

    SZONE_MAGAZINE_PTR_LOCK(szone, tiny_mag_ptr);

#if TINY_CACHE
    ptr = tiny_mag_ptr->mag_last_free;

    if ((((uintptr_t)ptr) & (TINY_QUANTUM - 1)) == msize) {
        // we have a winner
        tiny_mag_ptr->mag_last_free = NULL;
        SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
        CHECK(szone, __PRETTY_FUNCTION__);
        ptr = (void *)((uintptr_t)ptr & ~ (TINY_QUANTUM - 1));
        if (cleared_requested) {
            memset(ptr, 0, TINY_BYTES_FOR_MSIZE(msize));
        }
#if DEBUG_MALLOC
        if (LOG(szone,ptr)) {
            malloc_printf("in tiny_malloc_should_clear(), tiny cache ptr=%p, msize=%d\n", ptr, msize);
        }
#endif
        return ptr;
    }
#endif /* TINY_CACHE */

    ptr = tiny_malloc_from_free_list(szone, tiny_mag_ptr, mag_index, msize);
    if (ptr) {
        SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
        CHECK(szone, __PRETTY_FUNCTION__);
        if (cleared_requested) {
            memset(ptr, 0, TINY_BYTES_FOR_MSIZE(msize));
        }
        return ptr;
    }

    if (tiny_get_region_from_depot(szone, tiny_mag_ptr, mag_index)) {
        ptr = tiny_malloc_from_free_list(szone, tiny_mag_ptr, mag_index, msize);
        if (ptr) {
            SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
            CHECK(szone, __PRETTY_FUNCTION__);
            if (cleared_requested) {
                memset(ptr, 0, TINY_BYTES_FOR_MSIZE(msize));
            }
            return ptr;
        }
    }

    ptr = tiny_malloc_from_region_no_lock(szone, tiny_mag_ptr, mag_index, msize);
    // we don't clear because this freshly allocated space is pristine
    SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
    CHECK(szone, __PRETTY_FUNCTION__);
    return ptr;
}

static NOINLINE void
free_tiny_botch(szone_t *szone, free_list_t *ptr)
{
    mag_index_t mag_index = MAGAZINE_INDEX_FOR_TINY_REGION(TINY_REGION_FOR_PTR(ptr));
    magazine_t  *tiny_mag_ptr = &(szone->tiny_magazines[mag_index]);
    SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
    szone_error(szone, 1, "double free", ptr, NULL);
}

static INLINE void
free_tiny(szone_t *szone, void *ptr, region_t tiny_region, size_t known_size)
{
    msize_t     msize;
    boolean_t   is_free;
    mag_index_t mag_index = MAGAZINE_INDEX_FOR_TINY_REGION(tiny_region);
    magazine_t  *tiny_mag_ptr = &(szone->tiny_magazines[mag_index]);

    // ptr is known to be in tiny_region
    if (known_size) {
        msize = TINY_MSIZE_FOR_BYTES(known_size + TINY_QUANTUM - 1);
    } else {
        msize = get_tiny_meta_header(ptr, &is_free);
        if (is_free) {
            free_tiny_botch(szone, ptr);
            return;
        }
    }
#if DEBUG_MALLOC
    if (!msize) {
        malloc_printf("*** free_tiny() block in use is too large: %p\n", ptr);
        return;
    }
#endif
    
    SZONE_MAGAZINE_PTR_LOCK(szone, tiny_mag_ptr);

#if TINY_CACHE
    // Depot does not participate in TINY_CACHE since it can't be directly malloc()'d
    if (DEPOT_MAGAZINE_INDEX != mag_index) {
        if (msize < TINY_QUANTUM) { // to see if the bits fit in the last 4 bits
            void *ptr2 = tiny_mag_ptr->mag_last_free; // Might be NULL
            region_t rgn2 = tiny_mag_ptr->mag_last_free_rgn;
        
            /* check that we don't already have this pointer in the cache */
            if (ptr == (void *)((uintptr_t)ptr2 & ~ (TINY_QUANTUM - 1))) {
                free_tiny_botch(szone, ptr);
                return;
            }
            
            if ((szone->debug_flags & SCALABLE_MALLOC_DO_SCRIBBLE) && msize)
                memset(ptr, 0x55, TINY_BYTES_FOR_MSIZE(msize));
                
            tiny_mag_ptr->mag_last_free = (void *)(((uintptr_t)ptr) | msize);
            tiny_mag_ptr->mag_last_free_rgn = tiny_region;
    
            if (!ptr2) {
                SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
                CHECK(szone, __PRETTY_FUNCTION__);
                return;
            }
        
            msize = (uintptr_t)ptr2 & (TINY_QUANTUM - 1);
            ptr = (void *)(((uintptr_t)ptr2) & ~(TINY_QUANTUM - 1));
            tiny_region = rgn2;
        }
    }
#endif /* TINY_CACHE */

    // Now in the time it took to acquire the lock, the region may have migrated
    // from one magazine to another. I.e. trailer->mag_index is volatile.
    // In which case the magazine lock we obtained (namely magazines[mag_index].mag_lock) 
    // is stale. If so, keep on tryin' ...
    region_trailer_t *trailer = REGION_TRAILER_FOR_TINY_REGION(tiny_region);
    mag_index_t refreshed_index;
    
    while (mag_index != (refreshed_index = trailer->mag_index)) { // Note assignment
    
        SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
        
        mag_index = refreshed_index;
        tiny_mag_ptr = &(szone->tiny_magazines[mag_index]);
        SZONE_MAGAZINE_PTR_LOCK(szone, tiny_mag_ptr);
    }
                            
    tiny_free_no_lock(szone, tiny_mag_ptr, mag_index, tiny_region, ptr, msize);
    SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
    CHECK(szone, __PRETTY_FUNCTION__);
}

static void
print_tiny_free_list(szone_t *szone)
{
    free_list_t *ptr;
    _SIMPLE_STRING b = _simple_salloc();
    mag_index_t mag_index;

    if (b) {
        _simple_sappend(b, "tiny free sizes:\n");
        for (mag_index = -1; mag_index < szone->num_tiny_magazines; mag_index++) {
            grain_t     slot = 0;
            _simple_sprintf(b,"\tMagazine %d: ", mag_index);
            while (slot < NUM_TINY_SLOTS) {
                ptr = szone->tiny_magazines[mag_index].mag_free_list[slot];
                if (ptr) {
                    _simple_sprintf(b, "%s%y[%d]; ", (slot == NUM_TINY_SLOTS-1) ? ">=" : "", 
                                    (slot+1)*TINY_QUANTUM, free_list_count(szone, ptr));
                }
                slot++;
            }
            _simple_sappend(b,"\n");
        }
        _malloc_printf(MALLOC_PRINTF_NOLOG | MALLOC_PRINTF_NOPREFIX, "%s\n", _simple_string(b));
        _simple_sfree(b);
    }
}

static void
print_tiny_region(boolean_t verbose, region_t region, size_t bytes_at_end)
{
    unsigned            counts[1024];
    unsigned            in_use = 0;
    uintptr_t           start = (uintptr_t)TINY_REGION_ADDRESS(region);
    uintptr_t           current = start;
    uintptr_t           limit =  (uintptr_t)TINY_REGION_END(region) - bytes_at_end;
    boolean_t           is_free;
    msize_t             msize;
    unsigned            ci;
    _SIMPLE_STRING      b;
    uintptr_t           pgTot = 0;
    
    if (region == HASHRING_REGION_DEALLOCATED) {
        if ((b = _simple_salloc()) != NULL) {
            _simple_sprintf(b, "Tiny region [unknown address] was returned to the OS\n");
            _malloc_printf(MALLOC_PRINTF_NOLOG | MALLOC_PRINTF_NOPREFIX, "%s\n", _simple_string(b));
            _simple_sfree(b);
        }
        return;
    }
    
    memset(counts, 0, sizeof(counts));
    while (current < limit) {
        msize = get_tiny_meta_header((void *)current, &is_free);
        if (is_free & !msize && (current == start)) {
            // first block is all free
            uintptr_t pgLo = round_page(start + sizeof(free_list_t) + sizeof(msize_t));
            uintptr_t pgHi = trunc_page(start + TINY_REGION_SIZE - sizeof(msize_t));
            
            if (pgLo < pgHi) {
                pgTot += (pgHi - pgLo);
            }
            break;
        }
        if (!msize) {
            malloc_printf("*** error with %p: msize=%d\n", (void *)current, (unsigned)msize);
            break;
        }
        if (!is_free) {
            // block in use
            if (msize > NUM_TINY_SLOTS)
                malloc_printf("*** error at %p msize for in_use is %d\n", (void *)current, msize);
            if (msize < 1024)
                counts[msize]++;
            in_use++;
        } else {
            uintptr_t pgLo = round_page(current + sizeof(free_list_t) + sizeof(msize_t));
            uintptr_t pgHi = trunc_page(current + TINY_BYTES_FOR_MSIZE(msize) - sizeof(msize_t));
            
            if (pgLo < pgHi) {
                pgTot += (pgHi - pgLo);
            }
        }
        current += TINY_BYTES_FOR_MSIZE(msize);
    }
    if ((b = _simple_salloc()) != NULL) {
        _simple_sprintf(b, "Tiny region [%p-%p, %y] \t", (void *)start, TINY_REGION_END(region), (int)TINY_REGION_SIZE);
        _simple_sprintf(b, "Magazine=%d \t", MAGAZINE_INDEX_FOR_TINY_REGION(region));
        _simple_sprintf(b, "Allocations in use=%d \t Bytes in use=%ly \t", in_use, BYTES_USED_FOR_TINY_REGION(region));
        if (bytes_at_end)
            _simple_sprintf(b, "Untouched=%ly ", bytes_at_end);
        if (DEPOT_MAGAZINE_INDEX == MAGAZINE_INDEX_FOR_TINY_REGION(region)) {
            _simple_sprintf(b, "Advised MADV_FREE=%ly", pgTot);
        } else {
            _simple_sprintf(b, "Fragments subject to reclamation=%ly", pgTot);
        }
        if (verbose && in_use) {
            _simple_sappend(b, "\n\tSizes in use: ");
            for (ci = 0; ci < 1024; ci++)
                if (counts[ci])
                    _simple_sprintf(b, "%d[%d] ", TINY_BYTES_FOR_MSIZE(ci), counts[ci]);
        }
        _malloc_printf(MALLOC_PRINTF_NOLOG | MALLOC_PRINTF_NOPREFIX, "%s\n", _simple_string(b));
        _simple_sfree(b);
    }
}

static boolean_t
tiny_free_list_check(szone_t *szone, grain_t slot)
{
    mag_index_t mag_index;

    for (mag_index = -1; mag_index < szone->num_tiny_magazines; mag_index++) {
        magazine_t      *tiny_mag_ptr = &(szone->tiny_magazines[mag_index]);
        SZONE_MAGAZINE_PTR_LOCK(szone, tiny_mag_ptr);

        unsigned        count = 0;
        free_list_t     *ptr = szone->tiny_magazines[mag_index].mag_free_list[slot];
        boolean_t       is_free;
        free_list_t     *previous = NULL;
      
        while (ptr) {
            is_free = tiny_meta_header_is_free(ptr);
            if (! is_free) {
                malloc_printf("*** in-use ptr in free list slot=%d count=%d ptr=%p\n", slot, count, ptr);
                SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
                return 0;
            }
            if (((uintptr_t)ptr) & (TINY_QUANTUM - 1)) {
                malloc_printf("*** unaligned ptr in free list slot=%d  count=%d ptr=%p\n", slot, count, ptr);
                SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
                return 0;
            }
            if (!tiny_region_for_ptr_no_lock(szone, ptr)) {
                malloc_printf("*** ptr not in szone slot=%d  count=%d ptr=%p\n", slot, count, ptr);
                SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
                return 0;
            }
            if (free_list_unchecksum_ptr(szone, &ptr->previous) != previous) {
                malloc_printf("*** previous incorrectly set slot=%d  count=%d ptr=%p\n", slot, count, ptr);
                SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
                return 0;
            }
            previous = ptr;
            ptr = free_list_unchecksum_ptr(szone, &ptr->next);
            count++;
        }

        SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
    }
    return 1;
}

/*********************  SMALL FREE LIST UTILITIES       ************************/

/*
 * Mark a block as free.  Only the first quantum of a block is marked thusly,
 * the remainder are marked "middle".
 */
static INLINE void
small_meta_header_set_is_free(msize_t *meta_headers, unsigned index, msize_t msize)
{
    meta_headers[index] = msize | SMALL_IS_FREE;
}

/*
 * Mark a block as in use.  Only the first quantum of a block is marked thusly,
 * the remainder are marked "middle".
 */
static INLINE void
small_meta_header_set_in_use(msize_t *meta_headers, msize_t index, msize_t msize)
{
    meta_headers[index] = msize;
}

/*
 * Mark a quantum as being the second or later in a block.
 */
static INLINE void
small_meta_header_set_middle(msize_t *meta_headers, msize_t index)
{
    meta_headers[index] = 0;
}

/* 
 * Adds an item to the proper free list, and also marks the meta-header of the 
 * block properly.
 * Assumes szone has been locked
 */ 
static void
small_free_list_add_ptr(szone_t *szone, magazine_t *small_mag_ptr, void *ptr, msize_t msize)
{
    grain_t     slot = (msize <= szone->num_small_slots) ? msize - 1 : szone->num_small_slots - 1;
    free_list_t *free_ptr = ptr;
    free_list_t *free_head = small_mag_ptr->mag_free_list[slot];
    void        *follower;

#if DEBUG_MALLOC
    if (LOG(szone,ptr)) {
        malloc_printf("in %s, ptr=%p, msize=%d\n", __FUNCTION__, ptr, msize);
    }
    if (((uintptr_t)ptr) & (SMALL_QUANTUM - 1)) {
        szone_error(szone, 1, "small_free_list_add_ptr: Unaligned ptr", ptr, NULL);
    }
#endif
    small_meta_header_set_is_free(SMALL_META_HEADER_FOR_PTR(ptr), SMALL_META_INDEX_FOR_PTR(ptr), msize);

    if (free_head) {
#if DEBUG_MALLOC
        if (free_list_unchecksum_ptr(szone, &free_head->previous)) {
            szone_error(szone, 1, "small_free_list_add_ptr: Internal invariant broken (free_head->previous)", ptr,
            "ptr=%p slot=%d free_head=%p previous=%p\n", ptr, slot, (void *)free_head, free_head->previous.p);
        }
        if (!SMALL_PTR_IS_FREE(free_head)) {
            szone_error(szone, 1, "small_free_list_add_ptr: Internal invariant broken (free_head is not a free pointer)", ptr,
            "ptr=%p slot=%d free_head=%p\n", ptr, slot, (void *)free_head);
        }
#endif
        free_head->previous.u = free_list_checksum_ptr(szone, free_ptr);
    } else {
        BITMAPN_SET(small_mag_ptr->mag_bitmap, slot);
    }
    free_ptr->previous.u = free_list_checksum_ptr(szone, NULL);
    free_ptr->next.u = free_list_checksum_ptr(szone, free_head);
    
    small_mag_ptr->mag_free_list[slot] = free_ptr;

    // Store msize at the end of the block denoted by "ptr" (i.e. at a negative offset from "follower")
    follower = (void *)((uintptr_t)ptr + SMALL_BYTES_FOR_MSIZE(msize));
    SMALL_PREVIOUS_MSIZE(follower) = msize;
}

/* 
 * Removes the item pointed to by ptr in the proper free list. 
 * Assumes szone has been locked
 */ 
static void
small_free_list_remove_ptr(szone_t *szone, magazine_t *small_mag_ptr, void *ptr, msize_t msize)
{
    grain_t     slot = (msize <= szone->num_small_slots) ? msize - 1 : szone->num_small_slots - 1;
    free_list_t *free_ptr = ptr, *next, *previous;
    
    next = free_list_unchecksum_ptr(szone, &free_ptr->next);
    previous = free_list_unchecksum_ptr(szone, &free_ptr->previous);

#if DEBUG_MALLOC
    if (LOG(szone,ptr)) {
        malloc_printf("In %s, ptr=%p, msize=%d\n", __FUNCTION__, ptr, msize);
    }
#endif

    if (!previous) { 
    // The block to remove is the head of the free list
#if DEBUG_MALLOC
        if (small_mag_ptr->mag_free_list[slot] != ptr) {
            szone_error(szone, 1, "small_free_list_remove_ptr: Internal invariant broken (small_mag_ptr->mag_free_list[slot])", ptr,
              "ptr=%p slot=%d msize=%d small_mag_ptr->mag_free_list[slot]=%p\n",
              ptr, slot, msize, (void *)small_mag_ptr->mag_free_list[slot]);
            return;
        }
#endif
        small_mag_ptr->mag_free_list[slot] = next;
        if (!next) BITMAPN_CLR(small_mag_ptr->mag_bitmap, slot);
    } else {
        // We know free_ptr is already checksummed, so we don't need to do it
        // again.
        previous->next = free_ptr->next;
    }
    if (next) {
        // We know free_ptr is already checksummed, so we don't need to do it
        // again.
        next->previous = free_ptr->previous;
    }
}

/*
 * small_region_for_ptr_no_lock - Returns the small region containing the pointer,
 * or NULL if not found.
 */
static INLINE region_t
small_region_for_ptr_no_lock(szone_t *szone, const void *ptr)
{
    rgnhdl_t r = hash_lookup_region_no_lock(szone->small_region_generation->hashed_regions,
        szone->small_region_generation->num_regions_allocated,
        szone->small_region_generation->num_regions_allocated_shift,
        SMALL_REGION_FOR_PTR(ptr));
    return r ? *r : r;
}

static void
small_finalize_region(szone_t *szone, magazine_t *small_mag_ptr) {
    void      *last_block, *previous_block;
    msize_t   last_msize, previous_msize, last_index;

    last_block = SMALL_REGION_END(small_mag_ptr->mag_last_region) - small_mag_ptr->mag_bytes_free_at_end;
    last_msize = SMALL_MSIZE_FOR_BYTES(small_mag_ptr->mag_bytes_free_at_end);
    
    // It is possible that the block prior to the last block in the region has
    // been free'd, but was not coalesced with the free bytes at the end of the
    // block, since we treat the bytes at the end of the region as "in use" in
    // the meta headers. Attempt to coalesce the last block with the previous 
    // block, so we don't violate the "no consecutive free blocks" invariant.
    //
    // FIXME: If we could calculate the previous small free size in the same
    //        manner as tiny_previous_preceding_free, it would eliminate the
    //        index & previous msize checks, which are a guard against reading 
    //        bogus data out of in-use or written-on-freed memory.
    //
    // FIXME: Need to investigate how much work would be required to increase
    //        'mag_bytes_free_at_end' when freeing the preceding block, rather
    //        than performing this workaround.
    //
    last_index = SMALL_META_INDEX_FOR_PTR(last_block);
    previous_msize = SMALL_PREVIOUS_MSIZE(last_block);

    if (last_index && (previous_msize <= last_index)) {
        previous_block = (void *)((uintptr_t)last_block - SMALL_BYTES_FOR_MSIZE(previous_msize));
        if (*SMALL_METADATA_FOR_PTR(previous_block) == (previous_msize | SMALL_IS_FREE)) {
            msize_t *meta_headers = SMALL_META_HEADER_FOR_PTR(last_block);
            
            small_meta_header_set_middle(meta_headers, last_index);
            small_free_list_remove_ptr(szone, small_mag_ptr, previous_block, previous_msize);
            last_block = (void *)((uintptr_t)last_block - SMALL_BYTES_FOR_MSIZE(previous_msize));
            last_msize += previous_msize;
        }
    }
    
    // splice last_block into the free list
    small_free_list_add_ptr(szone, small_mag_ptr, last_block, last_msize);
    small_mag_ptr->mag_bytes_free_at_end = 0;
    small_mag_ptr->mag_last_region = NULL;
}

static int
small_free_detach_region(szone_t *szone, magazine_t *small_mag_ptr, region_t r) {
    unsigned char       *ptr = SMALL_REGION_ADDRESS(r);
    msize_t             *meta_headers = SMALL_META_HEADER_FOR_PTR(ptr);
    uintptr_t           start = (uintptr_t)SMALL_REGION_ADDRESS(r);
    uintptr_t           current = start;
    uintptr_t           limit =  (uintptr_t)SMALL_REGION_END(r);
    int                 total_alloc = 0;

    while (current < limit) {
        unsigned        index = SMALL_META_INDEX_FOR_PTR(current);
        msize_t         msize_and_free = meta_headers[index];
        boolean_t       is_free = msize_and_free & SMALL_IS_FREE;
        msize_t         msize = msize_and_free & ~ SMALL_IS_FREE;
        
        if (!msize) {
#if DEBUG_MALLOC
            malloc_printf("*** small_free_detach_region error with %p: msize=%d is_free =%d\n", 
            (void *)current, msize, is_free);
#endif
            break;
        }
        if (is_free) {
            small_free_list_remove_ptr(szone, small_mag_ptr, (void *)current, msize);
        } else {
            total_alloc++;
        }
        current += SMALL_BYTES_FOR_MSIZE(msize);
    }
    return total_alloc;
}

static size_t
small_free_reattach_region(szone_t *szone, magazine_t *small_mag_ptr, region_t r) {
    unsigned char       *ptr = SMALL_REGION_ADDRESS(r);
    msize_t             *meta_headers = SMALL_META_HEADER_FOR_PTR(ptr);
    uintptr_t           start = (uintptr_t)SMALL_REGION_ADDRESS(r);
    uintptr_t           current = start;
    uintptr_t           limit =  (uintptr_t)SMALL_REGION_END(r);
    size_t              total_alloc = 0;

    while (current < limit) {
        unsigned        index = SMALL_META_INDEX_FOR_PTR(current);
        msize_t         msize_and_free = meta_headers[index];
        boolean_t       is_free = msize_and_free & SMALL_IS_FREE;
        msize_t         msize = msize_and_free & ~ SMALL_IS_FREE;
        
        if (!msize) {
#if DEBUG_MALLOC
            malloc_printf("*** small_free_reattach_region error with %p: msize=%d is_free =%d\n", 
            (void *)current, msize, is_free);
#endif
            break;
        }
        if (is_free) {
            small_free_list_add_ptr(szone, small_mag_ptr, (void *)current, msize);
        } else {
            total_alloc += SMALL_BYTES_FOR_MSIZE(msize);
        }
        current += SMALL_BYTES_FOR_MSIZE(msize);
    }
    return total_alloc;
}

static void 
small_free_scan_depot_madvise_free(szone_t *szone, magazine_t *depot_ptr, region_t r) {
    uintptr_t   start = (uintptr_t)SMALL_REGION_ADDRESS(r);
    uintptr_t   current = start;
    uintptr_t   limit =  (uintptr_t)SMALL_REGION_END(r);
    msize_t     *meta_headers = SMALL_META_HEADER_FOR_PTR(start);
    boolean_t   did_advise = FALSE;

    // Scan the metadata identifying blocks which span one or more pages. Mark the pages MADV_FREE taking care to preserve free list
    // management data. 
    while (current < limit) {
        unsigned        index = SMALL_META_INDEX_FOR_PTR(current);
        msize_t         msize_and_free = meta_headers[index];
        boolean_t       is_free = msize_and_free & SMALL_IS_FREE;
        msize_t         msize = msize_and_free & ~ SMALL_IS_FREE;

        if (is_free && !msize && (current == start)) {
#if DEBUG_MALLOC
            // first block is all free
            malloc_printf("*** small_free_scan_depot_madvise_free first block is all free! %p: msize=%d is_free =%d\n", 
            (void *)current, msize, is_free);
#endif      
            uintptr_t pgLo = round_page(start + sizeof(free_list_t) + sizeof(msize_t));
            uintptr_t pgHi = trunc_page(start + SMALL_REGION_SIZE - sizeof(msize_t));
            
            if (pgLo < pgHi) {
                madvise_free_range(szone, r, pgLo, pgHi);
                did_advise = TRUE;
            }
            break;
        }
        if (!msize) {
#if DEBUG_MALLOC
            malloc_printf("*** small_free_scan_depot_madvise_free error with %p: msize=%d is_free =%d\n", 
            (void *)current, msize, is_free);
#endif
            break;
        }
        if (is_free) {
            uintptr_t pgLo = round_page(current + sizeof(free_list_t) + sizeof(msize_t));
            uintptr_t pgHi = trunc_page(current + SMALL_BYTES_FOR_MSIZE(msize) - sizeof(msize_t));
            
            if (pgLo < pgHi) {
                madvise_free_range(szone, r, pgLo, pgHi);
                did_advise = TRUE;
            }
        }
        current += SMALL_BYTES_FOR_MSIZE(msize);
    }
    
    if (did_advise) {
        /* Move the node to the tail of the Deopt's recirculation list to delay its re-use. */
        region_trailer_t *node = REGION_TRAILER_FOR_SMALL_REGION(r);
        recirc_list_extract(szone, depot_ptr, node); // excise node from list
        recirc_list_splice_last(szone, depot_ptr, node); // connect to magazine as last node
    }
}

static void
small_free_try_depot_unmap_no_lock(szone_t *szone, magazine_t *depot_ptr, region_trailer_t *node)
{
#warning Tune Depot headroom    
    if (0 < node->bytes_used || 
        depot_ptr->recirculation_entries < (szone->num_small_magazines * 2)) {
        return;
    }
    
    // disconnect first node from Depot
    recirc_list_extract(szone, depot_ptr, node);
    
    // Iterate the region pulling its free entries off the (locked) Depot's free list
    region_t sparse_region = SMALL_REGION_FOR_PTR(node);
    int objects_in_use = small_free_detach_region(szone, depot_ptr, sparse_region);
    
    if (0 == objects_in_use) {
        // Invalidate the hash table entry for this region with HASHRING_REGION_DEALLOCATED.
        // Using HASHRING_REGION_DEALLOCATED preserves the collision chain, using HASHRING_OPEN_ENTRY (0) would not.
        rgnhdl_t pSlot = hash_lookup_region_no_lock(szone->small_region_generation->hashed_regions, 
                                     szone->small_region_generation->num_regions_allocated, 
                                     szone->small_region_generation->num_regions_allocated_shift, sparse_region); 
        *pSlot = HASHRING_REGION_DEALLOCATED;
        depot_ptr->num_bytes_in_magazine -= SMALL_REGION_PAYLOAD_BYTES;
#if ((__GNUC__ > 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ >= 1))) /* GCC 4.1 and forward supports atomic builtins */
        __sync_fetch_and_add( &(szone->num_small_regions_dealloc), 1); // Atomically increment num_small_regions_dealloc
#else
#ifdef __LP64__
        OSAtomicIncrement64( (volatile int64_t *)&(szone->num_small_regions_dealloc) );
#else
        OSAtomicIncrement32( (volatile int32_t *)&(szone->num_small_regions_dealloc) );
#endif
#endif
        
        // Transfer ownership of the region back to the OS
        SZONE_MAGAZINE_PTR_UNLOCK(szone, depot_ptr); // Avoid denial of service to Depot while in kernel
        deallocate_pages(szone, sparse_region, SMALL_REGION_SIZE, 0);
        SZONE_MAGAZINE_PTR_LOCK(szone, depot_ptr);

        MAGMALLOC_DEALLOCREGION((void *)szone, (void *)sparse_region); // DTrace USDT Probe
        
    } else {
        szone_error(szone, 1, "small_free_try_depot_unmap_no_lock objects_in_use not zero:", NULL, "%d\n", objects_in_use);
    }
}

static void
small_free_do_recirc_to_depot(szone_t *szone, magazine_t *small_mag_ptr, mag_index_t mag_index)
{
        // The entire magazine crossed the "emptiness threshold". Transfer a region
        // from this magazine to the Depot. Choose a region that itself has crossed the emptiness threshold (i.e
        // is at least fraction "f" empty.) Such a region will be marked "suitable" on the recirculation list.
        region_trailer_t *node = small_mag_ptr->firstNode;
        
        while (node && !node->recirc_suitable) {
            node = node->next;
        }
        
        if (NULL == node) {
#if DEBUG_MALLOC
            malloc_printf("*** small_free_do_recirc_to_depot end of list\n");
#endif
            return;
        }
        
        region_t sparse_region = SMALL_REGION_FOR_PTR(node);

        // Deal with unclaimed memory -- mag_bytes_free_at_end
        if (sparse_region == small_mag_ptr->mag_last_region && small_mag_ptr->mag_bytes_free_at_end) {
            small_finalize_region(szone, small_mag_ptr);
        }
        
        // disconnect first node from magazine
        recirc_list_extract(szone, small_mag_ptr, node);

        // Iterate the region pulling its free entries off its (locked) magazine's free list
        int objects_in_use = small_free_detach_region(szone, small_mag_ptr, sparse_region);
        magazine_t *depot_ptr = &(szone->small_magazines[DEPOT_MAGAZINE_INDEX]);
        
        // hand over the region to the (locked) Depot
        SZONE_MAGAZINE_PTR_LOCK(szone,depot_ptr);
        // this will cause small_free_list_add_ptr called by small_free_reattach_region to use 
        // the depot as its target magazine, rather than magazine formerly associated with sparse_region
        MAGAZINE_INDEX_FOR_SMALL_REGION(sparse_region) = DEPOT_MAGAZINE_INDEX;

        // Iterate the region putting its free entries on Depot's free list
        size_t bytes_inplay = small_free_reattach_region(szone, depot_ptr, sparse_region);
        
        small_mag_ptr->mag_num_bytes_in_objects -= bytes_inplay;
        small_mag_ptr->num_bytes_in_magazine -= SMALL_REGION_PAYLOAD_BYTES;
        small_mag_ptr->mag_num_objects -= objects_in_use;
        
        depot_ptr->mag_num_bytes_in_objects += bytes_inplay;
        depot_ptr->num_bytes_in_magazine += SMALL_REGION_PAYLOAD_BYTES;
        depot_ptr->mag_num_objects += objects_in_use;
        
        // connect to Depot as first node
        recirc_list_splice_first(szone, depot_ptr, node);

        MAGMALLOC_RECIRCREGION((void *)szone, (int)mag_index, (int)BYTES_USED_FOR_SMALL_REGION(sparse_region)); // DTrace USDT Probe

        // Mark free'd dirty pages with MADV_FREE to reduce memory pressure
        small_free_scan_depot_madvise_free(szone, depot_ptr, sparse_region);

        // If the region is entirely empty vm_deallocate() it
        small_free_try_depot_unmap_no_lock(szone, depot_ptr, node);
        
        SZONE_MAGAZINE_PTR_UNLOCK(szone,depot_ptr);
}    

static boolean_t
small_get_region_from_depot(szone_t *szone, magazine_t *small_mag_ptr, mag_index_t mag_index)
{
    magazine_t *depot_ptr = &(szone->small_magazines[DEPOT_MAGAZINE_INDEX]);

    /* FIXME: Would Uniprocessor benefit from recirc and MADV_FREE? */
    if (szone->num_small_magazines == 1) // Uniprocessor, single magazine, so no recirculation necessary
        return 0;
        
#if DEBUG_MALLOC
    if (DEPOT_MAGAZINE_INDEX == mag_index) {
        szone_error(szone, 1, "small_get_region_from_depot called for magazine index -1", NULL, NULL);
        return 0;
    }
#endif
        
    SZONE_MAGAZINE_PTR_LOCK(szone,depot_ptr);

    // Appropriate one of the Depot's regions. Prefer LIFO selection for best cache utilization.
    region_trailer_t *node = depot_ptr->firstNode;
        
    if (NULL == node) { // Depot empty?
        SZONE_MAGAZINE_PTR_UNLOCK(szone,depot_ptr);
        return 0;
    }
    
    // disconnect first node from Depot
    recirc_list_extract(szone, depot_ptr, node);
    
    // Iterate the region pulling its free entries off the (locked) Depot's free list
    region_t sparse_region = SMALL_REGION_FOR_PTR(node);
    int objects_in_use = small_free_detach_region(szone, depot_ptr, sparse_region);

    // Transfer ownership of the region
    MAGAZINE_INDEX_FOR_SMALL_REGION(sparse_region) = mag_index;
    
    // Iterate the region putting its free entries on its new (locked) magazine's free list
    size_t bytes_inplay = small_free_reattach_region(szone, small_mag_ptr, sparse_region);
    
    depot_ptr->mag_num_bytes_in_objects -= bytes_inplay;
    depot_ptr->num_bytes_in_magazine -= SMALL_REGION_PAYLOAD_BYTES;
    depot_ptr->mag_num_objects -= objects_in_use;
    
    small_mag_ptr->mag_num_bytes_in_objects += bytes_inplay;
    small_mag_ptr->num_bytes_in_magazine += SMALL_REGION_PAYLOAD_BYTES;
    small_mag_ptr->mag_num_objects += objects_in_use;
    
    // connect to magazine as first node (it's maximally sparse at this moment)
    recirc_list_splice_first(szone, small_mag_ptr, node);
        
    SZONE_MAGAZINE_PTR_UNLOCK(szone,depot_ptr);

    MAGMALLOC_DEPOTREGION((void *)szone, (int)mag_index, (int)BYTES_USED_FOR_SMALL_REGION(sparse_region)); // DTrace USDT Probe
        
    if (-1 == madvise((void *)sparse_region, SMALL_REGION_PAYLOAD_BYTES, MADV_FREE_REUSE)) {
        /* -1 return: VM map entry change makes this unfit for reuse. Something evil lurks. */
#if DEBUG_MALLOC
        szone_error(szone, 1, "small_get_region_from_depot madvise(..., MADV_FREE_REUSE) failed", sparse_region, NULL);
#endif
        return 0;
    }

    return 1;
}

#warning Tune K and f!
#define K 1.5 // headroom measured in number of 8Mb regions
#define DENSITY_THRESHOLD(a) \
    ((a) - ((a) >> 2)) // "Emptiness" f = 0.25, so "Density" is (1 - f)*a. Generally: ((a) - ((a) >> -log2(f))) 

static INLINE void
small_free_no_lock(szone_t *szone, magazine_t *small_mag_ptr, mag_index_t mag_index, region_t region, void *ptr, msize_t msize)
{
    msize_t             *meta_headers = SMALL_META_HEADER_FOR_PTR(ptr);
    unsigned            index = SMALL_META_INDEX_FOR_PTR(ptr);
    void                *original_ptr = ptr;
    size_t              original_size = SMALL_BYTES_FOR_MSIZE(msize);
    unsigned char       *next_block = ((unsigned char *)ptr + original_size);
    msize_t             next_index = index + msize;
    msize_t             previous_msize, next_msize;
    void                *previous;
    boolean_t           did_prepend = FALSE;
    boolean_t           did_append = FALSE;

#if DEBUG_MALLOC
    if (LOG(szone,ptr)) {
        malloc_printf("in small_free_no_lock(), ptr=%p, msize=%d\n", ptr, msize);
    }
    if (! msize) {
        szone_error(szone, 1, "trying to free small block that is too small", ptr,
            "in small_free_no_lock(), ptr=%p, msize=%d\n", ptr, msize);
    }
#endif

    // We try to coalesce this block with the preceeding one
    if (index && (SMALL_PREVIOUS_MSIZE(ptr) <= index)) {
        previous_msize = SMALL_PREVIOUS_MSIZE(ptr);
        if (meta_headers[index - previous_msize] == (previous_msize | SMALL_IS_FREE)) {
            previous = (void *)((uintptr_t)ptr - SMALL_BYTES_FOR_MSIZE(previous_msize));
            // previous is really to be coalesced
            did_prepend = TRUE;
#if DEBUG_MALLOC
            if (LOG(szone, ptr) || LOG(szone,previous)) { 
                malloc_printf("in small_free_no_lock(), coalesced backwards for %p previous=%p\n", ptr, previous);
            }
#endif
            small_free_list_remove_ptr(szone, small_mag_ptr, previous, previous_msize);
            small_meta_header_set_middle(meta_headers, index);
            ptr = previous;
            msize += previous_msize;
            index -= previous_msize;
        }
    }
    // We try to coalesce with the next block
    if ((next_block < SMALL_REGION_END(region)) && (meta_headers[next_index] & SMALL_IS_FREE)) {
        // next block is free, we coalesce
        did_append = TRUE;
        next_msize = meta_headers[next_index] & ~ SMALL_IS_FREE;
#if DEBUG_MALLOC
        if (LOG(szone,ptr)) 
            malloc_printf("In small_free_no_lock(), for ptr=%p, msize=%d coalesced next block=%p next_msize=%d\n",
                          ptr, msize, next_block, next_msize);
#endif
        small_free_list_remove_ptr(szone, small_mag_ptr, next_block, next_msize);
        small_meta_header_set_middle(meta_headers, next_index);
        msize += next_msize;
    }
    if (szone->debug_flags & SCALABLE_MALLOC_DO_SCRIBBLE) {
        if (!msize) {
            szone_error(szone, 1, "incorrect size information - block header was damaged", ptr, NULL);
        } else {
            memset(ptr, 0x55, SMALL_BYTES_FOR_MSIZE(msize));
        }
    }
    small_free_list_add_ptr(szone, small_mag_ptr, ptr, msize);
    small_mag_ptr->mag_num_objects--;
    // we use original_size and not msize to avoid double counting the coalesced blocks
    small_mag_ptr->mag_num_bytes_in_objects -= original_size;
    
    // Update this region's bytes in use count
    region_trailer_t *node = REGION_TRAILER_FOR_SMALL_REGION(region);
    size_t bytes_used = node->bytes_used - original_size;
    node->bytes_used = bytes_used;
    
    /* FIXME: Would Uniprocessor benefit from recirc and MADV_FREE? */
    if (szone->num_small_magazines == 1) { // Uniprocessor, single magazine, so no recirculation necessary
        /* NOTHING */
    } else if (DEPOT_MAGAZINE_INDEX != mag_index) {
        // Emptiness discriminant
        if (bytes_used < DENSITY_THRESHOLD(SMALL_REGION_PAYLOAD_BYTES)) {
            /* Region has crossed threshold from density to sparsity. Mark it "suitable" on the
               recirculation candidates list. */
            node->recirc_suitable = TRUE;
        } else {
            /* After this free, we've found the region is still dense, so it must have been even more so before
               the free. That implies the region is already correctly marked. Do nothing. */
        }
    
        // Has the entire magazine crossed the "emptiness threshold"? If so, transfer a region
        // from this magazine to the Depot. Choose a region that itself has crossed the emptiness threshold (i.e
        // is at least fraction "f" empty.) Such a region will be marked "suitable" on the recirculation list.

        size_t a = small_mag_ptr->num_bytes_in_magazine; // Total bytes allocated to this magazine
        size_t u = small_mag_ptr->mag_num_bytes_in_objects; // In use (malloc'd) from this magaqzine

        if (a - u > ((3 * SMALL_REGION_PAYLOAD_BYTES) / 2) && u < DENSITY_THRESHOLD(a))
            small_free_do_recirc_to_depot(szone, small_mag_ptr, mag_index);

    } else {
        // Freed to Depot. N.B. Lock on small_magazines[DEPOT_MAGAZINE_INDEX] is already held
        uintptr_t safe_ptr = (uintptr_t)ptr + sizeof(free_list_t) + sizeof(msize_t);
        uintptr_t round_safe = round_page(safe_ptr);
        
        uintptr_t safe_extent = (uintptr_t)ptr + SMALL_BYTES_FOR_MSIZE(msize) - sizeof(msize_t);
        uintptr_t trunc_extent = trunc_page(safe_extent);
                
        // The newly freed block may complete a span of bytes that cover a page. Mark it with MADV_FREE.
        if (round_safe < trunc_extent) { // Safe area covers a page (perhaps many)
            if (did_prepend & did_append) { // Coalesced preceding with original_ptr *and* with following
                uintptr_t trunc_safe_prev = trunc_page((uintptr_t)original_ptr - sizeof(msize_t));
                uintptr_t rnd_safe_follow = 
                    round_page((uintptr_t)original_ptr + original_size + sizeof(free_list_t) + sizeof(msize_t));
                
                madvise_free_range(szone, region, MAX(round_safe, trunc_safe_prev), MIN(rnd_safe_follow, trunc_extent));
            } else if (did_prepend) { // Coalesced preceding with original_ptr
                uintptr_t trunc_safe_prev = trunc_page((uintptr_t)original_ptr - sizeof(msize_t));
                
                madvise_free_range(szone, region, MAX(round_safe, trunc_safe_prev), trunc_extent);
            } else if (did_append) { // Coalesced original_ptr with following
                uintptr_t rnd_safe_follow = 
                    round_page((uintptr_t)original_ptr + original_size + sizeof(free_list_t) + sizeof(msize_t));
                
                madvise_free_range(szone, region, round_safe, MIN(rnd_safe_follow, trunc_extent));
            } else // Isolated free
                madvise_free_range(szone, region, round_safe, trunc_extent);
        }

        if (0 < bytes_used) {
            /* Depot'd region is still live. Leave it in place on the Depot's recirculation list
               so as to avoid thrashing between the Depot's free list and a magazines's free list
               with detach_region/reattach_region */
        } else {
            /* Depot'd region is just now empty. Consider return to OS. */
            region_trailer_t *node = REGION_TRAILER_FOR_SMALL_REGION(region);
            magazine_t *depot_ptr = &(szone->small_magazines[DEPOT_MAGAZINE_INDEX]);
            small_free_try_depot_unmap_no_lock(szone, depot_ptr, node);
        }
    }
}

// Allocates from the last region or a freshly allocated region
static void *
small_malloc_from_region_no_lock(szone_t *szone, magazine_t *small_mag_ptr, mag_index_t mag_index, msize_t msize)
{
    void        *ptr, *aligned_address;

    // Before anything we transform the mag_bytes_free_at_end - if any - to a regular free block
    /* FIXME: last_block needs to be coalesced with previous entry if free, <rdar://5462322> */
    if (small_mag_ptr->mag_bytes_free_at_end)
        small_finalize_region(szone, small_mag_ptr);

    // time to create a new region
    aligned_address = allocate_pages(szone, SMALL_REGION_SIZE, SMALL_BLOCKS_ALIGN, 0, VM_MEMORY_MALLOC_SMALL);
    if (!aligned_address)
        return NULL;

    MAGMALLOC_ALLOCREGION((void *)szone, (int)mag_index); // DTrace USDT Probe
        
    // Here find the only place in smallville that (infrequently) takes the small_regions_lock.
    // Only one thread at a time should be permitted to assess the density of the hash
    // ring and adjust if needed. 
    // Only one thread at a time should be permitted to insert its new region on
    // the hash ring.
    // It is safe for all other threads to read the hash ring (hashed_regions) and
    // the associated sizes (num_regions_allocated and num_small_regions).
    
    LOCK(szone->small_regions_lock);
    // Check to see if the hash ring of small regions needs to grow.  Try to
    // avoid the hash ring becoming too dense.
    if (szone->small_region_generation->num_regions_allocated < (2 * szone->num_small_regions)) {
        region_t *new_regions;
        size_t new_size;
        size_t new_shift = szone->small_region_generation->num_regions_allocated_shift; // In/Out parameter
        new_regions = hash_regions_grow_no_lock(szone, szone->small_region_generation->hashed_regions,
                                              szone->small_region_generation->num_regions_allocated,
                                              &new_shift,
                                              &new_size);
        // Do not deallocate the current hashed_regions allocation since someone
        // may be iterating it.  Instead, just leak it.

        // Prepare to advance to the "next generation" of the hash ring.
        szone->small_region_generation->nextgen->hashed_regions = new_regions;
        szone->small_region_generation->nextgen->num_regions_allocated = new_size;
        szone->small_region_generation->nextgen->num_regions_allocated_shift = new_shift;

        // Throw the switch to atomically advance to the next generation.
        szone->small_region_generation = szone->small_region_generation->nextgen;
        // Ensure everyone sees the advance.
#if ((__GNUC__ > 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ >= 1))) /* GCC 4.1 and forward supports atomic builtins */
        __sync_synchronize();
#else
        OSMemoryBarrier();
#endif
    }
    // Tag the region at "aligned_address" as belonging to us, 
    // and so put it under the protection of the magazine lock we are holding.
    // Do this before advertising "aligned_address" on the hash ring(!)
    MAGAZINE_INDEX_FOR_SMALL_REGION(aligned_address) = mag_index;

    // Insert the new region into the hash ring, and update malloc statistics
    hash_region_insert_no_lock(szone->small_region_generation->hashed_regions, 
                               szone->small_region_generation->num_regions_allocated,
                               szone->small_region_generation->num_regions_allocated_shift,
                               aligned_address);
                               
    szone->num_small_regions++;

    UNLOCK(szone->small_regions_lock);

    small_mag_ptr->mag_last_region = aligned_address;
    BYTES_USED_FOR_SMALL_REGION(aligned_address) = SMALL_BYTES_FOR_MSIZE(msize);
    ptr = aligned_address;
    small_meta_header_set_in_use(SMALL_META_HEADER_FOR_PTR(ptr), 0, msize);
    small_mag_ptr->mag_num_objects++;
    small_mag_ptr->mag_num_bytes_in_objects += SMALL_BYTES_FOR_MSIZE(msize);
    small_mag_ptr->num_bytes_in_magazine += SMALL_REGION_PAYLOAD_BYTES;
    
    // add a big free block
    small_meta_header_set_in_use(SMALL_META_HEADER_FOR_PTR(ptr) , msize, NUM_SMALL_BLOCKS - msize);    
    small_mag_ptr->mag_bytes_free_at_end = SMALL_BYTES_FOR_MSIZE(NUM_SMALL_BLOCKS - msize);

    // connect to magazine as first node (it's maximally sparse at this moment)
    recirc_list_splice_first(szone, small_mag_ptr, REGION_TRAILER_FOR_SMALL_REGION(aligned_address));

    return ptr;
}

static INLINE boolean_t
small_try_realloc_in_place(szone_t *szone, void *ptr, size_t old_size, size_t new_size)
{
    // returns 1 on success
    msize_t     *meta_headers = SMALL_META_HEADER_FOR_PTR(ptr);
    unsigned    index;
    msize_t     old_msize, new_msize;
    unsigned    next_index;
    void        *next_block;
    msize_t     next_msize_and_free;
    boolean_t   is_free;
    msize_t     next_msize, leftover_msize;
    void        *leftover;

    index = SMALL_META_INDEX_FOR_PTR(ptr);
    old_msize = SMALL_MSIZE_FOR_BYTES(old_size);
    new_msize = SMALL_MSIZE_FOR_BYTES(new_size + SMALL_QUANTUM - 1);
    next_index = index + old_msize;
    
    if (next_index >= NUM_SMALL_BLOCKS) {
        return 0;
    }
    next_block = (char *)ptr + old_size;
    
#if DEBUG_MALLOC
    if ((uintptr_t)next_block & (SMALL_QUANTUM - 1)) {
        szone_error(szone, 1, "internal invariant broken in realloc(next_block)", next_block, NULL);
    }
    if (meta_headers[index] != old_msize)
        malloc_printf("*** small_try_realloc_in_place incorrect old %d %d\n",
          meta_headers[index], old_msize);
#endif

    magazine_t  *small_mag_ptr = mag_lock_zine_for_region_trailer(szone, szone->small_magazines,
                                    REGION_TRAILER_FOR_SMALL_REGION(SMALL_REGION_FOR_PTR(ptr)), 
                                    MAGAZINE_INDEX_FOR_SMALL_REGION(SMALL_REGION_FOR_PTR(ptr)));    

    /*
     * Look for a free block immediately afterwards.  If it's large enough, we can consume (part of)
     * it.
     */
    next_msize_and_free = meta_headers[next_index];
    is_free = next_msize_and_free & SMALL_IS_FREE;
    if (!is_free) {
        SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
        return 0; // next_block is in use;
    }
    next_msize = next_msize_and_free & ~ SMALL_IS_FREE;
    if (old_msize + next_msize < new_msize) {
        SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
        return 0; // even with next block, not enough
    }
    /*
     * The following block is big enough; pull it from its freelist and chop off enough to satisfy
     * our needs.
     */
    small_free_list_remove_ptr(szone, small_mag_ptr, next_block, next_msize);
    small_meta_header_set_middle(meta_headers, next_index);
    leftover_msize = old_msize + next_msize - new_msize;
    if (leftover_msize) {
        /* there's some left, so put the remainder back */
        leftover = (unsigned char *)ptr + SMALL_BYTES_FOR_MSIZE(new_msize);
        
        small_free_list_add_ptr(szone, small_mag_ptr, leftover, leftover_msize);
    }
#if DEBUG_MALLOC
    if (SMALL_BYTES_FOR_MSIZE(new_msize) > szone->large_threshold) {
        malloc_printf("*** realloc in place for %p exceeded msize=%d\n", new_msize);
    }
#endif
    small_meta_header_set_in_use(meta_headers, index, new_msize);
#if DEBUG_MALLOC
    if (LOG(szone,ptr)) {
        malloc_printf("in szone_realloc(), ptr=%p, msize=%d\n", ptr, *SMALL_METADATA_FOR_PTR(ptr));
    }
#endif
    small_mag_ptr->mag_num_bytes_in_objects += SMALL_BYTES_FOR_MSIZE(new_msize - old_msize);

    // Update this region's bytes in use count
    region_trailer_t *node = REGION_TRAILER_FOR_SMALL_REGION(SMALL_REGION_FOR_PTR(ptr));
    size_t bytes_used = node->bytes_used + SMALL_BYTES_FOR_MSIZE(new_msize - old_msize);
    node->bytes_used = bytes_used;

    // Emptiness discriminant
    if (bytes_used < DENSITY_THRESHOLD(SMALL_REGION_PAYLOAD_BYTES)) {
        /* After this reallocation the region is still sparse, so it must have been even more so before
           the reallocation. That implies the region is already correctly marked. Do nothing. */
    } else {
        /* Region has crossed threshold from sparsity to density. Mark it not "suitable" on the
           recirculation candidates list. */
        node->recirc_suitable = FALSE;
   }

    SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
    CHECK(szone, __PRETTY_FUNCTION__);
    return 1;
}

static boolean_t
small_check_region(szone_t *szone, region_t region)
{
    unsigned char       *ptr = SMALL_REGION_ADDRESS(region);
    msize_t             *meta_headers = SMALL_META_HEADER_FOR_PTR(ptr);
    unsigned char       *region_end = SMALL_REGION_END(region);
    msize_t             prev_free = 0;
    unsigned            index;
    msize_t             msize_and_free;
    msize_t             msize;
    free_list_t         *free_head;
    void                *previous, *next;
    msize_t             *follower;
    mag_index_t         mag_index = MAGAZINE_INDEX_FOR_SMALL_REGION(SMALL_REGION_FOR_PTR(ptr));
    magazine_t          *small_mag_ptr = &(szone->small_magazines[mag_index]);

    // Assumes locked
    CHECK_MAGAZINE_PTR_LOCKED(szone, small_mag_ptr, __PRETTY_FUNCTION__);

    if (region == small_mag_ptr->mag_last_region) 
        region_end -= small_mag_ptr->mag_bytes_free_at_end;
        
    while (ptr < region_end) {
        index = SMALL_META_INDEX_FOR_PTR(ptr);
        msize_and_free = meta_headers[index];
        if (!(msize_and_free & SMALL_IS_FREE)) {
            // block is in use
            msize = msize_and_free;
            if (!msize) {
                malloc_printf("*** invariant broken: null msize ptr=%p num_small_regions=%d end=%p\n",
                  ptr, szone->num_small_regions, region_end);
                return 0;
            }
            if (SMALL_BYTES_FOR_MSIZE(msize) > szone->large_threshold) {
                malloc_printf("*** invariant broken for %p this small msize=%d - size is too large\n",
                  ptr, msize_and_free);
                return 0;
            }
            ptr += SMALL_BYTES_FOR_MSIZE(msize);
            prev_free = 0;
        } else {
            // free pointer
            msize = msize_and_free & ~ SMALL_IS_FREE;
            free_head = (free_list_t *)ptr;
            follower = (msize_t *)FOLLOWING_SMALL_PTR(ptr, msize);
            if (!msize) {
                malloc_printf("*** invariant broken for free block %p this msize=%d\n", ptr, msize);
                return 0;
            }
            if (prev_free) {
                malloc_printf("*** invariant broken for %p (2 free in a row)\n", ptr);
                return 0;
            }
            previous = free_list_unchecksum_ptr(szone, &free_head->previous);
            next = free_list_unchecksum_ptr(szone, &free_head->next);
            if (previous && !SMALL_PTR_IS_FREE(previous)) {
                malloc_printf("*** invariant broken for %p (previous %p is not a free pointer)\n",
                  ptr, free_head->previous);
                return 0;
            }
            if (next && !SMALL_PTR_IS_FREE(next)) {
                malloc_printf("*** invariant broken for %p (next is not a free pointer)\n", ptr);
                return 0;
            }
            if (SMALL_PREVIOUS_MSIZE(follower) != msize) {
                malloc_printf("*** invariant broken for small free %p followed by %p in region [%p-%p] "
                  "(end marker incorrect) should be %d; in fact %d\n",
                  ptr, follower, SMALL_REGION_ADDRESS(region), region_end, msize, SMALL_PREVIOUS_MSIZE(follower));
                return 0;
            }
            ptr = (unsigned char *)follower;
            prev_free = SMALL_IS_FREE;
        }
    }
    return 1;
}

static kern_return_t
small_in_use_enumerator(task_t task, void *context, unsigned type_mask, szone_t *szone, 
                        memory_reader_t reader, vm_range_recorder_t recorder)
{
    size_t              num_regions;
    size_t              index;
    region_t            *regions;
    vm_range_t          buffer[MAX_RECORDER_BUFFER];
    unsigned            count = 0;
    kern_return_t       err;
    region_t            region;
    vm_range_t          range;
    vm_range_t          admin_range;
    vm_range_t          ptr_range;
    unsigned char       *mapped_region;
    msize_t             *block_header;
    unsigned            block_index;
    unsigned            block_limit;
    msize_t             msize_and_free;
    msize_t             msize;
    vm_address_t        mag_last_free_ptr = 0;
    msize_t             mag_last_free_msize = 0;

    region_hash_generation_t *srg_ptr;
    err = reader(task, (vm_address_t)szone->small_region_generation, sizeof(region_hash_generation_t), (void **)&srg_ptr);
    if (err) return err;

    num_regions = srg_ptr->num_regions_allocated;
    err = reader(task, (vm_address_t)srg_ptr->hashed_regions, sizeof(region_t) * num_regions, (void **)&regions);
    if (err) return err;

    for (index = 0; index < num_regions; ++index) {
        region = regions[index];
        if (HASHRING_OPEN_ENTRY != region && HASHRING_REGION_DEALLOCATED != region) {
            range.address = (vm_address_t)SMALL_REGION_ADDRESS(region);
            range.size = SMALL_REGION_SIZE;
            if (type_mask & MALLOC_ADMIN_REGION_RANGE_TYPE) {
                admin_range.address = range.address + SMALL_METADATA_START;
                admin_range.size = SMALL_METADATA_SIZE;
                recorder(task, context, MALLOC_ADMIN_REGION_RANGE_TYPE, &admin_range, 1);
            }
            if (type_mask & (MALLOC_PTR_REGION_RANGE_TYPE | MALLOC_ADMIN_REGION_RANGE_TYPE)) {
                ptr_range.address = range.address;
                ptr_range.size = NUM_SMALL_BLOCKS * SMALL_QUANTUM;
                recorder(task, context, MALLOC_PTR_REGION_RANGE_TYPE, &ptr_range, 1);
            }
            if (type_mask & MALLOC_PTR_IN_USE_RANGE_TYPE) {
                err = reader(task, range.address, range.size, (void **)&mapped_region);
                if (err) 
                    return err;
                    
                mag_index_t mag_index = MAGAZINE_INDEX_FOR_SMALL_REGION(mapped_region);
                magazine_t *small_mag_ptr;
                err = reader(task, (vm_address_t)&(szone->small_magazines[mag_index]), sizeof(magazine_t), 
                        (void **)&small_mag_ptr);
                if (err) return err;

                void *mag_last_free = small_mag_ptr->mag_last_free;
                if (mag_last_free) {
                    mag_last_free_ptr = (uintptr_t) mag_last_free & ~(SMALL_QUANTUM - 1);
                    mag_last_free_msize = (uintptr_t) mag_last_free & (SMALL_QUANTUM - 1);
                }

                block_header = (msize_t *)(mapped_region + SMALL_METADATA_START + sizeof(region_trailer_t));
                block_index = 0;
                block_limit = NUM_SMALL_BLOCKS;
                if (region == small_mag_ptr->mag_last_region)
                    block_limit -= SMALL_MSIZE_FOR_BYTES(small_mag_ptr->mag_bytes_free_at_end);
                while (block_index < block_limit) {
                    msize_and_free = block_header[block_index];
                    msize = msize_and_free & ~ SMALL_IS_FREE;
                    if (! (msize_and_free & SMALL_IS_FREE) &&
                        range.address + SMALL_BYTES_FOR_MSIZE(block_index) != mag_last_free_ptr) {
                        // Block in use
                        buffer[count].address = range.address + SMALL_BYTES_FOR_MSIZE(block_index);
                        buffer[count].size = SMALL_BYTES_FOR_MSIZE(msize);
                        count++;
                        if (count >= MAX_RECORDER_BUFFER) {
                            recorder(task, context, MALLOC_PTR_IN_USE_RANGE_TYPE, buffer, count);
                            count = 0;
                        }
                    }
                    block_index += msize;
                }
                if (count) {
                    recorder(task, context, MALLOC_PTR_IN_USE_RANGE_TYPE, buffer, count);
                    count = 0;
                }
            }
        }
    }
    return 0;
}

static void *
small_malloc_from_free_list(szone_t *szone, magazine_t *small_mag_ptr, mag_index_t mag_index, msize_t msize)
{
    free_list_t         *ptr;
    msize_t             this_msize;
    grain_t             slot = (msize <= szone->num_small_slots) ? msize - 1 : szone->num_small_slots - 1;
    free_list_t         **free_list = small_mag_ptr->mag_free_list;
    free_list_t         **the_slot = free_list + slot;
    free_list_t         *next;
    free_list_t         **limit;
    unsigned            bitmap;
    msize_t             leftover_msize;
    free_list_t         *leftover_ptr;

    // Assumes we've locked the region
    CHECK_MAGAZINE_PTR_LOCKED(szone, small_mag_ptr, __PRETTY_FUNCTION__);

    // Look for an exact match by checking the freelist for this msize.
    // 
    ptr = *the_slot;
    if (ptr) {
        next = free_list_unchecksum_ptr(szone, &ptr->next);
        if (next) {
            next->previous = ptr->previous;
        } else {
            BITMAPN_CLR(small_mag_ptr->mag_bitmap, slot);
        }
        *the_slot = next;
        this_msize = msize;
        goto return_small_alloc;
    }

    // Mask off the bits representing slots holding free blocks smaller than
    // the size we need.  If there are no larger free blocks, try allocating
    // from the free space at the end of the small region.
    if (szone->is_largemem) {
        // BITMAPN_CTZ implementation
        unsigned idx = slot >> 5;
        bitmap = 0;
        unsigned mask = ~ ((1 << (slot & 31)) - 1);
        for ( ; idx < SMALL_BITMAP_WORDS; ++idx ) {
            bitmap = small_mag_ptr->mag_bitmap[idx] & mask;
            if (bitmap != 0)
                break;
            mask = ~0U;
        }
        // Check for fallthrough: No bits set in bitmap
        if ((bitmap == 0) && (idx == SMALL_BITMAP_WORDS))
            goto try_small_from_end;

        // Start looking at the first set bit, plus 32 bits for every word of
        // zeroes or entries that were too small.
        slot = BITMAP32_CTZ((&bitmap)) + (idx * 32);
    } else {
        bitmap = small_mag_ptr->mag_bitmap[0] & ~ ((1 << slot) - 1);
        if (!bitmap) 
            goto try_small_from_end;

        slot = BITMAP32_CTZ((&bitmap));
    }
    // FIXME: Explain use of - 1 here, last slot has special meaning
    limit = free_list + szone->num_small_slots - 1;
    free_list += slot;

    if (free_list < limit) {
        ptr = *free_list;
        if (ptr) {

            next = free_list_unchecksum_ptr(szone, &ptr->next);
            *free_list = next;
            if (next) {
                next->previous = ptr->previous;
            } else {
                BITMAPN_CLR(small_mag_ptr->mag_bitmap, slot);
            }
            this_msize = SMALL_PTR_SIZE(ptr);
            goto add_leftover_and_proceed;
        }
#if DEBUG_MALLOC
        malloc_printf("in small_malloc_from_free_list(), mag_bitmap out of sync, slot=%d\n",slot);
#endif
    }

    // We are now looking at the last slot, which contains blocks equal to, or
    // due to coalescing of free blocks, larger than (num_small_slots - 1) * (small quantum size).
    // If the last freelist is not empty, and the head contains a block that is
    // larger than our request, then the remainder is put back on the free list.
    //
    ptr = *limit;
    if (ptr) {
        this_msize = SMALL_PTR_SIZE(ptr);
        next = free_list_unchecksum_ptr(szone, &ptr->next);
        if (this_msize - msize >= szone->num_small_slots) {
            // the leftover will go back to the free list, so we optimize by
            // modifying the free list rather than a pop and push of the head
            leftover_msize = this_msize - msize;
            leftover_ptr = (free_list_t *)((unsigned char *)ptr + SMALL_BYTES_FOR_MSIZE(msize));
            *limit = leftover_ptr;
            if (next) {
                next->previous.u = free_list_checksum_ptr(szone, leftover_ptr);
            }
            leftover_ptr->previous = ptr->previous;
            leftover_ptr->next = ptr->next;
            small_meta_header_set_is_free(SMALL_META_HEADER_FOR_PTR(leftover_ptr),
                                          SMALL_META_INDEX_FOR_PTR(leftover_ptr), leftover_msize);
            // Store msize at the end of the block denoted by "leftover_ptr" (i.e. at a negative offset from follower)
            SMALL_PREVIOUS_MSIZE(FOLLOWING_SMALL_PTR(leftover_ptr, leftover_msize)) = leftover_msize; // Access is safe
#if DEBUG_MALLOC
            if (LOG(szone,ptr)) {
            malloc_printf("in small_malloc_from_free_list(), last slot ptr=%p, msize=%d this_msize=%d\n", ptr, msize, this_msize);
            }
#endif
            this_msize = msize;
            goto return_small_alloc;
        }
        if (next) {
            next->previous = ptr->previous;
        }
        *limit = next;
        goto add_leftover_and_proceed;
    }
    
try_small_from_end:
    // Let's see if we can use small_mag_ptr->mag_bytes_free_at_end
    if (small_mag_ptr->mag_bytes_free_at_end >= SMALL_BYTES_FOR_MSIZE(msize)) {
        ptr = (free_list_t *)(SMALL_REGION_END(small_mag_ptr->mag_last_region) -
                              small_mag_ptr->mag_bytes_free_at_end);
        small_mag_ptr->mag_bytes_free_at_end -= SMALL_BYTES_FOR_MSIZE(msize);
        if (small_mag_ptr->mag_bytes_free_at_end) {
            // let's mark this block as in use to serve as boundary
            small_meta_header_set_in_use(SMALL_META_HEADER_FOR_PTR(ptr),
                                         SMALL_META_INDEX_FOR_PTR((unsigned char *)ptr + SMALL_BYTES_FOR_MSIZE(msize)),
                                         SMALL_MSIZE_FOR_BYTES(small_mag_ptr->mag_bytes_free_at_end));
        }
        this_msize = msize;
        goto return_small_alloc;
    }
    return NULL;
    
add_leftover_and_proceed:
    if (this_msize > msize) {
        leftover_msize = this_msize - msize;
        leftover_ptr = (free_list_t *)((unsigned char *)ptr + SMALL_BYTES_FOR_MSIZE(msize));
#if DEBUG_MALLOC
        if (LOG(szone,ptr)) {
            malloc_printf("in small_malloc_from_free_list(), adding leftover ptr=%p, this_msize=%d\n", ptr, this_msize);
        }
#endif
        small_free_list_add_ptr(szone, small_mag_ptr, leftover_ptr, leftover_msize);
        this_msize = msize;
    }
    
return_small_alloc:
    small_mag_ptr->mag_num_objects++;
    small_mag_ptr->mag_num_bytes_in_objects += SMALL_BYTES_FOR_MSIZE(this_msize);
    
    // Update this region's bytes in use count
    region_trailer_t *node = REGION_TRAILER_FOR_SMALL_REGION(SMALL_REGION_FOR_PTR(ptr));
    size_t bytes_used = node->bytes_used + SMALL_BYTES_FOR_MSIZE(this_msize);
    node->bytes_used = bytes_used;

    // Emptiness discriminant
    if (bytes_used < DENSITY_THRESHOLD(SMALL_REGION_PAYLOAD_BYTES)) {
        /* After this allocation the region is still sparse, so it must have been even more so before
           the allocation. That implies the region is already correctly marked. Do nothing. */
    } else {
        /* Region has crossed threshold from sparsity to density. Mark in not "suitable" on the
           recirculation candidates list. */
        node->recirc_suitable = FALSE;
   }
#if DEBUG_MALLOC
    if (LOG(szone,ptr)) {
        malloc_printf("in small_malloc_from_free_list(), ptr=%p, this_msize=%d, msize=%d\n", ptr, this_msize, msize);
    }
#endif
    small_meta_header_set_in_use(SMALL_META_HEADER_FOR_PTR(ptr), SMALL_META_INDEX_FOR_PTR(ptr), this_msize);
    return ptr;
}
#undef DENSITY_THRESHOLD
#undef K

static INLINE void *
small_malloc_should_clear(szone_t *szone, msize_t msize, boolean_t cleared_requested)
{
    void        *ptr;
    mag_index_t mag_index = mag_get_thread_index(szone);
    magazine_t  *small_mag_ptr = &(szone->small_magazines[mag_index]);
    
    SZONE_MAGAZINE_PTR_LOCK(szone, small_mag_ptr);
    
#if SMALL_CACHE
    ptr = (void *)small_mag_ptr->mag_last_free;
    
    if ((((uintptr_t)ptr) & (SMALL_QUANTUM - 1)) == msize) {
        // we have a winner
        small_mag_ptr->mag_last_free = NULL;
        SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
        CHECK(szone, __PRETTY_FUNCTION__);
        ptr = (void *)((uintptr_t)ptr & ~ (SMALL_QUANTUM - 1));
        if (cleared_requested) {
            memset(ptr, 0, SMALL_BYTES_FOR_MSIZE(msize));
        }
        return ptr;
    }
#endif /* SMALL_CACHE */

    ptr = small_malloc_from_free_list(szone, small_mag_ptr, mag_index, msize);
    if (ptr) {
        SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
        CHECK(szone, __PRETTY_FUNCTION__);
        if (cleared_requested) {
            memset(ptr, 0, SMALL_BYTES_FOR_MSIZE(msize));
        }
        return ptr;
    }
    
    if (small_get_region_from_depot(szone, small_mag_ptr, mag_index)) {
        ptr = small_malloc_from_free_list(szone, small_mag_ptr, mag_index, msize);
        if (ptr) {
            SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
            CHECK(szone, __PRETTY_FUNCTION__);
            if (cleared_requested) {
                memset(ptr, 0, SMALL_BYTES_FOR_MSIZE(msize));
            }
            return ptr;
        }
    }

    ptr = small_malloc_from_region_no_lock(szone, small_mag_ptr, mag_index, msize);
    // we don't clear because this freshly allocated space is pristine
    SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
    CHECK(szone, __PRETTY_FUNCTION__);
    return ptr;
}

static NOINLINE void
free_small_botch(szone_t *szone, free_list_t *ptr)
{
    mag_index_t mag_index = MAGAZINE_INDEX_FOR_SMALL_REGION(SMALL_REGION_FOR_PTR(ptr));
    magazine_t  *small_mag_ptr = &(szone->small_magazines[mag_index]);
    SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
    szone_error(szone, 1, "double free", ptr, NULL);
}

static INLINE void
free_small(szone_t *szone, void *ptr, region_t small_region, size_t known_size)
{
    msize_t     msize;
    mag_index_t mag_index = MAGAZINE_INDEX_FOR_SMALL_REGION(SMALL_REGION_FOR_PTR(ptr));
    magazine_t  *small_mag_ptr = &(szone->small_magazines[mag_index]);
    
    // ptr is known to be in small_region
    if (known_size) {
        msize = SMALL_MSIZE_FOR_BYTES(known_size + SMALL_QUANTUM - 1);
    } else {
        msize = SMALL_PTR_SIZE(ptr);
        if (SMALL_PTR_IS_FREE(ptr)) {
            free_small_botch(szone, ptr);
            return;
        }
    }
    
    SZONE_MAGAZINE_PTR_LOCK(szone, small_mag_ptr);

#if SMALL_CACHE
    // Depot does not participate in SMALL_CACHE since it can't be directly malloc()'d
    if (DEPOT_MAGAZINE_INDEX != mag_index) {
        
        void *ptr2 = small_mag_ptr->mag_last_free; // Might be NULL
        region_t rgn2 = small_mag_ptr->mag_last_free_rgn;
        
        /* check that we don't already have this pointer in the cache */
        if (ptr == (void *)((uintptr_t)ptr2 & ~ (SMALL_QUANTUM - 1))) {
            free_small_botch(szone, ptr);
            return;
        }

        if ((szone->debug_flags & SCALABLE_MALLOC_DO_SCRIBBLE) && msize)
            memset(ptr, 0x55, SMALL_BYTES_FOR_MSIZE(msize));

        small_mag_ptr->mag_last_free = (void *)(((uintptr_t)ptr) | msize);
        small_mag_ptr->mag_last_free_rgn = small_region;

        if (!ptr2) {
            SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
            CHECK(szone, __PRETTY_FUNCTION__);
            return;
        }

        msize = (uintptr_t)ptr2 & (SMALL_QUANTUM - 1);
        ptr = (void *)(((uintptr_t)ptr2) & ~(SMALL_QUANTUM - 1));
        small_region = rgn2;
    }
#endif /* SMALL_CACHE */

    // Now in the time it took to acquire the lock, the region may have migrated
    // from one magazine to another. I.e. trailer->mag_index is volatile.
    // In which case the magazine lock we obtained (namely magazines[mag_index].mag_lock) 
    // is stale. If so, keep on tryin' ...
    region_trailer_t *trailer = REGION_TRAILER_FOR_SMALL_REGION(small_region);
    mag_index_t refreshed_index;
    
    while (mag_index != (refreshed_index = trailer->mag_index)) { // Note assignment
    
        SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
        
        mag_index = refreshed_index;
        small_mag_ptr = &(szone->small_magazines[mag_index]);
        SZONE_MAGAZINE_PTR_LOCK(szone, small_mag_ptr);
    }
                            
    small_free_no_lock(szone, small_mag_ptr, mag_index, small_region, ptr, msize);
    SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
    CHECK(szone, __PRETTY_FUNCTION__);
}

static void
print_small_free_list(szone_t *szone)
{
    free_list_t         *ptr;
    _SIMPLE_STRING      b = _simple_salloc();
    mag_index_t mag_index;
    
    if (b) {
        _simple_sappend(b, "small free sizes:\n");
        for (mag_index = -1; mag_index < szone->num_small_magazines; mag_index++) {
            grain_t     slot = 0;
            _simple_sprintf(b,"\tMagazine %d: ", mag_index);
            while (slot < szone->num_small_slots) {
                ptr = szone->small_magazines[mag_index].mag_free_list[slot];
                if (ptr) {
                    _simple_sprintf(b, "%s%y[%d]; ", (slot == szone->num_small_slots-1) ? ">=" : "", 
                                (slot + 1) * SMALL_QUANTUM, free_list_count(szone, ptr));
                }
                slot++;
            }
            _simple_sappend(b,"\n");
        }
        _malloc_printf(MALLOC_PRINTF_NOLOG | MALLOC_PRINTF_NOPREFIX, "%s\n", _simple_string(b));
        _simple_sfree(b);
    }
}

static void
print_small_region(szone_t *szone, boolean_t verbose, region_t region, size_t bytes_at_end)
{
    unsigned            counts[1024];
    unsigned            in_use = 0;
    uintptr_t           start = (uintptr_t)SMALL_REGION_ADDRESS(region);
    uintptr_t           current = start;
    uintptr_t           limit = (uintptr_t)SMALL_REGION_END(region) - bytes_at_end;
    msize_t             msize_and_free;
    msize_t             msize;
    unsigned            ci;
    _SIMPLE_STRING      b;
    uintptr_t           pgTot = 0;

    if (region == HASHRING_REGION_DEALLOCATED) {
        if ((b = _simple_salloc()) != NULL) {
            _simple_sprintf(b, "Small region [unknown address] was returned to the OS\n");
            _malloc_printf(MALLOC_PRINTF_NOLOG | MALLOC_PRINTF_NOPREFIX, "%s\n", _simple_string(b));
            _simple_sfree(b);
        }
        return;
    }

    memset(counts, 0, sizeof(counts));
    while (current < limit) {
        msize_and_free = *SMALL_METADATA_FOR_PTR(current);
        msize = msize_and_free & ~ SMALL_IS_FREE;
        if (!msize) {
            malloc_printf("*** error with %p: msize=%d\n", (void *)current, (unsigned)msize);
            break;
        }
        if (!(msize_and_free & SMALL_IS_FREE)) {
            // block in use
            if (msize < 1024)
                counts[msize]++;
            in_use++;
        } else {
            uintptr_t pgLo = round_page(current + sizeof(free_list_t) + sizeof(msize_t));
            uintptr_t pgHi = trunc_page(current + TINY_BYTES_FOR_MSIZE(msize) - sizeof(msize_t));
            
            if (pgLo < pgHi) {
                pgTot += (pgHi - pgLo);
            }
        }
        current += SMALL_BYTES_FOR_MSIZE(msize);
    }
    if ((b = _simple_salloc()) != NULL) {
        _simple_sprintf(b, "Small region [%p-%p, %y] \t", (void *)start, SMALL_REGION_END(region), (int)SMALL_REGION_SIZE);
        _simple_sprintf(b, "Magazine=%d \t", MAGAZINE_INDEX_FOR_SMALL_REGION(region));
        _simple_sprintf(b, "Allocations in use=%d \t Bytes in use=%ly \t", in_use, BYTES_USED_FOR_SMALL_REGION(region));
        if (bytes_at_end)
            _simple_sprintf(b, "Untouched=%ly ", bytes_at_end);
        if (DEPOT_MAGAZINE_INDEX == MAGAZINE_INDEX_FOR_SMALL_REGION(region)) {
            _simple_sprintf(b, "Advised MADV_FREE=%ly", pgTot);
        } else {
            _simple_sprintf(b, "Fragments subject to reclamation=%ly", pgTot);
        }
        if (verbose && in_use) {
            _simple_sappend(b, "\n\tSizes in use: "); 
            for (ci = 0; ci < 1024; ci++)
                if (counts[ci])
                    _simple_sprintf(b, "%d[%d] ", SMALL_BYTES_FOR_MSIZE(ci), counts[ci]);
        }
        _malloc_printf(MALLOC_PRINTF_NOLOG | MALLOC_PRINTF_NOPREFIX, "%s\n", _simple_string(b));
        _simple_sfree(b);
    }
}

static boolean_t
small_free_list_check(szone_t *szone, grain_t slot)
{
    mag_index_t mag_index;

    for (mag_index = -1; mag_index < szone->num_small_magazines; mag_index++) {
        magazine_t      *small_mag_ptr = &(szone->small_magazines[mag_index]);
        SZONE_MAGAZINE_PTR_LOCK(szone, small_mag_ptr);

        unsigned        count = 0;
        free_list_t     *ptr = szone->small_magazines[mag_index].mag_free_list[slot];
        msize_t         msize_and_free;
        free_list_t     *previous = NULL;
      
        while (ptr) {
            msize_and_free = *SMALL_METADATA_FOR_PTR(ptr);
            if (!(msize_and_free & SMALL_IS_FREE)) {
                malloc_printf("*** in-use ptr in free list slot=%d count=%d ptr=%p\n", slot, count, ptr);
                SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
                return 0;
            }
            if (((uintptr_t)ptr) & (SMALL_QUANTUM - 1)) {
                malloc_printf("*** unaligned ptr in free list slot=%d  count=%d ptr=%p\n", slot, count, ptr);
                SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
                return 0;
            }
            if (!small_region_for_ptr_no_lock(szone, ptr)) {
                malloc_printf("*** ptr not in szone slot=%d  count=%d ptr=%p\n", slot, count, ptr);
                SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
                return 0;
            }
            if (free_list_unchecksum_ptr(szone, &ptr->previous) != previous) {
                malloc_printf("*** previous incorrectly set slot=%d  count=%d ptr=%p\n", slot, count, ptr);
                SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
                return 0;
            }
            previous = ptr;
            ptr = free_list_unchecksum_ptr(szone, &ptr->next);
            count++;
        }
    
        SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
    }
    return 1;
}

/*******************************************************************************
 * Large allocator implementation
 ******************************************************************************/
#pragma mark large allocator

#if DEBUG_MALLOC

static void
large_debug_print(szone_t *szone)
{
    unsigned            index;
    large_entry_t       *range;
    _SIMPLE_STRING      b = _simple_salloc();

    if (b) {
        for (index = 0, range = szone->large_entries; index < szone->num_large_entries; index++, range++)
            if (range->address)
                _simple_sprintf(b, "%d: %p(%y);  ", index, range->address, range->size);

        _malloc_printf(MALLOC_PRINTF_NOLOG | MALLOC_PRINTF_NOPREFIX, "%s\n", _simple_string(b));
        _simple_sfree(b);
    }
}
#endif

/*
 * Scan the hash ring looking for an entry for the given pointer.
 */
static large_entry_t *
large_entry_for_pointer_no_lock(szone_t *szone, const void *ptr)
{
    // result only valid with lock held
    unsigned            num_large_entries = szone->num_large_entries;
    unsigned            hash_index;
    unsigned            index;
    large_entry_t       *range;

    if (!num_large_entries)
        return NULL;
        
    hash_index = ((uintptr_t)ptr >> vm_page_shift) % num_large_entries;
    index = hash_index;
    
    do {
        range = szone->large_entries + index;
        if (range->address == (vm_address_t)ptr)
            return range;
        if (0 == range->address)
            return NULL; // end of chain
        index++;
        if (index == num_large_entries)
            index = 0;
    } while (index != hash_index);
    
    return NULL;
}

static void
large_entry_insert_no_lock(szone_t *szone, large_entry_t range)
{
    unsigned            num_large_entries = szone->num_large_entries;
    unsigned            hash_index = (((uintptr_t)(range.address)) >> vm_page_shift) % num_large_entries;
    unsigned            index = hash_index;
    large_entry_t       *entry;

    // assert(szone->num_large_objects_in_use < szone->num_large_entries); /* must be called with room to spare */
    
    do {
        entry = szone->large_entries + index;
        if (0 == entry->address) {
            *entry = range;
            return; // end of chain
        }
        index++;
        if (index == num_large_entries)
            index = 0;
    } while (index != hash_index);
    
    // assert(0); /* must not fallthrough! */
}

// FIXME: can't we simply swap the (now empty) entry with the last entry on the collision chain for this hash slot?
static INLINE void
large_entries_rehash_after_entry_no_lock(szone_t *szone, large_entry_t *entry)
{
    unsigned            num_large_entries = szone->num_large_entries;
    unsigned            hash_index = entry - szone->large_entries;
    unsigned            index = hash_index;
    large_entry_t       range;

    // assert(entry->address == 0) /* caller must have cleared *entry */
    
    do {
        index++;
        if (index == num_large_entries)
            index = 0;
        range = szone->large_entries[index];
        if (0 == range.address)
            return;
        szone->large_entries[index].address = (vm_address_t)0;
        szone->large_entries[index].size = 0;
        szone->large_entries[index].did_madvise_reusable = FALSE;
        large_entry_insert_no_lock(szone, range); // this will reinsert in the
                                                  // proper place
    } while (index != hash_index);
    
    // assert(0); /* since entry->address == 0, must not fallthrough! */
}

// FIXME: num should probably be a size_t, since you can theoretically allocate
// more than 2^32-1 large_threshold objects in 64 bit.
static INLINE large_entry_t *
large_entries_alloc_no_lock(szone_t *szone, unsigned num)
{
    size_t      size = num * sizeof(large_entry_t);
    
    // Note that we allocate memory (via a system call) under a spin lock
    // That is certainly evil, however it's very rare in the lifetime of a process
    // The alternative would slow down the normal case
    return allocate_pages(szone, round_page(size), 0, 0, VM_MEMORY_MALLOC_LARGE);
}

static void
large_entries_free_no_lock(szone_t *szone, large_entry_t *entries, unsigned num, vm_range_t *range_to_deallocate)
{
    size_t      size = num * sizeof(large_entry_t);

    range_to_deallocate->address = (vm_address_t)entries;
    range_to_deallocate->size = round_page(size);
}

static large_entry_t *
large_entries_grow_no_lock(szone_t *szone, vm_range_t *range_to_deallocate)
{
    // sets range_to_deallocate
    unsigned            old_num_entries = szone->num_large_entries;
    large_entry_t       *old_entries = szone->large_entries;
    // always an odd number for good hashing
    unsigned            new_num_entries = (old_num_entries) ? old_num_entries * 2 + 1 : 
                                                              ((vm_page_size / sizeof(large_entry_t)) - 1);
    large_entry_t       *new_entries = large_entries_alloc_no_lock(szone, new_num_entries);
    unsigned            index = old_num_entries;
    large_entry_t       oldRange;

    // if the allocation of new entries failed, bail
    if (new_entries == NULL)
        return NULL;

    szone->num_large_entries = new_num_entries;
    szone->large_entries = new_entries;

    /* rehash entries into the new list */
    while (index--) {
        oldRange = old_entries[index];
        if (oldRange.address) {
            large_entry_insert_no_lock(szone, oldRange);
        }
    }
    
    if (old_entries) {
        large_entries_free_no_lock(szone, old_entries, old_num_entries, range_to_deallocate);
    } else {
        range_to_deallocate->address = (vm_address_t)0;
        range_to_deallocate->size = 0;
    }
    
    return new_entries;
}

// frees the specific entry in the size table
// returns a range to truly deallocate
static vm_range_t
large_entry_free_no_lock(szone_t *szone, large_entry_t *entry)
{
    vm_range_t          range;
    
    range.address = entry->address;
    range.size = entry->size;
    
    if (szone->debug_flags & SCALABLE_MALLOC_ADD_GUARD_PAGES) {
        protect((void *)range.address, range.size, VM_PROT_READ | VM_PROT_WRITE, szone->debug_flags);
        range.address -= vm_page_size;
        range.size += 2 * vm_page_size;
    }
    
    entry->address = 0;
    entry->size = 0;
    entry->did_madvise_reusable = FALSE;
    large_entries_rehash_after_entry_no_lock(szone, entry);
    
#if DEBUG_MALLOC
    if (large_entry_for_pointer_no_lock(szone, (void *)range.address)) {
        malloc_printf("*** freed entry %p still in use; num_large_entries=%d\n",
          range.address, szone->num_large_entries);
        large_debug_print(szone);
        szone_sleep();
    }
#endif
    return range;
}

static NOINLINE kern_return_t
large_in_use_enumerator(task_t task, void *context, unsigned type_mask, vm_address_t large_entries_address,
                        unsigned num_entries, memory_reader_t reader, vm_range_recorder_t recorder)
{
    unsigned            index = 0;
    vm_range_t          buffer[MAX_RECORDER_BUFFER];
    unsigned            count = 0;
    large_entry_t       *entries;
    kern_return_t       err;
    vm_range_t          range;
    large_entry_t       entry;

    err = reader(task, large_entries_address, sizeof(large_entry_t) * num_entries, (void **)&entries);
    if (err)
        return err;
        
    index = num_entries;
    if (type_mask & MALLOC_ADMIN_REGION_RANGE_TYPE) {
        range.address = large_entries_address;
        range.size = round_page(num_entries * sizeof(large_entry_t));
        recorder(task, context, MALLOC_ADMIN_REGION_RANGE_TYPE, &range, 1);
    }
    if (type_mask & (MALLOC_PTR_IN_USE_RANGE_TYPE | MALLOC_PTR_REGION_RANGE_TYPE)) {
        while (index--) {
            entry = entries[index];
            if (entry.address) {
                range.address = entry.address;
                range.size = entry.size;
                buffer[count++] = range;
                if (count >= MAX_RECORDER_BUFFER) {
                    recorder(task, context, MALLOC_PTR_IN_USE_RANGE_TYPE | MALLOC_PTR_REGION_RANGE_TYPE,
                             buffer, count);
                    count = 0;
                }
            }
        }
    }
    if (count) {
        recorder(task, context, MALLOC_PTR_IN_USE_RANGE_TYPE | MALLOC_PTR_REGION_RANGE_TYPE, 
                 buffer, count);
    }
    return 0;
}

static void *
large_malloc(szone_t *szone, size_t num_pages, unsigned char alignment,
        boolean_t cleared_requested)
{
    void                *addr;
    vm_range_t          range_to_deallocate;
    size_t              size;
    large_entry_t       large_entry;
    
    if (!num_pages)
        num_pages = 1; // minimal allocation size for this szone
    size = (size_t)num_pages << vm_page_shift;
    range_to_deallocate.size = 0;
    range_to_deallocate.address = 0;
    
#if LARGE_CACHE
    if (size < LARGE_CACHE_SIZE_ENTRY_LIMIT) { // Look for a large_entry_t on the death-row cache?    
    SZONE_LOCK(szone);

    int i, best = -1, idx = szone->large_entry_cache_newest, stop_idx = szone->large_entry_cache_oldest;
    size_t best_size = SIZE_T_MAX;
    
    while (1) { // Scan large_entry_cache for best fit, starting with most recent entry
        size_t this_size = szone->large_entry_cache[idx].size;
        
        if (size == this_size) { // size match!
            best = idx;
            best_size = this_size;
            break;
        }
        
        if (size <= this_size && this_size < best_size) { // improved fit?
            best = idx;
            best_size = this_size;
        }
        
        if (idx == stop_idx) // exhausted live ring?
            break;
            
        if (idx)
            idx--; // bump idx down
        else
            idx = LARGE_ENTRY_CACHE_SIZE - 1; // wrap idx
    }
    
    if (best > -1 && (best_size - size) < size) { //limit fragmentation to 50%
        addr = (void *)szone->large_entry_cache[best].address;
        boolean_t was_madvised_reusable = szone->large_entry_cache[best].did_madvise_reusable;
        
        // Compact live ring to fill entry now vacated at large_entry_cache[best]
        // while preserving time-order
        if (szone->large_entry_cache_oldest < szone->large_entry_cache_newest) {
        
            // Ring hasn't wrapped. Fill in from right.
            for (i = best; i < szone->large_entry_cache_newest; ++i) 
                szone->large_entry_cache[i] = szone->large_entry_cache[i + 1];
                
            szone->large_entry_cache_newest--; // Pull in right endpoint. 
            
        } else if (szone->large_entry_cache_newest < szone->large_entry_cache_oldest) {
        
            // Ring has wrapped. Arrange to fill in from the contiguous side.
            if (best <= szone->large_entry_cache_newest) {
                // Fill from right.
                for (i = best; i < szone->large_entry_cache_newest; ++i)
                    szone->large_entry_cache[i] = szone->large_entry_cache[i + 1];
                    
                if (0 < szone->large_entry_cache_newest)
                    szone->large_entry_cache_newest--;
                else
                    szone->large_entry_cache_newest = LARGE_ENTRY_CACHE_SIZE - 1;
            } else {
                // Fill from left.
                for ( i = best; i > szone->large_entry_cache_oldest; --i)
                    szone->large_entry_cache[i] = szone->large_entry_cache[i - 1];
                    
                if (szone->large_entry_cache_oldest < LARGE_ENTRY_CACHE_SIZE - 1)
                    szone->large_entry_cache_oldest++;
                else
                    szone->large_entry_cache_oldest = 0;
            }
            
        } else {
            // By trichotomy, large_entry_cache_newest == large_entry_cache_oldest.
            // That implies best == large_entry_cache_newest == large_entry_cache_oldest
            // and the ring is now empty.
            szone->large_entry_cache[best].address = 0;
            szone->large_entry_cache[best].size = 0;
            szone->large_entry_cache[best].did_madvise_reusable = FALSE;
        }
        
        if ((szone->num_large_objects_in_use + 1) * 4 > szone->num_large_entries) {
            // density of hash table too high; grow table
            // we do that under lock to avoid a race
            large_entry_t *entries = large_entries_grow_no_lock(szone, &range_to_deallocate);
            if (entries == NULL) {
                SZONE_UNLOCK(szone);
                return NULL;
            }
        }
        
        large_entry.address = (vm_address_t)addr;
        large_entry.size = best_size;
        large_entry.did_madvise_reusable = FALSE; 
        large_entry_insert_no_lock(szone, large_entry);

        szone->num_large_objects_in_use ++;
        szone->num_bytes_in_large_objects += best_size;
        if (!was_madvised_reusable)
            szone->large_entry_cache_hoard_bytes -= best_size;
        SZONE_UNLOCK(szone);
        
        if (range_to_deallocate.size) {
            // we deallocate outside the lock
            deallocate_pages(szone, (void *)range_to_deallocate.address, range_to_deallocate.size, 0);
        }

        // Perform the madvise() outside the lock.
        // Typically the madvise() is successful and we'll quickly return from this routine.
        // In the unusual case of failure, reacquire the lock to unwind.
        if (was_madvised_reusable && -1 == madvise(addr, size, MADV_FREE_REUSE)) {
            /* -1 return: VM map entry change makes this unfit for reuse. */
#if DEBUG_MALLOC
            szone_error(szone, 1, "large_malloc madvise(..., MADV_FREE_REUSE) failed", addr, NULL);
#endif
            
            SZONE_LOCK(szone);
            szone->num_large_objects_in_use--;
            szone->num_bytes_in_large_objects -= large_entry.size;
    
            // Re-acquire "entry" after interval just above where we let go the lock.
            large_entry_t *entry = large_entry_for_pointer_no_lock(szone, addr);
            if (NULL == entry) {
                szone_error(szone, 1, "entry for pointer being discarded from death-row vanished", addr, NULL);
                SZONE_UNLOCK(szone);
            } else {
            
                    range_to_deallocate = large_entry_free_no_lock(szone, entry);
                    SZONE_UNLOCK(szone);
                    
                    if (range_to_deallocate.size) {
                        // we deallocate outside the lock
                        deallocate_pages(szone, (void *)range_to_deallocate.address, range_to_deallocate.size, 0);
                    }
            }
            /* Fall through to allocate_pages() afresh. */
        } else {
            if (cleared_requested) {
                memset(addr, 0, size);
            }
        
            return addr;
        }
    } else {
        SZONE_UNLOCK(szone);
    }
    }

    range_to_deallocate.size = 0;
    range_to_deallocate.address = 0;
#endif /* LARGE_CACHE */

    addr = allocate_pages(szone, size, alignment, szone->debug_flags, VM_MEMORY_MALLOC_LARGE);
    if (addr == NULL) {
        return NULL;
    }

    SZONE_LOCK(szone);
    if ((szone->num_large_objects_in_use + 1) * 4 > szone->num_large_entries) {
        // density of hash table too high; grow table
        // we do that under lock to avoid a race
        large_entry_t *entries = large_entries_grow_no_lock(szone, &range_to_deallocate);
        if (entries == NULL) {
            SZONE_UNLOCK(szone);
            return NULL;
        }
    }

    large_entry.address = (vm_address_t)addr;
    large_entry.size = size;
    large_entry.did_madvise_reusable = FALSE;
    large_entry_insert_no_lock(szone, large_entry);

    szone->num_large_objects_in_use ++;
    szone->num_bytes_in_large_objects += size;
    SZONE_UNLOCK(szone);
    
    if (range_to_deallocate.size) {
        // we deallocate outside the lock
        deallocate_pages(szone, (void *)range_to_deallocate.address, range_to_deallocate.size, 0);
    }
    return addr;
}

static NOINLINE void
free_large(szone_t *szone, void *ptr)
{
    // We have established ptr is page-aligned and neither tiny nor small
    large_entry_t       *entry;
    vm_range_t          vm_range_to_deallocate;
    
    SZONE_LOCK(szone);
    entry = large_entry_for_pointer_no_lock(szone, ptr);
    if (entry) {
#if LARGE_CACHE
#ifndef MADV_CAN_REUSE
#define MADV_CAN_REUSE 9 /* per Francois, for testing until xnu is resubmitted to B&I */
#endif
        if (entry->size < LARGE_CACHE_SIZE_ENTRY_LIMIT && 
            -1 != madvise((void *)(entry->address), entry->size, MADV_CAN_REUSE)) { // Put the large_entry_t on the death-row cache?    
            int idx = szone->large_entry_cache_newest, stop_idx = szone->large_entry_cache_oldest;
            large_entry_t this_entry = *entry; // Make a local copy, "entry" is volatile when lock is let go.
            boolean_t reusable = TRUE;
            boolean_t should_madvise = szone->large_entry_cache_hoard_bytes + this_entry.size > szone->large_entry_cache_hoard_lmit;
    
            // Already freed?
            // [Note that repeated entries in death-row risk vending the same entry subsequently
            // to two different malloc() calls. By checking here the (illegal) double free
            // is accommodated, matching the behavior of the previous implementation.]
            while (1) { // Scan large_entry_cache starting with most recent entry
                if (szone->large_entry_cache[idx].address == entry->address) {
                    szone_error(szone, 1, "pointer being freed already on death-row", ptr, NULL);
                    SZONE_UNLOCK(szone);
                    return;
                }
        
                if (idx == stop_idx) // exhausted live ring?
                    break;
            
                if (idx)
                    idx--; // bump idx down
                else
                    idx = LARGE_ENTRY_CACHE_SIZE - 1; // wrap idx
            }

            SZONE_UNLOCK(szone);
            
            if (szone->debug_flags & SCALABLE_MALLOC_PURGEABLE) { // Are we a purgable zone?
                int state = VM_PURGABLE_NONVOLATILE; // restore to default condition

                if (KERN_SUCCESS != vm_purgable_control(mach_task_self(), this_entry.address, VM_PURGABLE_SET_STATE, &state)) {
                    malloc_printf("*** can't vm_purgable_control(..., VM_PURGABLE_SET_STATE) for large freed block at %p\n", this_entry.address);
                    reusable = FALSE;
                }
            }

            if (szone->large_legacy_reset_mprotect) { // Linked for Leopard?
                // Accomodate Leopard apps that (illegally) mprotect() their own guard pages on large malloc'd allocations
                kern_return_t err = vm_protect(mach_task_self(), (vm_address_t)(this_entry.address), this_entry.size,
                                0, PROT_READ | PROT_WRITE);
                if (err) {
                    malloc_printf("*** can't reset protection for large freed block at %p\n", this_entry.address);
                    reusable = FALSE;
                }
            }
            
            // madvise(..., MADV_REUSABLE) death-row arrivals if hoarding would exceed large_entry_cache_hoard_lmit 
            if (should_madvise) {
                // Issue madvise to avoid paging out the dirtied free()'d pages in "entry"
                MAGMALLOC_MADVFREEREGION((void *)szone, (void *)0, (void *)(this_entry.address), this_entry.size); // DTrace USDT Probe

                if (-1 == madvise((void *)(this_entry.address), this_entry.size, MADV_FREE_REUSABLE)) {
                    /* -1 return: VM map entry change makes this unfit for reuse. */
#if DEBUG_MALLOC
                    szone_error(szone, 1, "free_large madvise(..., MADV_FREE_REUSABLE) failed", (void *)this_entry.address, NULL);
#endif
                    reusable = FALSE;
                }
            }
            
            SZONE_LOCK(szone);

            // Re-acquire "entry" after interval just above where we let go the lock.
            entry = large_entry_for_pointer_no_lock(szone, ptr);
            if (NULL == entry) {
                szone_error(szone, 1, "entry for pointer being freed from death-row vanished", ptr, NULL);
                SZONE_UNLOCK(szone);
                return;
            }

            // Add "entry" to death-row ring
            if (reusable) {
                int idx = szone->large_entry_cache_newest; // Most recently occupied
                vm_address_t addr;
                size_t adjsize;
                
                if (szone->large_entry_cache_newest == szone->large_entry_cache_oldest &&
                    0 == szone->large_entry_cache[idx].address) {
                    // Ring is empty, idx is good as it stands
                    addr = 0;
                    adjsize = 0;
                } else {
                    // Extend the queue to the "right" by bumping up large_entry_cache_newest
                    if (idx == LARGE_ENTRY_CACHE_SIZE - 1)
                        idx = 0; // Wrap index
                    else
                        idx++; // Bump index
                            
                    if (idx == szone->large_entry_cache_oldest) { // Fully occupied
                        // Drop this entry from the cache and deallocate the VM
                        addr = szone->large_entry_cache[idx].address;
                        adjsize = szone->large_entry_cache[idx].size;
                        if (!szone->large_entry_cache[idx].did_madvise_reusable)
                            szone->large_entry_cache_hoard_bytes -= adjsize;
                    } else {
                        // Using an unoccupied cache slot
                        addr = 0;
                        adjsize = 0;
                    }
                }

                if ((szone->debug_flags & SCALABLE_MALLOC_DO_SCRIBBLE))
                    memset((void *)(entry->address), 0x55, entry->size);
                        
                entry->did_madvise_reusable = should_madvise; // Was madvise()'d above?
                if (!should_madvise) // Entered on death-row without madvise() => up the hoard total
                    szone->large_entry_cache_hoard_bytes += entry->size;
                
                szone->large_entry_cache[idx] = *entry;
                szone->large_entry_cache_newest = idx;
                
                szone->num_large_objects_in_use--;
                szone->num_bytes_in_large_objects -= entry->size;
                
                (void)large_entry_free_no_lock(szone, entry);
                    
                if (0 == addr) {
                    SZONE_UNLOCK(szone);
                    return;
                }
                    
                // Fall through to drop large_entry_cache_oldest from the cache,
                // and then deallocate its pages.

                // Trim the queue on the "left" by bumping up large_entry_cache_oldest
                if (szone->large_entry_cache_oldest == LARGE_ENTRY_CACHE_SIZE - 1)
                    szone->large_entry_cache_oldest = 0;
                else
                    szone->large_entry_cache_oldest++;
                    
                // we deallocate_pages, including guard pages, outside the lock
                SZONE_UNLOCK(szone);
                deallocate_pages(szone, (void *)addr, (size_t)adjsize, 0);
                return;
            } else {
                /* fall through to discard an allocation that is not reusable */
            }
        }
#endif /* LARGE_CACHE */

        szone->num_large_objects_in_use--;
        szone->num_bytes_in_large_objects -= entry->size;
    
        vm_range_to_deallocate = large_entry_free_no_lock(szone, entry);
    } else {
#if DEBUG_MALLOC
        large_debug_print(szone);
#endif
        szone_error(szone, 1, "pointer being freed was not allocated", ptr, NULL);
        SZONE_UNLOCK(szone);
        return;
    }
    SZONE_UNLOCK(szone); // we release the lock asap
    CHECK(szone, __PRETTY_FUNCTION__);
    
    // we deallocate_pages, including guard pages, outside the lock
    if (vm_range_to_deallocate.address) {
#if DEBUG_MALLOC
        // FIXME: large_entry_for_pointer_no_lock() needs the lock held ...
        if (large_entry_for_pointer_no_lock(szone, (void *)vm_range_to_deallocate.address)) {
            malloc_printf("*** invariant broken: %p still in use num_large_entries=%d\n", 
                          vm_range_to_deallocate.address, szone->num_large_entries);
            large_debug_print(szone);
            szone_sleep();
        }
#endif
        deallocate_pages(szone, (void *)vm_range_to_deallocate.address, (size_t)vm_range_to_deallocate.size, 0);
    }
}

static INLINE int
large_try_realloc_in_place(szone_t *szone, void *ptr, size_t old_size, size_t new_size)
{
    vm_address_t        addr = (vm_address_t)ptr + old_size;
    large_entry_t       *large_entry;
    kern_return_t       err;

    SZONE_LOCK(szone);
    large_entry = large_entry_for_pointer_no_lock(szone, (void *)addr);
    SZONE_UNLOCK(szone);
    
    if (large_entry) { // check if "addr = ptr + old_size" is already spoken for
        return 0; // large pointer already exists in table - extension is not going to work
    }
    
    new_size = round_page(new_size);
    /*
     * Ask for allocation at a specific address, and mark as realloc
     * to request coalescing with previous realloc'ed extensions.
     */
    err = vm_allocate(mach_task_self(), &addr, new_size - old_size, VM_MAKE_TAG(VM_MEMORY_REALLOC));
    if (err != KERN_SUCCESS) {
        return 0;
    }
    
    SZONE_LOCK(szone);
    /* extend existing large entry */
    large_entry = large_entry_for_pointer_no_lock(szone, ptr);
    if (!large_entry) {
        szone_error(szone, 1, "large entry reallocated is not properly in table", ptr, NULL);
        SZONE_UNLOCK(szone);
        return 0; // Bail, leaking "addr"
    }

    large_entry->address = (vm_address_t)ptr;
    large_entry->size = new_size;
    szone->num_bytes_in_large_objects += new_size - old_size;
    SZONE_UNLOCK(szone); // we release the lock asap
    
    return 1;
}

/*********************  Zone call backs ************************/
/*
 * Mark these NOINLINE to avoid bloating the purgeable zone call backs
 */
static NOINLINE void
szone_free(szone_t *szone, void *ptr)
{
    region_t    tiny_region;
    region_t    small_region;

#if DEBUG_MALLOC
    if (LOG(szone, ptr))
        malloc_printf("in szone_free with %p\n", ptr);
#endif
    if (!ptr)
        return;
    /*
     * Try to free to a tiny region.
     */
    if ((uintptr_t)ptr & (TINY_QUANTUM - 1)) {
        szone_error(szone, 1, "Non-aligned pointer being freed", ptr, NULL);
        return;
    }
    if ((tiny_region = tiny_region_for_ptr_no_lock(szone, ptr)) != NULL) {
        if (TINY_INDEX_FOR_PTR(ptr) >= NUM_TINY_BLOCKS) {
            szone_error(szone, 1, "Pointer to metadata being freed", ptr, NULL);
            return;
        }
        free_tiny(szone, ptr, tiny_region, 0);
        return;
    }

    /*
     * Try to free to a small region.
     */
    if ((uintptr_t)ptr & (SMALL_QUANTUM - 1)) {
        szone_error(szone, 1, "Non-aligned pointer being freed (2)", ptr, NULL);
        return;
    }
    if ((small_region = small_region_for_ptr_no_lock(szone, ptr)) != NULL) {
        if (SMALL_META_INDEX_FOR_PTR(ptr) >= NUM_SMALL_BLOCKS) {
            szone_error(szone, 1, "Pointer to metadata being freed (2)", ptr, NULL);
            return;
        }
        free_small(szone, ptr, small_region, 0);
        return;
    }

    /* check that it's a legal large allocation */
    if ((uintptr_t)ptr & (vm_page_size - 1)) {
        szone_error(szone, 1, "non-page-aligned, non-allocated pointer being freed", ptr, NULL);
        return;
    }
    free_large(szone, ptr);
}

static NOINLINE void
szone_free_definite_size(szone_t *szone, void *ptr, size_t size)
{
#if DEBUG_MALLOC
    if (LOG(szone, ptr))
        malloc_printf("in szone_free_definite_size with %p\n", ptr);
        
    if (0 == size) {
        szone_error(szone, 1, "pointer of size zero being freed", ptr, NULL);
        return;
    }
    
#endif
    if (!ptr)
        return;
        
    /*
     * Try to free to a tiny region.
     */
    if ((uintptr_t)ptr & (TINY_QUANTUM - 1)) {
        szone_error(szone, 1, "Non-aligned pointer being freed", ptr, NULL);
        return;
    }
    if (size <= (NUM_TINY_SLOTS - 1)*TINY_QUANTUM) {
        if (TINY_INDEX_FOR_PTR(ptr) >= NUM_TINY_BLOCKS) {
            szone_error(szone, 1, "Pointer to metadata being freed", ptr, NULL);
            return;
        }
        free_tiny(szone, ptr, TINY_REGION_FOR_PTR(ptr), size);
        return;
    }

    /*
     * Try to free to a small region.
     */
    if ((uintptr_t)ptr & (SMALL_QUANTUM - 1)) {
        szone_error(szone, 1, "Non-aligned pointer being freed (2)", ptr, NULL);
        return;
    }
    if (!((szone->debug_flags & SCALABLE_MALLOC_ADD_GUARD_PAGES) && PROTECT_SMALL) && 
                (size <= szone->large_threshold)) {
        if (SMALL_META_INDEX_FOR_PTR(ptr) >= NUM_SMALL_BLOCKS) {
            szone_error(szone, 1, "Pointer to metadata being freed (2)", ptr, NULL);
            return;
        }
        free_small(szone, ptr, SMALL_REGION_FOR_PTR(ptr), size);
        return;
    }

    /* check that it's a legal large allocation */
    if ((uintptr_t)ptr & (vm_page_size - 1)) {
        szone_error(szone, 1, "non-page-aligned, non-allocated pointer being freed", ptr, NULL);
        return;
    }
    free_large(szone, ptr);
}

static NOINLINE void *
szone_malloc_should_clear(szone_t *szone, size_t size, boolean_t cleared_requested)
{
    void        *ptr;
    msize_t     msize;

    if (size <= (NUM_TINY_SLOTS - 1)*TINY_QUANTUM) {
        // think tiny
        msize = TINY_MSIZE_FOR_BYTES(size + TINY_QUANTUM - 1);
        if (!msize)
            msize = 1;
        ptr = tiny_malloc_should_clear(szone, msize, cleared_requested);
    } else if (!((szone->debug_flags & SCALABLE_MALLOC_ADD_GUARD_PAGES) && PROTECT_SMALL) && 
                (size <= szone->large_threshold)) {
        // think small
        msize = SMALL_MSIZE_FOR_BYTES(size + SMALL_QUANTUM - 1);
        if (! msize) 
            msize = 1;
        ptr = small_malloc_should_clear(szone, msize, cleared_requested);
    } else {
        // large
        size_t num_pages = round_page(size) >> vm_page_shift;
        if (num_pages == 0)     /* Overflowed */
            ptr = 0;
        else
            ptr = large_malloc(szone, num_pages, 0, cleared_requested);
    }
#if DEBUG_MALLOC
    if (LOG(szone, ptr))
        malloc_printf("szone_malloc returned %p\n", ptr);
#endif
    /*
     * If requested, scribble on allocated memory.
     */
    if ((szone->debug_flags & SCALABLE_MALLOC_DO_SCRIBBLE) && ptr && !cleared_requested && size)
        memset(ptr, 0xaa, size);

    return ptr;
}

static NOINLINE void *
szone_malloc(szone_t *szone, size_t size) {
    return szone_malloc_should_clear(szone, size, 0);
}

static NOINLINE void *
szone_calloc(szone_t *szone, size_t num_items, size_t size)
{
    size_t total_bytes = num_items * size;
    
    // Check for overflow of integer multiplication
    if (num_items > 1) {
#if __LP64__ /* size_t is uint64_t */
        if ((num_items | size) & 0xffffffff00000000ul) {
            // num_items or size equals or exceeds sqrt(2^64) == 2^32, appeal to wider arithmetic
            __uint128_t product = ((__uint128_t)num_items) * ((__uint128_t)size);
            if ((uint64_t)(product >> 64)) // compiles to test on upper register of register pair
                return NULL;
        }
#else /* size_t is uint32_t */
        if ((num_items | size) & 0xffff0000ul) {
            // num_items or size equals or exceeds sqrt(2^32) == 2^16, appeal to wider arithmetic
            uint64_t product = ((uint64_t)num_items) * ((uint64_t)size);
            if ((uint32_t)(product >> 32)) // compiles to test on upper register of register pair
                return NULL;
        }
#endif
    }
    
    return szone_malloc_should_clear(szone, total_bytes, 1);
}

static NOINLINE void *
szone_valloc(szone_t *szone, size_t size)
{
    void        *ptr;
    
    if (size <= szone->large_threshold) {
        ptr = szone_memalign(szone, vm_page_size, size);
    } else {
        size_t  num_pages;

        num_pages = round_page(size) >> vm_page_shift;
        ptr = large_malloc(szone, num_pages, 0, 0);
    }
    
#if DEBUG_MALLOC
    if (LOG(szone, ptr))
        malloc_printf("szone_valloc returned %p\n", ptr);
#endif
    return ptr;
}

/* Isolate PIC-base load (for __is_threaded) here. */
static NOINLINE size_t
szone_size_try_large(szone_t *szone, const void *ptr)
{
    size_t              size = 0;
    large_entry_t       *entry;

    SZONE_LOCK(szone);
    entry = large_entry_for_pointer_no_lock(szone, ptr);
    if (entry) {
        size = entry->size;
    }
    SZONE_UNLOCK(szone); 
#if DEBUG_MALLOC
    if (LOG(szone, ptr)) {
        malloc_printf("szone_size for %p returned %d\n", ptr, (unsigned)size);
    }
#endif
    return size;
}

static NOINLINE size_t
szone_size(szone_t *szone, const void *ptr)
{
    boolean_t           is_free;
    msize_t             msize, msize_and_free;
    
    if (!ptr)
        return 0;
#if DEBUG_MALLOC
    if (LOG(szone, ptr)) {
        malloc_printf("in szone_size for %p (szone=%p)\n", ptr, szone);
    }
#endif

    /*
     * Look for it in a tiny region.
     */
    if ((uintptr_t)ptr & (TINY_QUANTUM - 1))
        return 0;
    if (tiny_region_for_ptr_no_lock(szone, ptr)) {
        if (TINY_INDEX_FOR_PTR(ptr) >= NUM_TINY_BLOCKS)
            return 0;
        msize = get_tiny_meta_header(ptr, &is_free);
        if (is_free)
            return 0;
#if TINY_CACHE
        {
            mag_index_t mag_index = MAGAZINE_INDEX_FOR_TINY_REGION(TINY_REGION_FOR_PTR(ptr));
            magazine_t  *tiny_mag_ptr = &(szone->tiny_magazines[mag_index]);

            if (msize < TINY_QUANTUM && ptr == (void *)((uintptr_t)(tiny_mag_ptr->mag_last_free) & ~ (TINY_QUANTUM - 1)))
                return 0;
        }
#endif
        return TINY_BYTES_FOR_MSIZE(msize);
    }
    
    /*
     * Look for it in a small region.
     */
    if ((uintptr_t)ptr & (SMALL_QUANTUM - 1))
        return 0;
    if (small_region_for_ptr_no_lock(szone, ptr)) {
        if (SMALL_META_INDEX_FOR_PTR(ptr) >= NUM_SMALL_BLOCKS)
            return 0;
        msize_and_free = *SMALL_METADATA_FOR_PTR(ptr);
        if (msize_and_free & SMALL_IS_FREE)
            return 0;
#if SMALL_CACHE
        {
            mag_index_t mag_index = MAGAZINE_INDEX_FOR_SMALL_REGION(SMALL_REGION_FOR_PTR(ptr));
            magazine_t  *small_mag_ptr = &(szone->small_magazines[mag_index]);

            if (ptr == (void *)((uintptr_t)(small_mag_ptr->mag_last_free) & ~ (SMALL_QUANTUM - 1)))
                return 0;
        }
#endif
        return SMALL_BYTES_FOR_MSIZE(msize_and_free);
    }

    /*
     * If not page-aligned, it cannot have come from a large allocation.
     */
    if ((uintptr_t)ptr & (vm_page_size - 1))
        return 0;

    /*
     * Look for it in a large entry.
     */
    return szone_size_try_large(szone, ptr);
}

static NOINLINE void *
szone_realloc(szone_t *szone, void *ptr, size_t new_size)
{
    size_t      old_size;
    void        *new_ptr;
    
#if DEBUG_MALLOC
    if (LOG(szone, ptr)) {
        malloc_printf("in szone_realloc for %p, %d\n", ptr, (unsigned)new_size);
    }
#endif
    if (!ptr) {
        ptr = szone_malloc(szone, new_size);
        return ptr;
    }
    old_size = szone_size(szone, ptr);
    if (!old_size) {
        szone_error(szone, 1, "pointer being reallocated was not allocated", ptr, NULL);
        return NULL;
    }
    /* we never shrink an allocation */
    if (old_size >= new_size)
        return ptr;

    /*
     * If the new size suits the tiny allocator and the pointer being resized
     * belongs to a tiny region, try to reallocate in-place.
     */
    if ((new_size + TINY_QUANTUM - 1) <= (NUM_TINY_SLOTS - 1) * TINY_QUANTUM) {
        if (tiny_region_for_ptr_no_lock(szone, ptr) != NULL) {
            if (tiny_try_realloc_in_place(szone, ptr, old_size, new_size)) {
                return ptr;
            }
        }

    /*
     * If the new size suits the small allocator and the pointer being resized
     * belongs to a small region, and we're not protecting the small allocations
     * try to reallocate in-place.
     */
    } else if (!((szone->debug_flags & SCALABLE_MALLOC_ADD_GUARD_PAGES) && PROTECT_SMALL) &&
        ((new_size + SMALL_QUANTUM - 1) <= szone->large_threshold) &&
        (small_region_for_ptr_no_lock(szone, ptr) != NULL)) {
            if (small_try_realloc_in_place(szone, ptr, old_size, new_size)) {
                return ptr;
            }

    /*
     * If the allocation's a large allocation, try to reallocate in-place there.
     */
    } else if (!((szone->debug_flags & SCALABLE_MALLOC_ADD_GUARD_PAGES) && PROTECT_SMALL) && 
                !(szone->debug_flags & SCALABLE_MALLOC_PURGEABLE) &&
                (old_size > szone->large_threshold)) {
        if (large_try_realloc_in_place(szone, ptr, old_size, new_size)) {
            return ptr;
        }
    }

    /*
     * Can't reallocate in place for whatever reason; allocate a new buffer and copy.
     */
    new_ptr = szone_malloc(szone, new_size);
    if (new_ptr == NULL)
        return NULL;

    /*
     * If the allocation's large enough, try to copy using VM.  If that fails, or
     * if it's too small, just copy by hand.
     */
    if ((old_size < szone->vm_copy_threshold) ||
      vm_copy(mach_task_self(), (vm_address_t)ptr, old_size, (vm_address_t)new_ptr))
        memcpy(new_ptr, ptr, old_size);
    szone_free(szone, ptr);
    
#if DEBUG_MALLOC
    if (LOG(szone, ptr)) {
        malloc_printf("szone_realloc returned %p for %d\n", new_ptr, (unsigned)new_size);
    }
#endif
    return new_ptr;
}

static NOINLINE void *
szone_memalign(szone_t *szone, size_t alignment, size_t size)
{
    if ((size + alignment) < size) // size_t arithmetic wrapped!
        return NULL;

    // alignment is gauranteed a power of 2 at least as large as sizeof(void *), hence non-zero.
    // Since size + alignment didn't wrap, 0 <= size + alignment - 1 < size + alignment
    size_t span = size + alignment - 1;
    
    if (alignment <= TINY_QUANTUM) {
        return szone_malloc(szone, size); // Trivially satisfied by tiny, small, or large
        
    } else if (span <= (NUM_TINY_SLOTS - 1)*TINY_QUANTUM) {
        msize_t mspan = TINY_MSIZE_FOR_BYTES(span + TINY_QUANTUM - 1);
        void *p = szone_malloc(szone, span); // avoids inlining tiny_malloc_should_clear(szone, mspan, 0);
        
        if (NULL == p)
            return NULL;

        size_t offset = ((uintptr_t) p) & (alignment - 1); // p % alignment
        size_t pad = (0 == offset) ? 0 : alignment - offset; // p + pad achieves desired alignment
        
        msize_t msize = TINY_MSIZE_FOR_BYTES(size + TINY_QUANTUM - 1);
        msize_t mpad = TINY_MSIZE_FOR_BYTES(pad + TINY_QUANTUM - 1);
        msize_t mwaste = mspan - msize - mpad; // excess blocks

        if (mpad > 0) {
            void *q = (void *)(((uintptr_t) p) + pad);
            
            // Mark q as a block header and in-use, thus creating two blocks.
            magazine_t  *tiny_mag_ptr = mag_lock_zine_for_region_trailer(szone, szone->tiny_magazines,
                                    REGION_TRAILER_FOR_TINY_REGION(TINY_REGION_FOR_PTR(p)),
                                    MAGAZINE_INDEX_FOR_TINY_REGION(TINY_REGION_FOR_PTR(p)));    
            set_tiny_meta_header_in_use(q, msize);
            
            // set_tiny_meta_header_in_use() "reaffirms" the block_header on the *following* block, so
            // now set its in_use bit as well. But only if its within the original allocation made above.
            if (mwaste > 0)
                BITARRAY_SET(TINY_INUSE_FOR_HEADER(TINY_BLOCK_HEADER_FOR_PTR(q)), TINY_INDEX_FOR_PTR(q) + msize);
            SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
            
            // Give up mpad blocks beginning at p to the tiny free list
            // region_t r = TINY_REGION_FOR_PTR(p);
            szone_free(szone, p); // avoids inlining free_tiny(szone, p, &r);
            
            p = q; // advance p to the desired alignment
        }
        
        if (mwaste > 0) {
            void *q = (void *)(((uintptr_t) p) + TINY_BYTES_FOR_MSIZE(msize));
            // Mark q as block header and in-use, thus creating two blocks.
            magazine_t  *tiny_mag_ptr = mag_lock_zine_for_region_trailer(szone, szone->tiny_magazines,
                                    REGION_TRAILER_FOR_TINY_REGION(TINY_REGION_FOR_PTR(p)),
                                    MAGAZINE_INDEX_FOR_TINY_REGION(TINY_REGION_FOR_PTR(p)));    
            set_tiny_meta_header_in_use(q, mwaste);
            SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
            
            // Give up mwaste blocks beginning at q to the tiny free list
            // region_t r = TINY_REGION_FOR_PTR(q);
            szone_free(szone, q); // avoids inlining free_tiny(szone, q, &r);
        }
        
        return p; // p has the desired size and alignment, and can later be free()'d
        
    } else if ((NUM_TINY_SLOTS - 1)*TINY_QUANTUM < size && alignment <= SMALL_QUANTUM) {
        return szone_malloc(szone, size); // Trivially satisfied by small or large
        
    } else if (!((szone->debug_flags & SCALABLE_MALLOC_ADD_GUARD_PAGES) && PROTECT_SMALL) && (span <= szone->large_threshold)) {

        if (size <= (NUM_TINY_SLOTS - 1)*TINY_QUANTUM) {
            size = (NUM_TINY_SLOTS - 1)*TINY_QUANTUM + TINY_QUANTUM; // ensure block allocated by small does not have a tiny-possible size
            span = size + alignment - 1;
        }

        msize_t mspan = SMALL_MSIZE_FOR_BYTES(span + SMALL_QUANTUM - 1);
        void *p = szone_malloc(szone, span); // avoid inlining small_malloc_should_clear(szone, mspan, 0);

        if (NULL == p)
            return NULL;

        size_t offset = ((uintptr_t) p) & (alignment - 1); // p % alignment
        size_t pad = (0 == offset) ? 0 : alignment - offset; // p + pad achieves desired alignment
        
        msize_t msize = SMALL_MSIZE_FOR_BYTES(size + SMALL_QUANTUM - 1);
        msize_t mpad = SMALL_MSIZE_FOR_BYTES(pad + SMALL_QUANTUM - 1);
        msize_t mwaste = mspan - msize - mpad; // excess blocks
        
        if (mpad > 0) {
            void *q = (void *)(((uintptr_t) p) + pad);
            
            // Mark q as block header and in-use, thus creating two blocks.
            magazine_t  *small_mag_ptr = mag_lock_zine_for_region_trailer(szone, szone->small_magazines,
                                    REGION_TRAILER_FOR_SMALL_REGION(SMALL_REGION_FOR_PTR(p)),
                                    MAGAZINE_INDEX_FOR_SMALL_REGION(SMALL_REGION_FOR_PTR(p)));    
            small_meta_header_set_in_use(SMALL_META_HEADER_FOR_PTR(p), SMALL_META_INDEX_FOR_PTR(p), mpad);
            small_meta_header_set_in_use(SMALL_META_HEADER_FOR_PTR(q), SMALL_META_INDEX_FOR_PTR(q), msize + mwaste);
            SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
            
            // Give up mpad blocks beginning at p to the small free list
            // region_t r = SMALL_REGION_FOR_PTR(p);
            szone_free(szone, p); // avoid inlining free_small(szone, p, &r);
            
            p = q; // advance p to the desired alignment
        }
        if (mwaste > 0) {
            void *q = (void *)(((uintptr_t) p) + SMALL_BYTES_FOR_MSIZE(msize));
            // Mark q as block header and in-use, thus creating two blocks.
            magazine_t  *small_mag_ptr = mag_lock_zine_for_region_trailer(szone, szone->small_magazines,
                                    REGION_TRAILER_FOR_SMALL_REGION(SMALL_REGION_FOR_PTR(p)),
                                    MAGAZINE_INDEX_FOR_SMALL_REGION(SMALL_REGION_FOR_PTR(p)));    
            small_meta_header_set_in_use(SMALL_META_HEADER_FOR_PTR(p), SMALL_META_INDEX_FOR_PTR(p), msize);
            small_meta_header_set_in_use(SMALL_META_HEADER_FOR_PTR(q), SMALL_META_INDEX_FOR_PTR(q), mwaste);
            SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
            
            // Give up mwaste blocks beginning at q to the small free list
            // region_t r = SMALL_REGION_FOR_PTR(q);
            szone_free(szone, q); // avoid inlining free_small(szone, q, &r);
        }

        return p; // p has the desired size and alignment, and can later be free()'d
        
    } else if (szone->large_threshold < size && alignment <= vm_page_size) {
        return szone_malloc(szone, size); // Trivially satisfied by large
        
    } else {
        // ensure block allocated by large does not have a small-possible size
        size_t num_pages = round_page(MAX(szone->large_threshold + 1, size)) >> vm_page_shift;
        void *p;
        
        if (num_pages == 0)     /* Overflowed */
            p = NULL;
        else
            p = large_malloc(szone, num_pages, MAX(vm_page_shift, __builtin_ctz(alignment)), 0);
            
        return p;
    }
    /* NOTREACHED */
}

// given a size, returns the number of pointers allocated capable of holding
// that size, up to the limit specified by the 'count' argument.  These pointers
// are stored in the 'results' array, which must be allocated by the caller.
// may return zero, since this function is only a best attempt at allocating
// the pointers.  clients should be prepared to call malloc for any additional
// blocks they need.
static NOINLINE unsigned
szone_batch_malloc(szone_t *szone, size_t size, void **results, unsigned count)
{
    msize_t     msize = TINY_MSIZE_FOR_BYTES(size + TINY_QUANTUM - 1);
    unsigned    found = 0;
    mag_index_t mag_index = mag_get_thread_index(szone);
    magazine_t  *tiny_mag_ptr = &(szone->tiny_magazines[mag_index]);

    // only bother implementing this for tiny
    if (size > (NUM_TINY_SLOTS - 1)*TINY_QUANTUM)
        return 0;
    // make sure to return objects at least one quantum in size
    if (!msize)
        msize = 1;

    CHECK(szone, __PRETTY_FUNCTION__);

    // We must lock the zone now, since tiny_malloc_from_free_list assumes that 
    // the caller has done so.
    SZONE_MAGAZINE_PTR_LOCK(szone, tiny_mag_ptr);

    // with the zone locked, allocate objects from the free list until all
    // sufficiently large objects have been exhausted, or we have met our quota
    // of objects to allocate.
    while (found < count) {
        void *ptr = tiny_malloc_from_free_list(szone, tiny_mag_ptr, mag_index, msize);
        if (!ptr)
            break;

        *results++ = ptr;
        found++;
    }
    SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
    return found;
}

/* Try caching the tiny_region and checking if the next ptr hits there. */
static NOINLINE void
szone_batch_free(szone_t *szone, void **to_be_freed, unsigned count)
{
    unsigned    cc = 0;
    void        *ptr;
    region_t    tiny_region = NULL;
    boolean_t   is_free;
    msize_t     msize;
    magazine_t  *tiny_mag_ptr = NULL;
    mag_index_t mag_index = -1;
    
    // frees all the pointers in to_be_freed
    // note that to_be_freed may be overwritten during the process
    if (!count)
        return;
        
    CHECK(szone, __PRETTY_FUNCTION__);
    while (cc < count) {
        ptr = to_be_freed[cc];
        if (ptr) {
            if (NULL == tiny_region || tiny_region != TINY_REGION_FOR_PTR(ptr)) { // region same as last iteration?
                if (tiny_mag_ptr) { // non-NULL iff magazine lock taken
                    SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
                    tiny_mag_ptr = NULL;
                }
                    
                tiny_region = tiny_region_for_ptr_no_lock(szone, ptr);
                
                if (tiny_region) {
                    tiny_mag_ptr = mag_lock_zine_for_region_trailer(szone, szone->tiny_magazines,
                                                REGION_TRAILER_FOR_TINY_REGION(tiny_region),
                                                MAGAZINE_INDEX_FOR_TINY_REGION(tiny_region));
                    mag_index = MAGAZINE_INDEX_FOR_TINY_REGION(tiny_region);
                }               
            }
            if (tiny_region) {
                // this is a tiny pointer
                if (TINY_INDEX_FOR_PTR(ptr) >= NUM_TINY_BLOCKS)
                    break; // pointer to metadata; let the standard free deal with it
                msize = get_tiny_meta_header(ptr, &is_free);
                if (is_free)
                    break; // a double free; let the standard free deal with it
                    
                tiny_free_no_lock(szone, tiny_mag_ptr, mag_index, tiny_region, ptr, msize);
                to_be_freed[cc] = NULL;
            } else {
                // No region in this zone claims ptr; let the standard free deal with it
                break;
            }
        }
        cc++;
    }
    
    if (tiny_mag_ptr) {
        SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
        tiny_mag_ptr = NULL;
    }

    CHECK(szone, __PRETTY_FUNCTION__);
    while (count--) {
        ptr = to_be_freed[count];
        if (ptr)
            szone_free(szone, ptr);
    }
}

// FIXME: Suppose one of the locks is held?
static void
szone_destroy(szone_t *szone)
{
    size_t              index;
    large_entry_t       *large;
    vm_range_t          range_to_deallocate;

    /* destroy large entries */
    index = szone->num_large_entries;
    while (index--) {
        large = szone->large_entries + index;
        if (large->address) {
            // we deallocate_pages, including guard pages
            deallocate_pages(szone, (void *)(large->address), large->size, szone->debug_flags);
        }
    }
    large_entries_free_no_lock(szone, szone->large_entries, szone->num_large_entries, &range_to_deallocate);
    if (range_to_deallocate.size)
        deallocate_pages(szone, (void *)range_to_deallocate.address, (size_t)range_to_deallocate.size, 0);

    /* destroy tiny regions */
    for (index = 0; index < szone->tiny_region_generation->num_regions_allocated; ++index)
      if ((HASHRING_OPEN_ENTRY != szone->tiny_region_generation->hashed_regions[index]) && 
          (HASHRING_REGION_DEALLOCATED != szone->tiny_region_generation->hashed_regions[index]))
        deallocate_pages(szone, szone->tiny_region_generation->hashed_regions[index], TINY_REGION_SIZE, 0);

    /* destroy small regions */
    for (index = 0; index < szone->small_region_generation->num_regions_allocated; ++index)
      if ((HASHRING_OPEN_ENTRY != szone->small_region_generation->hashed_regions[index]) && 
          (HASHRING_REGION_DEALLOCATED != szone->small_region_generation->hashed_regions[index]))
        deallocate_pages(szone, szone->small_region_generation->hashed_regions[index], SMALL_REGION_SIZE, 0);

    /* destroy region hash rings, if any */
    if (szone->tiny_region_generation->hashed_regions != szone->initial_tiny_regions) {
      size_t size = round_page(szone->tiny_region_generation->num_regions_allocated * sizeof(region_t));
      deallocate_pages(szone, szone->tiny_region_generation->hashed_regions, size, 0);
    }
    if (szone->small_region_generation->hashed_regions != szone->initial_small_regions) {
      size_t size = round_page(szone->small_region_generation->num_regions_allocated * sizeof(region_t));
      deallocate_pages(szone, szone->small_region_generation->hashed_regions, size, 0);
    }
    
    /* Now destroy the separate szone region */
    if (szone->cpu_id_key != (pthread_key_t) -1)
        (void)pthread_key_delete(szone->cpu_id_key);
    deallocate_pages(szone, (void *)&(szone->tiny_magazines[-1]), TINY_MAGAZINE_PAGED_SIZE, SCALABLE_MALLOC_ADD_GUARD_PAGES);
    deallocate_pages(szone, (void *)&(szone->small_magazines[-1]), SMALL_MAGAZINE_PAGED_SIZE, SCALABLE_MALLOC_ADD_GUARD_PAGES);
    deallocate_pages(szone, (void *)szone, SZONE_PAGED_SIZE, SCALABLE_MALLOC_ADD_GUARD_PAGES);
}

static NOINLINE size_t
szone_good_size(szone_t *szone, size_t size)
{
    msize_t msize;
    int guard_small = (szone->debug_flags & SCALABLE_MALLOC_ADD_GUARD_PAGES) && PROTECT_SMALL;

    // Find a good size for this tiny allocation.
    if (size <= (NUM_TINY_SLOTS - 1) * TINY_QUANTUM) {
        msize = TINY_MSIZE_FOR_BYTES(size + TINY_QUANTUM - 1);
        if (!msize)
            msize = 1;
        return TINY_BYTES_FOR_MSIZE(msize);
    }
    
    // Find a good size for this small allocation.
    if (!guard_small && (size <= szone->large_threshold)) {
        msize = SMALL_MSIZE_FOR_BYTES(size + SMALL_QUANTUM - 1);
        if (!msize)
            msize = 1;
        return SMALL_BYTES_FOR_MSIZE(msize);
    }
    
    // Check for integer overflow on the size, since unlike the two cases above,
    // there is no upper bound on allocation size at this point.
    if (size > round_page(size))
        return (size_t)(-1LL);

#if DEBUG_MALLOC
    // It is not acceptable to see a size of zero here, since that means we
    // failed to catch a request for zero bytes in the tiny check, or the size
    // overflowed to zero during some arithmetic.
    if (size == 0)
        malloc_printf("szone_good_size() invariant broken %y\n", size);
#endif
    return round_page(size);
}

unsigned szone_check_counter = 0;
unsigned szone_check_start = 0;
unsigned szone_check_modulo = 1;

static NOINLINE boolean_t
szone_check_all(szone_t *szone, const char *function)
{
    size_t index;

    /* check tiny regions - chould check region count */
    for (index = 0; index < szone->tiny_region_generation->num_regions_allocated; ++index) {
        region_t tiny = szone->tiny_region_generation->hashed_regions[index];
      
        if (HASHRING_REGION_DEALLOCATED == tiny)
            continue;
      
        if (tiny) {
            magazine_t *tiny_mag_ptr = mag_lock_zine_for_region_trailer(szone, szone->tiny_magazines,
                REGION_TRAILER_FOR_TINY_REGION(tiny), MAGAZINE_INDEX_FOR_TINY_REGION(tiny));

            if (!tiny_check_region(szone, tiny)) {
                SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
                szone->debug_flags &= ~ CHECK_REGIONS;
                szone_error(szone, 1, "check: tiny region incorrect", NULL,
                            "*** tiny region %ld incorrect szone_check_all(%s) counter=%d\n",
                            index, function, szone_check_counter);
                return 0;
            }
            SZONE_MAGAZINE_PTR_UNLOCK(szone, tiny_mag_ptr);
        }
    }
    /* check tiny free lists */
    for (index = 0; index < NUM_TINY_SLOTS; ++index) {
        if (!tiny_free_list_check(szone, index)) {
            szone->debug_flags &= ~ CHECK_REGIONS;
            szone_error(szone, 1, "check: tiny free list incorrect", NULL,
                        "*** tiny free list incorrect (slot=%ld) szone_check_all(%s) counter=%d\n",
                        index, function, szone_check_counter);
            return 0;
        }
    }
 
    /* check small regions - could check region count */
    for (index = 0; index < szone->small_region_generation->num_regions_allocated; ++index) {
        region_t small = szone->small_region_generation->hashed_regions[index];
      
        if (HASHRING_REGION_DEALLOCATED == small)
            continue;
      
        if (small) {
            magazine_t *small_mag_ptr = mag_lock_zine_for_region_trailer(szone, szone->small_magazines,
                REGION_TRAILER_FOR_SMALL_REGION(small), MAGAZINE_INDEX_FOR_SMALL_REGION(small));
                
            if (!small_check_region(szone, small)) {
                SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
                szone->debug_flags &= ~ CHECK_REGIONS;
                szone_error(szone, 1, "check: small region incorrect", NULL,
                            "*** small region %ld incorrect szone_check_all(%s) counter=%d\n",
                            index, function, szone_check_counter);
                return 0;
            }
            SZONE_MAGAZINE_PTR_UNLOCK(szone, small_mag_ptr);
        }
    }
    /* check small free lists */
    for (index = 0; index < szone->num_small_slots; ++index) {
        if (!small_free_list_check(szone, index)) {
            szone->debug_flags &= ~ CHECK_REGIONS;
            szone_error(szone, 1, "check: small free list incorrect", NULL,
                        "*** small free list incorrect (slot=%ld) szone_check_all(%s) counter=%d\n",
                        index, function, szone_check_counter);
            return 0;
        }
    }

    return 1;
}

static boolean_t
szone_check(szone_t *szone)
{
    if ((++szone_check_counter % 10000) == 0)
        _malloc_printf(ASL_LEVEL_NOTICE, "at szone_check counter=%d\n", szone_check_counter);
        
    if (szone_check_counter < szone_check_start)
        return 1;
        
    if (szone_check_counter % szone_check_modulo)
        return 1;
        
    return szone_check_all(szone, "");
}

static kern_return_t
szone_ptr_in_use_enumerator(task_t task, void *context, unsigned type_mask, vm_address_t zone_address, 
                            memory_reader_t reader, vm_range_recorder_t recorder)
{
    szone_t             *szone;
    kern_return_t       err;
    
    if (!reader) reader = _szone_default_reader;
    
    err = reader(task, zone_address, sizeof(szone_t), (void **)&szone);
    if (err) return err;
    
    err = tiny_in_use_enumerator(task, context, type_mask, szone, reader, recorder);
    if (err) return err;
    
    err = small_in_use_enumerator(task, context, type_mask, szone, reader, recorder);
    if (err) return err;
    
    err = large_in_use_enumerator(task, context, type_mask,
      (vm_address_t)szone->large_entries, szone->num_large_entries, reader, recorder);
    return err;
}

// Following method is deprecated:  use scalable_zone_statistics instead
void
scalable_zone_info(malloc_zone_t *zone, unsigned *info_to_fill, unsigned count)
{
    szone_t     *szone = (void *)zone;
    unsigned    info[13];

    // We do not lock to facilitate debug
    
    size_t      s = 0;
    unsigned    t = 0;
    size_t      u = 0;
    mag_index_t mag_index;
    
    for (mag_index = -1; mag_index < szone->num_tiny_magazines; mag_index++) {
        s += szone->tiny_magazines[mag_index].mag_bytes_free_at_end;
        t += szone->tiny_magazines[mag_index].mag_num_objects;
        u += szone->tiny_magazines[mag_index].mag_num_bytes_in_objects;
    }
    
    info[4] = t;
    info[5] = u;
    
    for (t = 0, u = 0, mag_index = -1; mag_index < szone->num_small_magazines; mag_index++) {
        s += szone->small_magazines[mag_index].mag_bytes_free_at_end;
        t += szone->small_magazines[mag_index].mag_num_objects;
        u += szone->small_magazines[mag_index].mag_num_bytes_in_objects;
    }
    
    info[6] = t;
    info[7] = u;
    
    info[8] = szone->num_large_objects_in_use;
    info[9] = szone->num_bytes_in_large_objects;
    
    info[10] = 0; // DEPRECATED szone->num_huge_entries;
    info[11] = 0; // DEPRECATED szone->num_bytes_in_huge_objects;
    
    info[12] = szone->debug_flags;
    
    info[0] = info[4] + info[6] + info[8] + info[10];
    info[1] = info[5] + info[7] + info[9] + info[11];
    
    info[3] = (szone->num_tiny_regions - szone->num_tiny_regions_dealloc) * TINY_REGION_SIZE + 
              (szone->num_small_regions - szone->num_small_regions_dealloc) * SMALL_REGION_SIZE + info[9] + info[11];
    
    info[2] = info[3] - s;
    memcpy(info_to_fill, info, sizeof(unsigned)*count);
}

// FIXME: consistent picture requires locking!
static NOINLINE void
szone_print(szone_t *szone, boolean_t verbose)
{
    unsigned    info[13];
    size_t      index;
    region_t    region;
  
    scalable_zone_info((void *)szone, info, 13);
    _malloc_printf(MALLOC_PRINTF_NOLOG | MALLOC_PRINTF_NOPREFIX,
                 "Scalable zone %p: inUse=%d(%y) touched=%y allocated=%y flags=%d\n",
                 szone, info[0], info[1], info[2], info[3], info[12]);
    _malloc_printf(MALLOC_PRINTF_NOLOG | MALLOC_PRINTF_NOPREFIX,
                 "\ttiny=%d(%y) small=%d(%y) large=%d(%y) huge=%d(%y)\n",
                 info[4], info[5], info[6], info[7], info[8], info[9], info[10], info[11]);
    // tiny
    _malloc_printf(MALLOC_PRINTF_NOLOG | MALLOC_PRINTF_NOPREFIX,
                 "%d tiny regions:\n", szone->num_tiny_regions);
    if (szone->num_tiny_regions_dealloc)
        _malloc_printf(MALLOC_PRINTF_NOLOG | MALLOC_PRINTF_NOPREFIX,
            "[%d tiny regions have been vm_deallocate'd]\n", szone->num_tiny_regions_dealloc);
    for (index = 0; index < szone->tiny_region_generation->num_regions_allocated; ++index) {
        region = szone->tiny_region_generation->hashed_regions[index];
        if (HASHRING_OPEN_ENTRY != region && HASHRING_REGION_DEALLOCATED != region) {
            mag_index_t mag_index = MAGAZINE_INDEX_FOR_TINY_REGION(region);
            print_tiny_region(verbose, region, (region == szone->tiny_magazines[mag_index].mag_last_region) ? 
                                                szone->tiny_magazines[mag_index].mag_bytes_free_at_end : 0);
        }
    }
    if (verbose)
        print_tiny_free_list(szone);
    // small
    _malloc_printf(MALLOC_PRINTF_NOLOG | MALLOC_PRINTF_NOPREFIX,
                 "%d small regions:\n", szone->num_small_regions);
    if (szone->num_small_regions_dealloc)
        _malloc_printf(MALLOC_PRINTF_NOLOG | MALLOC_PRINTF_NOPREFIX,
            "[%d small regions have been vm_deallocate'd]\n", szone->num_small_regions_dealloc);
    for (index = 0; index < szone->small_region_generation->num_regions_allocated; ++index) {
        region = szone->small_region_generation->hashed_regions[index];
        if (HASHRING_OPEN_ENTRY != region && HASHRING_REGION_DEALLOCATED != region) {
            mag_index_t mag_index = MAGAZINE_INDEX_FOR_SMALL_REGION(region);
            print_small_region(szone, verbose, region,
                                (region == szone->small_magazines[mag_index].mag_last_region) ? 
                                szone->small_magazines[mag_index].mag_bytes_free_at_end : 0);
        }
    }
    if (verbose)
        print_small_free_list(szone);
}

static void
szone_log(malloc_zone_t *zone, void *log_address)
{
    szone_t     *szone = (szone_t *)zone;

    szone->log_address = log_address;
}

static void
szone_force_lock(szone_t *szone)
{
    mag_index_t i;
    
    for (i = 0; i < szone->num_tiny_magazines; ++i) {
        SZONE_MAGAZINE_PTR_LOCK(szone, (&(szone->tiny_magazines[i])));
    }
    SZONE_MAGAZINE_PTR_LOCK(szone, (&(szone->tiny_magazines[DEPOT_MAGAZINE_INDEX])));
    
    for (i = 0; i < szone->num_small_magazines; ++i) {
        SZONE_MAGAZINE_PTR_LOCK(szone, (&(szone->small_magazines[i])));
    }
    SZONE_MAGAZINE_PTR_LOCK(szone, (&(szone->small_magazines[DEPOT_MAGAZINE_INDEX])));
    
    SZONE_LOCK(szone);
}

static void
szone_force_unlock(szone_t *szone)
{
    mag_index_t i;
    
    SZONE_UNLOCK(szone);
    
    for (i = -1; i < szone->num_small_magazines; ++i) {
        SZONE_MAGAZINE_PTR_UNLOCK(szone, (&(szone->small_magazines[i])));
    }

    for (i = -1; i < szone->num_tiny_magazines; ++i) {
        SZONE_MAGAZINE_PTR_UNLOCK(szone, (&(szone->tiny_magazines[i])));
    }
}

static boolean_t
szone_locked(szone_t *szone) 
{
  mag_index_t i;
  int tookLock;

  tookLock = SZONE_TRY_LOCK(szone);
  if (tookLock == 0)
    return 1;
  SZONE_UNLOCK(szone);
  
  for (i = -1; i < szone->num_small_magazines; ++i) {
    tookLock = SZONE_MAGAZINE_PTR_TRY_LOCK(szone, (&(szone->small_magazines[i])));
    if (tookLock == 0)
      return 1;
    SZONE_MAGAZINE_PTR_UNLOCK(szone, (&(szone->small_magazines[i])));
  }
  
  for (i = -1; i < szone->num_tiny_magazines; ++i) {
    tookLock = SZONE_MAGAZINE_PTR_TRY_LOCK(szone, (&(szone->tiny_magazines[i])));
    if (tookLock == 0)
      return 1;
    SZONE_MAGAZINE_PTR_UNLOCK(szone, (&(szone->tiny_magazines[i])));
  }
  return 0;
}  

boolean_t
scalable_zone_statistics(malloc_zone_t *zone, malloc_statistics_t *stats, unsigned subzone)
{
    szone_t *szone = (szone_t *)zone;
    
    switch (subzone) {
        case 0:
        {       
            size_t      s = 0;
            unsigned    t = 0;
            size_t      u = 0;
            mag_index_t mag_index;
            
            for (mag_index = -1; mag_index < szone->num_tiny_magazines; mag_index++) {
                s += szone->tiny_magazines[mag_index].mag_bytes_free_at_end;
                t += szone->tiny_magazines[mag_index].mag_num_objects;
                u += szone->tiny_magazines[mag_index].mag_num_bytes_in_objects;
            }
    
            stats->blocks_in_use = t;
            stats->size_in_use = u;
            stats->size_allocated = (szone->num_tiny_regions - szone->num_tiny_regions_dealloc) * TINY_REGION_SIZE;
            stats->max_size_in_use = stats->size_allocated - s;
            return 1;
        }
        case 1: 
        {
            size_t      s = 0;
            unsigned    t = 0;
            size_t      u = 0;
            mag_index_t mag_index;
            
            for (mag_index = -1; mag_index < szone->num_small_magazines; mag_index++) {
                s += szone->small_magazines[mag_index].mag_bytes_free_at_end;
                t += szone->small_magazines[mag_index].mag_num_objects;
                u += szone->small_magazines[mag_index].mag_num_bytes_in_objects;
            }
    
            stats->blocks_in_use = t;
            stats->size_in_use = u;
            stats->size_allocated = (szone->num_small_regions - szone->num_small_regions_dealloc) * SMALL_REGION_SIZE;
            stats->max_size_in_use = stats->size_allocated - s;
            return 1;
        }
        case 2:
            stats->blocks_in_use = szone->num_large_objects_in_use;
            stats->size_in_use = szone->num_bytes_in_large_objects;
            stats->max_size_in_use = stats->size_allocated = stats->size_in_use;
            return 1;
        case 3:
            stats->blocks_in_use = 0; // DEPRECATED szone->num_huge_entries;
            stats->size_in_use = 0; // DEPRECATED szone->num_bytes_in_huge_objects;
            stats->max_size_in_use = stats->size_allocated = 0;
            return 1;
    }
    return 0;
}

static void
szone_statistics(szone_t *szone, malloc_statistics_t *stats)
{
    size_t      large;
    
    size_t      s = 0;
    unsigned    t = 0;
    size_t      u = 0;
    mag_index_t mag_index;
    
    for (mag_index = -1; mag_index < szone->num_tiny_magazines; mag_index++) {
        s += szone->tiny_magazines[mag_index].mag_bytes_free_at_end;
        t += szone->tiny_magazines[mag_index].mag_num_objects;
        u += szone->tiny_magazines[mag_index].mag_num_bytes_in_objects;
    }
    
    for (mag_index = -1; mag_index < szone->num_small_magazines; mag_index++) {
        s += szone->small_magazines[mag_index].mag_bytes_free_at_end;
        t += szone->small_magazines[mag_index].mag_num_objects;
        u += szone->small_magazines[mag_index].mag_num_bytes_in_objects;
    }
    
    large = szone->num_bytes_in_large_objects + 0; // DEPRECATED szone->num_bytes_in_huge_objects;
    
    stats->blocks_in_use = t + szone->num_large_objects_in_use + 0; // DEPRECATED szone->num_huge_entries;
    stats->size_in_use = u + large;
    stats->max_size_in_use = stats->size_allocated = 
        (szone->num_tiny_regions - szone->num_tiny_regions_dealloc) * TINY_REGION_SIZE +
        (szone->num_small_regions - szone->num_small_regions_dealloc) * SMALL_REGION_SIZE + large; 
    // Now we account for the untouched areas
    stats->max_size_in_use -= s;
}

static void *
legacy_zeroing_large_malloc(szone_t *szone, size_t size) {
    if (size > LARGE_THRESHOLD) // Leopard and earlier returned a ZFOD range, so ...
        return szone_calloc(szone, 1, size); // Clear to zero always, ham-handedly touching in each page
    else
        return szone_malloc(szone, size);
}

static void *
legacy_zeroing_large_valloc(szone_t *szone, size_t size) {
    void *p = szone_valloc(szone, size);
    
    // Leopard and earlier returned a ZFOD range, so ...
    memset(p, 0, size); // Clear to zero always, ham-handedly touching in each page
    return p;
}

void zeroify_scalable_zone(malloc_zone_t *zone)
{
    szone_t     *szone = (szone_t *)zone;
    
    if (szone) {
        szone->basic_zone.malloc = (void *)legacy_zeroing_large_malloc;
        szone->basic_zone.valloc = (void *)legacy_zeroing_large_valloc;
    }
}

static const struct malloc_introspection_t szone_introspect = {
    (void *)szone_ptr_in_use_enumerator,
    (void *)szone_good_size,
    (void *)szone_check,
    (void *)szone_print,
    szone_log,
    (void *)szone_force_lock,
    (void *)szone_force_unlock,
    (void *)szone_statistics,
    (void *)szone_locked,
}; // marked as const to spare the DATA section

malloc_zone_t *
create_scalable_zone(size_t initial_size, unsigned debug_flags)
{
    szone_t     *szone;
    uint64_t    hw_memsize = 0;
    size_t      uint64_t_size = sizeof(hw_memsize);
    int         err;

    /*
     * Sanity-check our build-time assumptions about the size of a page.
     * Since we have sized various things assuming the default page size,
     * attempting to determine it dynamically is not useful.
     */
    if ((vm_page_size != _vm_page_size) || (vm_page_shift != _vm_page_shift)) {
        malloc_printf("*** FATAL ERROR - machine page size does not match our assumptions.\n");
        exit(-1);
    }

#if defined(__i386__) || defined(__x86_64__)
    if (_COMM_PAGE_VERSION_REQD > (*((short *) _COMM_PAGE_VERSION))) { // _COMM_PAGE_CPU_NUMBER must be present at runtime
        malloc_printf("*** ERROR - comm page version mismatch.\n");
        exit(-1);
    }
#endif
    
    /* get memory for the zone, which is now separate from any region.
       add guard pages to prevent walking from any other vm allocations
       to here and overwriting the function pointers in basic_zone. */
    szone = allocate_pages(NULL, SZONE_PAGED_SIZE, 0, SCALABLE_MALLOC_ADD_GUARD_PAGES, VM_MEMORY_MALLOC);
    if (!szone)
        return NULL;

    /* set up the szone structure */
#if 0
#warning CHECK_REGIONS enabled
    debug_flags |= CHECK_REGIONS;
#endif
#if 0
#warning LOG enabled
    szone->log_address = ~0;
#endif
    szone->trg[0].nextgen = &(szone->trg[1]);
    szone->trg[1].nextgen = &(szone->trg[0]);
    szone->tiny_region_generation = &(szone->trg[0]);
    
    szone->tiny_region_generation->hashed_regions = szone->initial_tiny_regions;
    szone->tiny_region_generation->num_regions_allocated = INITIAL_NUM_REGIONS;
    szone->tiny_region_generation->num_regions_allocated_shift = INITIAL_NUM_REGIONS_SHIFT;
    
    szone->srg[0].nextgen = &(szone->srg[1]);
    szone->srg[1].nextgen = &(szone->srg[0]);
    szone->small_region_generation = &(szone->srg[0]);
    
    szone->small_region_generation->hashed_regions = szone->initial_small_regions;
    szone->small_region_generation->num_regions_allocated = INITIAL_NUM_REGIONS;
    szone->small_region_generation->num_regions_allocated_shift = INITIAL_NUM_REGIONS_SHIFT;
    
    
    /*
     * Initialize variables that size the free list for SMALL allocations based
     * upon the amount of memory in the system.  Switch to a larger number of
     * free list entries at 1GB.
     */
    if (0 == sysctlbyname("hw.memsize", &hw_memsize, &uint64_t_size, 0, 0) &&
        hw_memsize >= (1ULL << 30)) {
        szone->is_largemem = 1;
        szone->num_small_slots = NUM_SMALL_SLOTS_LARGEMEM;
        szone->large_threshold = LARGE_THRESHOLD_LARGEMEM;
        szone->vm_copy_threshold = VM_COPY_THRESHOLD_LARGEMEM;
    } else {
        szone->is_largemem = 0;
        szone->num_small_slots = NUM_SMALL_SLOTS;
        szone->large_threshold = LARGE_THRESHOLD;
        szone->vm_copy_threshold = VM_COPY_THRESHOLD;
    }
#if LARGE_CACHE
    szone->large_entry_cache_hoard_lmit = hw_memsize >> 10; // madvise(..., MADV_REUSABLE) death-row arrivals above this threshold [~0.1%]

    /* <rdar://problem/6610904> Reset protection when returning a previous large allocation? */
    int32_t libSystemVersion  = NSVersionOfLinkTimeLibrary("System");
    if ((-1 != libSystemVersion) && ((libSystemVersion >> 16) < 112) /* CFSystemVersionSnowLeopard */)
        szone->large_legacy_reset_mprotect = TRUE;
    else
        szone->large_legacy_reset_mprotect = FALSE;
#endif
    
    // Initialize the security token.
#if __LP64__
    szone->cookie = ((uintptr_t)arc4random() << 32) | (uintptr_t)arc4random();
#else
    szone->cookie = arc4random();
#endif
    
    szone->basic_zone.version = 6;
    szone->basic_zone.size = (void *)szone_size;
    szone->basic_zone.malloc = (void *)szone_malloc;
    szone->basic_zone.calloc = (void *)szone_calloc;
    szone->basic_zone.valloc = (void *)szone_valloc;
    szone->basic_zone.free = (void *)szone_free;
    szone->basic_zone.realloc = (void *)szone_realloc;
    szone->basic_zone.destroy = (void *)szone_destroy;
    szone->basic_zone.batch_malloc = (void *)szone_batch_malloc;
    szone->basic_zone.batch_free = (void *)szone_batch_free;
    szone->basic_zone.introspect = (struct malloc_introspection_t *)&szone_introspect;
    szone->basic_zone.memalign = (void *)szone_memalign;
    szone->basic_zone.free_definite_size = (void *)szone_free_definite_size;
    szone->debug_flags = debug_flags;
    LOCK_INIT(szone->large_szone_lock);
    
#if defined(__ppc__) || defined(__ppc64__)
    /*
     * In the interest of compatibility for PPC applications executing via Rosetta,
     * arrange to zero-fill allocations as occurred by side effect in Leopard and earlier.
     */
    zeroify_scalable_zone((malloc_zone_t *)szone);
#endif

    if ((err = pthread_key_create(&(szone->cpu_id_key), NULL))) {
        malloc_printf("*** ERROR -pthread_key_create failure err=%d.\n", err);
        szone->cpu_id_key = (pthread_key_t) -1;
    }

    // Query the number of configured processors.
    // Uniprocessor case gets just one tiny and one small magazine (whose index is zero). This gives
    // the same behavior as the original scalable malloc. MP gets per-CPU magazines
    // that scale (way) better.
    int nproc = sysconf(_SC_NPROCESSORS_CONF);
    szone->num_tiny_magazines = (nproc > 1) ? MIN(nproc, TINY_MAX_MAGAZINES) : 1;

    // FIXME vm_allocate() based on number of configured CPUs
    magazine_t *tiny_magazines = allocate_pages(NULL, TINY_MAGAZINE_PAGED_SIZE, 0, 
                                                    SCALABLE_MALLOC_ADD_GUARD_PAGES, VM_MEMORY_MALLOC);
    if (NULL == tiny_magazines)
        return NULL;

    szone->tiny_magazines = &(tiny_magazines[1]); // szone->tiny_magazines[-1] is the Depot

    // The magazines are indexed in [0 .. (num_tiny_magazines - 1)]
    // Find the smallest power of 2 that exceeds (num_tiny_magazines - 1)
    szone->num_tiny_magazines_mask_shift = 0;
    int i = 1;
     while( i <= (szone->num_tiny_magazines - 1) ) {
        szone->num_tiny_magazines_mask_shift++;
        i <<= 1;
    }
    
    // Now if i <= TINY_MAX_MAGAZINES we'll never access tiny_magazines[] out of bounds.
    if (i > TINY_MAX_MAGAZINES) {
        malloc_printf("*** FATAL ERROR - magazine mask exceeds allocated magazines.\n");
        exit(-1);
    }
    
    // Reduce i by 1 to obtain a mask covering [0 .. (num_tiny_magazines - 1)]
    szone->num_tiny_magazines_mask = i - 1; // A mask used for hashing to a magazine index (and a safety aid)
    
    // Init the tiny_magazine locks
    LOCK_INIT(szone->tiny_regions_lock);
    LOCK_INIT(szone->tiny_magazines[DEPOT_MAGAZINE_INDEX].magazine_lock);
    for (i = 0; i < szone->num_tiny_magazines; ++i) {
        LOCK_INIT(szone->tiny_magazines[i].magazine_lock);
    }

    szone->num_small_magazines = (nproc > 1) ? MIN(nproc, SMALL_MAX_MAGAZINES) : 1;

    // FIXME vm_allocate() based on number of configured CPUs
    magazine_t *small_magazines = allocate_pages(NULL, SMALL_MAGAZINE_PAGED_SIZE, 0, 
                                                    SCALABLE_MALLOC_ADD_GUARD_PAGES, VM_MEMORY_MALLOC);
    if (NULL == small_magazines)
        return NULL;

    szone->small_magazines = &(small_magazines[1]); // szone->small_magazines[-1] is the Depot
    
    // The magazines are indexed in [0 .. (num_small_magazines - 1)]
    // Find the smallest power of 2 that exceeds (num_small_magazines - 1)
    szone->num_small_magazines_mask_shift = 0;
     while( i <= (szone->num_small_magazines - 1) ) {
        szone->num_small_magazines_mask_shift++;
        i <<= 1;
    }
    
    // Now if i <= SMALL_MAX_MAGAZINES we'll never access small_magazines[] out of bounds.
    if (i > SMALL_MAX_MAGAZINES) {
        malloc_printf("*** FATAL ERROR - magazine mask exceeds allocated magazines.\n");
        exit(-1);
    }
    
    // Reduce i by 1 to obtain a mask covering [0 .. (num_small_magazines - 1)]
    szone->num_small_magazines_mask = i - 1; // A mask used for hashing to a magazine index (and a safety aid)
    
    // Init the small_magazine locks
    LOCK_INIT(szone->small_regions_lock);
    LOCK_INIT(szone->small_magazines[DEPOT_MAGAZINE_INDEX].magazine_lock);
    for (i = 0; i < szone->num_small_magazines; ++i) {
        LOCK_INIT(szone->small_magazines[i].magazine_lock);
    }

    CHECK(szone, __PRETTY_FUNCTION__);
    return (malloc_zone_t *)szone;
}

//
// purgeable zones have their own "large" allocation pool, but share "tiny" and "large" 
// heaps with a helper_zone identified in the call to create_purgeable_zone()
//
static size_t
purgeable_size(szone_t *szone, const void *ptr)
{
    size_t t = szone_size_try_large(szone, ptr);
    
    if (t)
        return t;
    else
        return szone_size(szone->helper_zone, ptr);
}

static void *
purgeable_malloc(szone_t *szone, size_t size) {
    if (size <= szone->large_threshold)
        return szone_malloc(szone->helper_zone, size);
    else
        return szone_malloc(szone, size);
}

static void *
purgeable_calloc(szone_t *szone, size_t num_items, size_t size)
{
    size_t total_bytes = num_items * size;
    
    // Check for overflow of integer multiplication
    if (num_items > 1) {
#if __LP64__ /* size_t is uint64_t */
        if ((num_items | size) & 0xffffffff00000000ul) {
            // num_items or size equals or exceeds sqrt(2^64) == 2^32, appeal to wider arithmetic
                __uint128_t product = ((__uint128_t)num_items) * ((__uint128_t)size);
            if ((uint64_t)(product >> 64)) // compiles to test on upper register of register pair
                return NULL;
        }
#else /* size_t is uint32_t */
        if ((num_items | size) & 0xffff0000ul) {
            // num_items or size equals or exceeds sqrt(2^32) == 2^16, appeal to wider arithmetic
                uint64_t product = ((uint64_t)num_items) * ((uint64_t)size);
            if ((uint32_t)(product >> 32)) // compiles to test on upper register of register pair
                return NULL;
        }
#endif
    }
    
    if (total_bytes <= szone->large_threshold)
        return szone_calloc(szone->helper_zone, 1, total_bytes);
    else
        return szone_calloc(szone, 1, total_bytes);
}

static void *
purgeable_valloc(szone_t *szone, size_t size)
{
    if (size <= szone->large_threshold)
        return szone_valloc(szone->helper_zone, size);
    else
        return szone_valloc(szone, size);
}

static void
purgeable_free(szone_t *szone, void *ptr)
{
    large_entry_t       *entry;

    SZONE_LOCK(szone);
    entry = large_entry_for_pointer_no_lock(szone, ptr);
    SZONE_UNLOCK(szone); 
    if (entry) {
        return free_large(szone, ptr);
    } else {
        return szone_free(szone->helper_zone, ptr);
    }
}
    
static void
purgeable_free_definite_size(szone_t *szone, void *ptr, size_t size)
{
    if (size <= szone->large_threshold)
        return szone_free_definite_size(szone->helper_zone, ptr, size);
    else
        return szone_free_definite_size(szone, ptr, size);
}

static void *
purgeable_realloc(szone_t *szone, void *ptr, size_t new_size)
{
    if (new_size <= szone->large_threshold)
        return szone_realloc(szone->helper_zone, ptr, new_size);
    else 
        return szone_realloc(szone, ptr, new_size);
}

static void
purgeable_destroy(szone_t *szone)
{
    /* destroy large entries */
    size_t index = szone->num_large_entries;
    large_entry_t       *large;
    vm_range_t          range_to_deallocate;

    while (index--) {
        large = szone->large_entries + index;
        if (large->address) {
            // we deallocate_pages, including guard pages
            deallocate_pages(szone, (void *)(large->address), large->size, szone->debug_flags);
        }
    }
    large_entries_free_no_lock(szone, szone->large_entries, szone->num_large_entries, &range_to_deallocate);
    if (range_to_deallocate.size)
        deallocate_pages(szone, (void *)range_to_deallocate.address, (size_t)range_to_deallocate.size, 0);
    
    /* Now destroy the separate szone region */
    deallocate_pages(szone, (void *)szone, SZONE_PAGED_SIZE, SCALABLE_MALLOC_ADD_GUARD_PAGES);
}

static unsigned
purgeable_batch_malloc(szone_t *szone, size_t size, void **results, unsigned count)
{
    return szone_batch_malloc(szone->helper_zone, size, results, count);
}

static void
purgeable_batch_free(szone_t *szone, void **to_be_freed, unsigned count)
{
    return szone_batch_free(szone->helper_zone, to_be_freed, count);
}

static void *
purgeable_memalign(szone_t *szone, size_t alignment, size_t size)
{
    if (size <= szone->large_threshold)
        return szone_memalign(szone->helper_zone, alignment, size);
    else
        return szone_memalign(szone, alignment, size);
}

static kern_return_t
purgeable_ptr_in_use_enumerator(task_t task, void *context, unsigned type_mask, vm_address_t zone_address, 
                            memory_reader_t reader, vm_range_recorder_t recorder)
{
    szone_t             *szone;
    kern_return_t       err;
    
    if (!reader) reader = _szone_default_reader;
    
    err = reader(task, zone_address, sizeof(szone_t), (void **)&szone);
    if (err) return err;

    err = large_in_use_enumerator(task, context, type_mask, 
      (vm_address_t)szone->large_entries, szone->num_large_entries, reader, recorder);
    return err;
}

static size_t
purgeable_good_size(szone_t *szone, size_t size)
{
    if (size <= szone->large_threshold)
        return szone_good_size(szone->helper_zone, size);
    else
        return szone_good_size(szone, size);
}

static boolean_t
purgeable_check(szone_t *szone)
{
    return 1;
}

static void
purgeable_print(szone_t *szone, boolean_t verbose)
{
    _malloc_printf(MALLOC_PRINTF_NOLOG | MALLOC_PRINTF_NOPREFIX,
                 "Scalable zone %p: inUse=%d(%y) flags=%d\n",
                 szone, szone->num_large_objects_in_use, szone->num_bytes_in_large_objects, szone->debug_flags);
}

static void
purgeable_log(malloc_zone_t *zone, void *log_address)
{
    szone_t     *szone = (szone_t *)zone;

    szone->log_address = log_address;
}

static void
purgeable_force_lock(szone_t *szone)
{    
    SZONE_LOCK(szone);
}

static void
purgeable_force_unlock(szone_t *szone)
{    
    SZONE_UNLOCK(szone);
}

static void
purgeable_statistics(szone_t *szone, malloc_statistics_t *stats)
{
    stats->blocks_in_use = szone->num_large_objects_in_use;
    stats->size_in_use = stats->max_size_in_use = stats->size_allocated = szone->num_bytes_in_large_objects;
}

static boolean_t
purgeable_locked(szone_t *szone) 
{
  int tookLock;

  tookLock = SZONE_TRY_LOCK(szone);
  if (tookLock == 0)
    return 1;
  SZONE_UNLOCK(szone);
  return 0;
}

static const struct malloc_introspection_t purgeable_introspect = {
    (void *)purgeable_ptr_in_use_enumerator,
    (void *)purgeable_good_size,
    (void *)purgeable_check,
    (void *)purgeable_print,
    purgeable_log,
    (void *)purgeable_force_lock,
    (void *)purgeable_force_unlock,
    (void *)purgeable_statistics,
    (void *)purgeable_locked,
}; // marked as const to spare the DATA section

malloc_zone_t *
create_purgeable_zone(size_t initial_size, malloc_zone_t *malloc_default_zone, unsigned debug_flags)
{
    szone_t     *szone;
        
    /* get memory for the zone, which is now separate from any region.
       add guard pages to prevent walking from any other vm allocations
       to here and overwriting the function pointers in basic_zone. */
    szone = allocate_pages(NULL, SZONE_PAGED_SIZE, 0, SCALABLE_MALLOC_ADD_GUARD_PAGES, VM_MEMORY_MALLOC);
    if (!szone)
        return NULL;

    /* set up the szone structure */
#if 0
#warning LOG enabled
    szone->log_address = ~0;
#endif
    
        /* Purgeable zone does not participate in the adaptive "largemem" sizing. */
    szone->is_largemem = 0;
    szone->large_threshold = LARGE_THRESHOLD;
    szone->vm_copy_threshold = VM_COPY_THRESHOLD;

#if LARGE_CACHE
    /* <rdar://problem/6610904> Reset protection when returning a previous large allocation? */
    int32_t libSystemVersion  = NSVersionOfLinkTimeLibrary("System");
    if ((-1 != libSystemVersion) && ((libSystemVersion >> 16) < 112) /* CFSystemVersionSnowLeopard */)
        szone->large_legacy_reset_mprotect = TRUE;
    else
        szone->large_legacy_reset_mprotect = FALSE;
#endif

    szone->basic_zone.version = 6;
    szone->basic_zone.size = (void *)purgeable_size;
    szone->basic_zone.malloc = (void *)purgeable_malloc;
    szone->basic_zone.calloc = (void *)purgeable_calloc;
    szone->basic_zone.valloc = (void *)purgeable_valloc;
    szone->basic_zone.free = (void *)purgeable_free;
    szone->basic_zone.realloc = (void *)purgeable_realloc;
    szone->basic_zone.destroy = (void *)purgeable_destroy;
    szone->basic_zone.batch_malloc = (void *)purgeable_batch_malloc;
    szone->basic_zone.batch_free = (void *)purgeable_batch_free;
    szone->basic_zone.introspect = (struct malloc_introspection_t *)&purgeable_introspect;
    szone->basic_zone.memalign = (void *)purgeable_memalign;
    szone->basic_zone.free_definite_size = (void *)purgeable_free_definite_size;
    
    szone->debug_flags = debug_flags | SCALABLE_MALLOC_PURGEABLE;
    
    /* Purgeable zone does not support SCALABLE_MALLOC_ADD_GUARD_PAGES. */
    if (szone->debug_flags & SCALABLE_MALLOC_ADD_GUARD_PAGES) {
        _malloc_printf(ASL_LEVEL_INFO, "purgeable zone does not support guard pages\n");
        szone->debug_flags &= ~SCALABLE_MALLOC_ADD_GUARD_PAGES;
    }

    LOCK_INIT(szone->large_szone_lock);

    szone->helper_zone = (struct szone_s *)malloc_default_zone;

    CHECK(szone, __PRETTY_FUNCTION__);
    return (malloc_zone_t *)szone;
}

/*
 * For use by CheckFix: create a new zone whose behavior is, apart from
 * the use of death-row and per-CPU magazines, that of Leopard.
 */
static NOINLINE void *
legacy_valloc(szone_t *szone, size_t size)
{
    void        *ptr;
    size_t num_pages;

    num_pages = round_page(size) >> vm_page_shift;
    ptr = large_malloc(szone, num_pages, 0, TRUE);
#if DEBUG_MALLOC
    if (LOG(szone, ptr))
    malloc_printf("legacy_valloc returned %p\n", ptr);
#endif
    return ptr;
}

malloc_zone_t *
create_legacy_scalable_zone(size_t initial_size, unsigned debug_flags)
{
    malloc_zone_t *mzone = create_scalable_zone(initial_size, debug_flags);
    szone_t     *szone = (szone_t *)mzone;

    if (!szone)
        return NULL;

    szone->is_largemem = 0;
    szone->num_small_slots = NUM_SMALL_SLOTS;
    szone->large_threshold = LARGE_THRESHOLD;
    szone->vm_copy_threshold = VM_COPY_THRESHOLD;
    
    szone->basic_zone.valloc = (void *)legacy_valloc;
    szone->basic_zone.free_definite_size = NULL;
    
    return mzone;
}

/********* Support code for emacs unexec ************/

/* History of freezedry version numbers:
 *
 * 1) Old malloc (before the scalable malloc implementation in this file
 *    existed).
 * 2) Original freezedrying code for scalable malloc.  This code was apparently
 *    based on the old freezedrying code and was fundamentally flawed in its
 *    assumption that tracking allocated memory regions was adequate to fake
 *    operations on freezedried memory.  This doesn't work, since scalable
 *    malloc does not store flags in front of large page-aligned allocations.
 * 3) Original szone-based freezedrying code.
 * 4) Fresher malloc with tiny zone
 * 5) 32/64bit compatible malloc
 * 6) Metadata within 1MB and 8MB region for tiny and small
 *
 * No version backward compatibility is provided, but the version number does
 * make it possible for malloc_jumpstart() to return an error if the application
 * was freezedried with an older version of malloc.
 */
#define MALLOC_FREEZEDRY_VERSION 6

typedef struct {
    unsigned    version;
    unsigned    nszones;
    szone_t     *szones;
} malloc_frozen;

static void *
frozen_malloc(szone_t *zone, size_t new_size)
{
    return malloc(new_size);
}

static void *
frozen_calloc(szone_t *zone, size_t num_items, size_t size)
{
    return calloc(num_items, size);
}

static void *
frozen_valloc(szone_t *zone, size_t new_size)
{
    return valloc(new_size);
}

static void *
frozen_realloc(szone_t *zone, void *ptr, size_t new_size)
{
    size_t      old_size = szone_size(zone, ptr);
    void        *new_ptr;
    
    if (new_size <= old_size) {
        return ptr;
    }
    new_ptr = malloc(new_size);
    if (old_size > 0) {
        memcpy(new_ptr, ptr, old_size);
    }
    return new_ptr;
}

static void
frozen_free(szone_t *zone, void *ptr)
{
}

static void
frozen_destroy(szone_t *zone)
{
}

/********* Pseudo-private API for emacs unexec ************/

/*
 * malloc_freezedry() records all of the szones in use, so that they can be
 * partially reconstituted by malloc_jumpstart().  Due to the differences
 * between reconstituted memory regions and those created by the szone code,
 * care is taken not to reallocate from the freezedried memory, except in the
 * case of a non-growing realloc().
 *
 * Due to the flexibility provided by the zone registration mechanism, it is
 * impossible to implement generic freezedrying for any zone type.  This code
 * only handles applications that use the szone allocator, so malloc_freezedry()
 * returns 0 (error) if any non-szone zones are encountered.
 */

uintptr_t
malloc_freezedry(void)
{
    extern unsigned malloc_num_zones;
    extern malloc_zone_t **malloc_zones;
    malloc_frozen *data;
    unsigned i;

    /* Allocate space in which to store the freezedry state. */
    data = (malloc_frozen *) malloc(sizeof(malloc_frozen));

    /* Set freezedry version number so that malloc_jumpstart() can check for compatibility. */
    data->version = MALLOC_FREEZEDRY_VERSION;

    /* Allocate the array of szone pointers. */
    data->nszones = malloc_num_zones;
    data->szones = (szone_t *) calloc(malloc_num_zones, sizeof(szone_t));

    /*
     * Fill in the array of szone structures.  They are copied rather than
     * referenced, since the originals are likely to be clobbered during malloc
     * initialization.
     */
    for (i = 0; i < malloc_num_zones; i++) {
        if (strcmp(malloc_zones[i]->zone_name, "DefaultMallocZone")) {
            /* Unknown zone type. */
            free(data->szones);
            free(data);
            return 0;
        }
        memcpy(&data->szones[i], malloc_zones[i], sizeof(szone_t));
    }

    return((uintptr_t)data);
}

int
malloc_jumpstart(uintptr_t cookie)
{
    malloc_frozen *data = (malloc_frozen *)cookie;
    unsigned i;

    if (data->version != MALLOC_FREEZEDRY_VERSION) {
        /* Unsupported freezedry version. */
        return 1;
    }

    for (i = 0; i < data->nszones; i++) {
        /* Set function pointers.  Even the functions that stay the same must be
         * set, since there are no guarantees that they will be mapped to the
         * same addresses. */
        data->szones[i].basic_zone.size = (void *) szone_size;
        data->szones[i].basic_zone.malloc = (void *) frozen_malloc;
        data->szones[i].basic_zone.calloc = (void *) frozen_calloc;
        data->szones[i].basic_zone.valloc = (void *) frozen_valloc;
        data->szones[i].basic_zone.free = (void *) frozen_free;
        data->szones[i].basic_zone.realloc = (void *) frozen_realloc;
        data->szones[i].basic_zone.destroy = (void *) frozen_destroy;
        data->szones[i].basic_zone.introspect = (struct malloc_introspection_t *)&szone_introspect;

        /* Register the freezedried zone. */
        malloc_zone_register(&data->szones[i].basic_zone);
    }

    return 0;
}