objc4-781.tar.gz

[apple/objc4.git] / runtime / objc-runtime-new.mm

/*
 * Copyright (c) 2005-2009 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@
 */

/***********************************************************************
* objc-runtime-new.m
* Support for new-ABI classes and images.
**********************************************************************/

#if __OBJC2__

#include "DenseMapExtras.h"
#include "objc-private.h"
#include "objc-runtime-new.h"
#include "objc-file.h"
#include "objc-cache.h"
#include "objc-zalloc.h"
#include <Block.h>
#include <objc/message.h>
#include <mach/shared_region.h>

#define newprotocol(p) ((protocol_t *)p)

static void disableTaggedPointers();
static void detach_class(Class cls, bool isMeta);
static void free_class(Class cls);
static IMP addMethod(Class cls, SEL name, IMP imp, const char *types, bool replace);
static void adjustCustomFlagsForMethodChange(Class cls, method_t *meth);
static method_t *search_method_list(const method_list_t *mlist, SEL sel);
static bool method_lists_contains_any(method_list_t * const *mlists, method_list_t * const *end,
        SEL sels[], size_t selcount);
static void flushCaches(Class cls);
static void initializeTaggedPointerObfuscator(void);
#if SUPPORT_FIXUP
static void fixupMessageRef(message_ref_t *msg);
#endif
static Class realizeClassMaybeSwiftAndUnlock(Class cls, mutex_t& lock);
static Class readClass(Class cls, bool headerIsBundle, bool headerIsPreoptimized);

struct locstamped_category_t {
    category_t *cat;
    struct header_info *hi;
};
enum {
    ATTACH_CLASS               = 1 << 0,
    ATTACH_METACLASS           = 1 << 1,
    ATTACH_CLASS_AND_METACLASS = 1 << 2,
    ATTACH_EXISTING            = 1 << 3,
};
static void attachCategories(Class cls, const struct locstamped_category_t *cats_list, uint32_t cats_count, int flags);


/***********************************************************************
* Lock management
**********************************************************************/
mutex_t runtimeLock;
mutex_t selLock;
#if CONFIG_USE_CACHE_LOCK
mutex_t cacheUpdateLock;
#endif
recursive_mutex_t loadMethodLock;

/***********************************************************************
* Class structure decoding
**********************************************************************/

const uintptr_t objc_debug_class_rw_data_mask = FAST_DATA_MASK;


/***********************************************************************
* Non-pointer isa decoding
**********************************************************************/
#if SUPPORT_INDEXED_ISA

// Indexed non-pointer isa.

// These are used to mask the ISA and see if its got an index or not.
const uintptr_t objc_debug_indexed_isa_magic_mask  = ISA_INDEX_MAGIC_MASK;
const uintptr_t objc_debug_indexed_isa_magic_value = ISA_INDEX_MAGIC_VALUE;

// die if masks overlap
STATIC_ASSERT((ISA_INDEX_MASK & ISA_INDEX_MAGIC_MASK) == 0);

// die if magic is wrong
STATIC_ASSERT((~ISA_INDEX_MAGIC_MASK & ISA_INDEX_MAGIC_VALUE) == 0);

// Then these are used to extract the index from the ISA.
const uintptr_t objc_debug_indexed_isa_index_mask  = ISA_INDEX_MASK;
const uintptr_t objc_debug_indexed_isa_index_shift  = ISA_INDEX_SHIFT;

asm("\n .globl _objc_absolute_indexed_isa_magic_mask"                   \
    "\n _objc_absolute_indexed_isa_magic_mask = " STRINGIFY2(ISA_INDEX_MAGIC_MASK));
asm("\n .globl _objc_absolute_indexed_isa_magic_value" \
    "\n _objc_absolute_indexed_isa_magic_value = " STRINGIFY2(ISA_INDEX_MAGIC_VALUE));
asm("\n .globl _objc_absolute_indexed_isa_index_mask"                   \
    "\n _objc_absolute_indexed_isa_index_mask = " STRINGIFY2(ISA_INDEX_MASK));
asm("\n .globl _objc_absolute_indexed_isa_index_shift" \
    "\n _objc_absolute_indexed_isa_index_shift = " STRINGIFY2(ISA_INDEX_SHIFT));


// And then we can use that index to get the class from this array.  Note
// the size is provided so that clients can ensure the index they get is in
// bounds and not read off the end of the array.
// Defined in the objc-msg-*.s files
// const Class objc_indexed_classes[]

// When we don't have enough bits to store a class*, we can instead store an
// index in to this array.  Classes are added here when they are realized.
// Note, an index of 0 is illegal.
uintptr_t objc_indexed_classes_count = 0;

// SUPPORT_INDEXED_ISA
#else
// not SUPPORT_INDEXED_ISA

// These variables exist but are all set to 0 so that they are ignored.
const uintptr_t objc_debug_indexed_isa_magic_mask  = 0;
const uintptr_t objc_debug_indexed_isa_magic_value = 0;
const uintptr_t objc_debug_indexed_isa_index_mask  = 0;
const uintptr_t objc_debug_indexed_isa_index_shift = 0;
Class objc_indexed_classes[1] = { nil };
uintptr_t objc_indexed_classes_count = 0;

// not SUPPORT_INDEXED_ISA
#endif


#if SUPPORT_PACKED_ISA

// Packed non-pointer isa.

asm("\n .globl _objc_absolute_packed_isa_class_mask" \
    "\n _objc_absolute_packed_isa_class_mask = " STRINGIFY2(ISA_MASK));

const uintptr_t objc_debug_isa_class_mask  = ISA_MASK;
const uintptr_t objc_debug_isa_magic_mask  = ISA_MAGIC_MASK;
const uintptr_t objc_debug_isa_magic_value = ISA_MAGIC_VALUE;

// die if masks overlap
STATIC_ASSERT((ISA_MASK & ISA_MAGIC_MASK) == 0);

// die if magic is wrong
STATIC_ASSERT((~ISA_MAGIC_MASK & ISA_MAGIC_VALUE) == 0);

// die if virtual address space bound goes up
STATIC_ASSERT((~ISA_MASK & MACH_VM_MAX_ADDRESS) == 0  ||  
              ISA_MASK + sizeof(void*) == MACH_VM_MAX_ADDRESS);

// SUPPORT_PACKED_ISA
#else
// not SUPPORT_PACKED_ISA

// These variables exist but enforce pointer alignment only.
const uintptr_t objc_debug_isa_class_mask  = (~WORD_MASK);
const uintptr_t objc_debug_isa_magic_mask  = WORD_MASK;
const uintptr_t objc_debug_isa_magic_value = 0;

// not SUPPORT_PACKED_ISA
#endif


/***********************************************************************
* Swift marker bits
**********************************************************************/
const uintptr_t objc_debug_swift_stable_abi_bit = FAST_IS_SWIFT_STABLE;


/***********************************************************************
* allocatedClasses
* A table of all classes (and metaclasses) which have been allocated
* with objc_allocateClassPair.
**********************************************************************/
namespace objc {
static ExplicitInitDenseSet<Class> allocatedClasses;
}

/***********************************************************************
* _firstRealizedClass
* The root of all realized classes
**********************************************************************/
static Class _firstRealizedClass = nil;

/***********************************************************************
* didInitialAttachCategories
* Whether the initial attachment of categories present at startup has
* been done.
**********************************************************************/
static bool didInitialAttachCategories = false;

/***********************************************************************
* didCallDyldNotifyRegister
* Whether the call to _dyld_objc_notify_register has completed.
**********************************************************************/
bool didCallDyldNotifyRegister = false;

/*
  Low two bits of mlist->entsize is used as the fixed-up marker.
  PREOPTIMIZED VERSION:
    Method lists from shared cache are 1 (uniqued) or 3 (uniqued and sorted).
    (Protocol method lists are not sorted because of their extra parallel data)
    Runtime fixed-up method lists get 3.
  UN-PREOPTIMIZED VERSION:
    Method lists from shared cache are 1 (uniqued) or 3 (uniqued and sorted)
    Shared cache's sorting and uniquing are not trusted, but do affect the 
    location of the selector name string.
    Runtime fixed-up method lists get 2.

  High two bits of protocol->flags is used as the fixed-up marker.
  PREOPTIMIZED VERSION:
    Protocols from shared cache are 1<<30.
    Runtime fixed-up protocols get 1<<30.
  UN-PREOPTIMIZED VERSION:
  Protocols from shared cache are 1<<30.
    Shared cache's fixups are not trusted.
    Runtime fixed-up protocols get 3<<30.
*/

static uint32_t fixed_up_method_list = 3;
static uint32_t uniqued_method_list = 1;
static uint32_t fixed_up_protocol = PROTOCOL_FIXED_UP_1;
static uint32_t canonical_protocol = PROTOCOL_IS_CANONICAL;

void
disableSharedCacheOptimizations(void)
{
    fixed_up_method_list = 2;
    // It is safe to set uniqued method lists to 0 as we'll never call it unless
    // the method list was already in need of being fixed up
    uniqued_method_list = 0;
    fixed_up_protocol = PROTOCOL_FIXED_UP_1 | PROTOCOL_FIXED_UP_2;
    // Its safe to just set canonical protocol to 0 as we'll never call
    // clearIsCanonical() unless isCanonical() returned true, which can't happen
    // with a 0 mask
    canonical_protocol = 0;
}

bool method_list_t::isUniqued() const {
    return (flags() & uniqued_method_list) != 0;
}

bool method_list_t::isFixedUp() const {
    return flags() == fixed_up_method_list;
}

void method_list_t::setFixedUp() {
    runtimeLock.assertLocked();
    ASSERT(!isFixedUp());
    entsizeAndFlags = entsize() | fixed_up_method_list;
}

bool protocol_t::isFixedUp() const {
    return (flags & PROTOCOL_FIXED_UP_MASK) == fixed_up_protocol;
}

void protocol_t::setFixedUp() {
    runtimeLock.assertLocked();
    ASSERT(!isFixedUp());
    flags = (flags & ~PROTOCOL_FIXED_UP_MASK) | fixed_up_protocol;
}

bool protocol_t::isCanonical() const {
    return (flags & canonical_protocol) != 0;
}

void protocol_t::clearIsCanonical() {
    runtimeLock.assertLocked();
    ASSERT(isCanonical());
    flags = flags & ~canonical_protocol;
}


method_list_t * const *method_array_t::endCategoryMethodLists(Class cls) const
{
    auto mlists = beginLists();
    auto mlistsEnd = endLists();
    
    if (mlists == mlistsEnd  ||  !cls->data()->ro()->baseMethods())
    {
        // No methods, or no base methods. 
        // Everything here is a category method.
        return mlistsEnd;
    }
    
    // Have base methods. Category methods are 
    // everything except the last method list.
    return mlistsEnd - 1;
}

static const char *sel_cname(SEL sel)
{
    return (const char *)(void *)sel;
}


static size_t protocol_list_size(const protocol_list_t *plist)
{
    return sizeof(protocol_list_t) + plist->count * sizeof(protocol_t *);
}


static void try_free(const void *p) 
{
    if (p && malloc_size(p)) free((void *)p);
}


using ClassCopyFixupHandler = void (*)(Class _Nonnull oldClass,
                                       Class _Nonnull newClass);
// Normally there's only one handler registered.
static GlobalSmallVector<ClassCopyFixupHandler, 1> classCopyFixupHandlers;

void _objc_setClassCopyFixupHandler(void (* _Nonnull newFixupHandler)
    (Class _Nonnull oldClass, Class _Nonnull newClass)) {
    mutex_locker_t lock(runtimeLock);
    
    classCopyFixupHandlers.append(newFixupHandler);
}

static Class 
alloc_class_for_subclass(Class supercls, size_t extraBytes)
{
    if (!supercls  ||  !supercls->isAnySwift()) {
        return _calloc_class(sizeof(objc_class) + extraBytes);
    }

    // Superclass is a Swift class. New subclass must duplicate its extra bits.

    // Allocate the new class, with space for super's prefix and suffix
    // and self's extraBytes.
    swift_class_t *swiftSupercls = (swift_class_t *)supercls;
    size_t superSize = swiftSupercls->classSize;
    void *superBits = swiftSupercls->baseAddress();
    void *bits = malloc(superSize + extraBytes);

    // Copy all of the superclass's data to the new class.
    memcpy(bits, superBits, superSize);

    // Erase the objc data and the Swift description in the new class.
    swift_class_t *swcls = (swift_class_t *)
        ((uint8_t *)bits + swiftSupercls->classAddressOffset);
    bzero(swcls, sizeof(objc_class));
    swcls->description = nil;

    for (auto handler : classCopyFixupHandlers) {
        handler(supercls, (Class)swcls);
    }

    // Mark this class as Swift-enhanced.
    if (supercls->isSwiftStable()) {
        swcls->bits.setIsSwiftStable();
    }
    if (supercls->isSwiftLegacy()) {
        swcls->bits.setIsSwiftLegacy();
    }
    
    return (Class)swcls;
}


/***********************************************************************
* object_getIndexedIvars.
**********************************************************************/
void *object_getIndexedIvars(id obj)
{
    uint8_t *base = (uint8_t *)obj;

    if (!obj) return nil;
    if (obj->isTaggedPointer()) return nil;

    if (!obj->isClass()) return base + obj->ISA()->alignedInstanceSize();

    Class cls = (Class)obj;
    if (!cls->isAnySwift()) return base + sizeof(objc_class);
    
    swift_class_t *swcls = (swift_class_t *)cls;
    return base - swcls->classAddressOffset + word_align(swcls->classSize);
}


/***********************************************************************
* make_ro_writeable
* Reallocates rw->ro if necessary to make it writeable.
* Locking: runtimeLock must be held by the caller.
**********************************************************************/
static class_ro_t *make_ro_writeable(class_rw_t *rw)
{
    runtimeLock.assertLocked();

    if (rw->flags & RW_COPIED_RO) {
        // already writeable, do nothing
    } else {
        rw->set_ro(rw->ro()->duplicate());
        rw->flags |= RW_COPIED_RO;
    }
    return const_cast<class_ro_t *>(rw->ro());
}


/***********************************************************************
* dataSegmentsContain
* Returns true if the given address lies within a data segment in any
* loaded image.
**********************************************************************/
NEVER_INLINE
static bool
dataSegmentsContain(Class cls)
{
    uint32_t index;
    if (objc::dataSegmentsRanges.find((uintptr_t)cls, index)) {
        // if the class is realized (hence has a class_rw_t),
        // memorize where we found the range
        if (cls->isRealized()) {
            cls->data()->witness = (uint16_t)index;
        }
        return true;
    }
    return false;
}


/***********************************************************************
* isKnownClass
* Return true if the class is known to the runtime (located within the
* shared cache, within the data segment of a loaded image, or has been
* allocated with obj_allocateClassPair).
*
* The result of this operation is cached on the class in a "witness"
* value that is cheaply checked in the fastpath.
**********************************************************************/
ALWAYS_INLINE
static bool
isKnownClass(Class cls)
{
    if (fastpath(objc::dataSegmentsRanges.contains(cls->data()->witness, (uintptr_t)cls))) {
        return true;
    }
    auto &set = objc::allocatedClasses.get();
    return set.find(cls) != set.end() || dataSegmentsContain(cls);
}


/***********************************************************************
* addClassTableEntry
* Add a class to the table of all classes. If addMeta is true,
* automatically adds the metaclass of the class as well.
* Locking: runtimeLock must be held by the caller.
**********************************************************************/
static void
addClassTableEntry(Class cls, bool addMeta = true)
{
    runtimeLock.assertLocked();

    // This class is allowed to be a known class via the shared cache or via
    // data segments, but it is not allowed to be in the dynamic table already.
    auto &set = objc::allocatedClasses.get();

    ASSERT(set.find(cls) == set.end());

    if (!isKnownClass(cls))
        set.insert(cls);
    if (addMeta)
        addClassTableEntry(cls->ISA(), false);
}


/***********************************************************************
* checkIsKnownClass
* Checks the given class against the list of all known classes. Dies
* with a fatal error if the class is not known.
* Locking: runtimeLock must be held by the caller.
**********************************************************************/
ALWAYS_INLINE
static void
checkIsKnownClass(Class cls)
{
    if (slowpath(!isKnownClass(cls))) {
        _objc_fatal("Attempt to use unknown class %p.", cls);
    }
}

/***********************************************************************
* classNSObject
* Returns class NSObject.
* Locking: none
**********************************************************************/
static Class classNSObject(void)
{
    extern objc_class OBJC_CLASS_$_NSObject;
    return (Class)&OBJC_CLASS_$_NSObject;
}

static Class metaclassNSObject(void)
{
    extern objc_class OBJC_METACLASS_$_NSObject;
    return (Class)&OBJC_METACLASS_$_NSObject;
}

/***********************************************************************
* printReplacements
* Implementation of PrintReplacedMethods / OBJC_PRINT_REPLACED_METHODS.
* Warn about methods from cats that override other methods in cats or cls.
* Assumes no methods from cats have been added to cls yet.
**********************************************************************/
__attribute__((cold, noinline))
static void
printReplacements(Class cls, const locstamped_category_t *cats_list, uint32_t cats_count)
{
    uint32_t c;
    bool isMeta = cls->isMetaClass();

    // Newest categories are LAST in cats
    // Later categories override earlier ones.
    for (c = 0; c < cats_count; c++) {
        category_t *cat = cats_list[c].cat;

        method_list_t *mlist = cat->methodsForMeta(isMeta);
        if (!mlist) continue;

        for (const auto& meth : *mlist) {
            SEL s = sel_registerName(sel_cname(meth.name));

            // Search for replaced methods in method lookup order.
            // Complain about the first duplicate only.

            // Look for method in earlier categories
            for (uint32_t c2 = 0; c2 < c; c2++) {
                category_t *cat2 = cats_list[c2].cat;

                const method_list_t *mlist2 = cat2->methodsForMeta(isMeta);
                if (!mlist2) continue;

                for (const auto& meth2 : *mlist2) {
                    SEL s2 = sel_registerName(sel_cname(meth2.name));
                    if (s == s2) {
                        logReplacedMethod(cls->nameForLogging(), s, 
                                          cls->isMetaClass(), cat->name, 
                                          meth2.imp, meth.imp);
                        goto complained;
                    }
                }
            }

            // Look for method in cls
            for (const auto& meth2 : cls->data()->methods()) {
                SEL s2 = sel_registerName(sel_cname(meth2.name));
                if (s == s2) {
                    logReplacedMethod(cls->nameForLogging(), s, 
                                      cls->isMetaClass(), cat->name, 
                                      meth2.imp, meth.imp);
                    goto complained;
                }
            }

        complained:
            ;
        }
    }
}


/***********************************************************************
* unreasonableClassCount
* Provides an upper bound for any iteration of classes,
* to prevent spins when runtime metadata is corrupted.
**********************************************************************/
static unsigned unreasonableClassCount()
{
    runtimeLock.assertLocked();

    int base = NXCountMapTable(gdb_objc_realized_classes) +
    getPreoptimizedClassUnreasonableCount();

    // Provide lots of slack here. Some iterations touch metaclasses too.
    // Some iterations backtrack (like realized class iteration).
    // We don't need an efficient bound, merely one that prevents spins.
    return (base + 1) * 16;
}


/***********************************************************************
* Class enumerators
* The passed in block returns `false` if subclasses can be skipped
* Locking: runtimeLock must be held by the caller.
**********************************************************************/
static inline void
foreach_realized_class_and_subclass_2(Class top, unsigned &count,
                                      bool skip_metaclass,
                                      bool (^code)(Class) __attribute((noescape)))
{
    Class cls = top;

    runtimeLock.assertLocked();
    ASSERT(top);

    while (1) {
        if (--count == 0) {
            _objc_fatal("Memory corruption in class list.");
        }

        bool skip_subclasses;

        if (skip_metaclass && cls->isMetaClass()) {
            skip_subclasses = true;
        } else {
            skip_subclasses = !code(cls);
        }

        if (!skip_subclasses && cls->data()->firstSubclass) {
            cls = cls->data()->firstSubclass;
        } else {
            while (!cls->data()->nextSiblingClass  &&  cls != top) {
                cls = cls->superclass;
                if (--count == 0) {
                    _objc_fatal("Memory corruption in class list.");
                }
            }
            if (cls == top) break;
            cls = cls->data()->nextSiblingClass;
        }
    }
}

// Enumerates a class and all of its realized subclasses.
static void
foreach_realized_class_and_subclass(Class top, bool (^code)(Class) __attribute((noescape)))
{
    unsigned int count = unreasonableClassCount();

    foreach_realized_class_and_subclass_2(top, count, false, code);
}

// Enumerates all realized classes and metaclasses.
static void
foreach_realized_class_and_metaclass(bool (^code)(Class) __attribute((noescape)))
{
    unsigned int count = unreasonableClassCount();

    for (Class top = _firstRealizedClass;
         top != nil;
         top = top->data()->nextSiblingClass)
    {
        foreach_realized_class_and_subclass_2(top, count, false, code);
    }
}

// Enumerates all realized classes (ignoring metaclasses).
static void
foreach_realized_class(bool (^code)(Class) __attribute((noescape)))
{
    unsigned int count = unreasonableClassCount();

    for (Class top = _firstRealizedClass;
         top != nil;
         top = top->data()->nextSiblingClass)
    {
        foreach_realized_class_and_subclass_2(top, count, true, code);
    }
}


/***********************************************************************
 * Method Scanners / Optimization tracking
 * Implementation of scanning for various implementations of methods.
 **********************************************************************/

namespace objc {

enum SelectorBundle {
    AWZ,
    RR,
    Core,
};

namespace scanner {

// The current state of NSObject swizzling for every scanner
//
// It allows for cheap checks of global swizzles, and also lets
// things like IMP Swizzling before NSObject has been initialized
// to be remembered, as setInitialized() would miss these.
//
// Every pair of bits describes a SelectorBundle.
// even bits: is NSObject class swizzled for this bundle
// odd bits:  is NSObject meta class swizzled for this bundle
static uintptr_t NSObjectSwizzledMask;

static ALWAYS_INLINE uintptr_t
swizzlingBit(SelectorBundle bundle, bool isMeta)
{
    return 1UL << (2 * bundle + isMeta);
}

static void __attribute__((cold, noinline))
printCustom(Class cls, SelectorBundle bundle, bool inherited)
{
    static char const * const SelectorBundleName[] = {
        [AWZ]  = "CUSTOM AWZ",
        [RR]   = "CUSTOM RR",
        [Core] = "CUSTOM Core",
    };

    _objc_inform("%s: %s%s%s", SelectorBundleName[bundle],
                 cls->nameForLogging(),
                 cls->isMetaClass() ? " (meta)" : "",
                 inherited ? " (inherited)" : "");
}

enum class Scope { Instances, Classes, Both };

template <typename Traits, SelectorBundle Bundle, bool &ShouldPrint, Scope Domain = Scope::Both>
class Mixin {

    // work around compiler being broken with templates using Class/objc_class,
    // probably some weird confusion with Class being builtin
    ALWAYS_INLINE static objc_class *as_objc_class(Class cls) {
        return (objc_class *)cls;
    }

    static void
    setCustomRecursively(Class cls, bool inherited = false)
    {
        foreach_realized_class_and_subclass(cls, [=](Class c){
            if (c != cls && !as_objc_class(c)->isInitialized()) {
                // Subclass not yet initialized. Wait for setInitialized() to do it
                return false;
            }
            if (Traits::isCustom(c)) {
                return false;
            }
            Traits::setCustom(c);
            if (ShouldPrint) {
                printCustom(cls, Bundle, inherited || c != cls);
            }
            return true;
        });
    }

    static bool
    isNSObjectSwizzled(bool isMeta)
    {
        return NSObjectSwizzledMask & swizzlingBit(Bundle, isMeta);
    }

    static void
    setNSObjectSwizzled(Class NSOClass, bool isMeta)
    {
        NSObjectSwizzledMask |= swizzlingBit(Bundle, isMeta);
        if (as_objc_class(NSOClass)->isInitialized()) {
            setCustomRecursively(NSOClass);
        }
    }

    static void
    scanChangedMethodForUnknownClass(const method_t *meth)
    {
        Class cls;

        cls = classNSObject();
        if (Domain != Scope::Classes && !isNSObjectSwizzled(NO)) {
            for (const auto &meth2: as_objc_class(cls)->data()->methods()) {
                if (meth == &meth2) {
                    setNSObjectSwizzled(cls, NO);
                    break;
                }
            }
        }

        cls = metaclassNSObject();
        if (Domain != Scope::Instances && !isNSObjectSwizzled(YES)) {
            for (const auto &meth2: as_objc_class(cls)->data()->methods()) {
                if (meth == &meth2) {
                    setNSObjectSwizzled(cls, YES);
                    break;
                }
            }
        }
    }

    static void
    scanAddedClassImpl(Class cls, bool isMeta)
    {
        Class NSOClass = (isMeta ? metaclassNSObject() : classNSObject());
        bool setCustom = NO, inherited = NO;

        if (isNSObjectSwizzled(isMeta)) {
            setCustom = YES;
        } else if (cls == NSOClass) {
            // NSObject is default but we need to check categories
            auto &methods = as_objc_class(cls)->data()->methods();
            setCustom = Traits::scanMethodLists(methods.beginCategoryMethodLists(),
                                                  methods.endCategoryMethodLists(cls));
        } else if (!isMeta && !as_objc_class(cls)->superclass) {
            // Custom Root class
            setCustom = YES;
        } else if (Traits::isCustom(as_objc_class(cls)->superclass)) {
            // Superclass is custom, therefore we are too.
            setCustom = YES;
            inherited = YES;
        } else {
            // Not NSObject.
            auto &methods = as_objc_class(cls)->data()->methods();
            setCustom = Traits::scanMethodLists(methods.beginLists(),
                                                  methods.endLists());
        }
        if (slowpath(setCustom)) {
            if (ShouldPrint) printCustom(cls, Bundle, inherited);
        } else {
            Traits::setDefault(cls);
        }
    }

public:
    // Scan a class that is about to be marked Initialized for particular
    // bundles of selectors, and mark the class and its children
    // accordingly.
    //
    // This also handles inheriting properties from its superclass.
    //
    // Caller: objc_class::setInitialized()
    static void
    scanInitializedClass(Class cls, Class metacls)
    {
        if (Domain != Scope::Classes) {
            scanAddedClassImpl(cls, false);
        }
        if (Domain != Scope::Instances) {
            scanAddedClassImpl(metacls, true);
        }
    }

    // Inherit various properties from the superclass when a class
    // is being added to the graph.
    //
    // Caller: addSubclass()
    static void
    scanAddedSubClass(Class subcls, Class supercls)
    {
        if (slowpath(Traits::isCustom(supercls) && !Traits::isCustom(subcls))) {
            setCustomRecursively(subcls, true);
        }
    }

    // Scan Method lists for selectors that would override things
    // in a Bundle.
    //
    // This is used to detect when categories override problematic selectors
    // are injected in a class after it has been initialized.
    //
    // Caller: prepareMethodLists()
    static void
    scanAddedMethodLists(Class cls, method_list_t **mlists, int count)
    {
        if (slowpath(Traits::isCustom(cls))) {
            return;
        }
        if (slowpath(Traits::scanMethodLists(mlists, mlists + count))) {
            setCustomRecursively(cls);
        }
    }

    // Handle IMP Swizzling (the IMP for an exisiting method being changed).
    //
    // In almost all cases, IMP swizzling does not affect custom bits.
    // Custom search will already find the method whether or not
    // it is swizzled, so it does not transition from non-custom to custom.
    //
    // The only cases where IMP swizzling can affect the custom bits is
    // if the swizzled method is one of the methods that is assumed to be
    // non-custom. These special cases are listed in setInitialized().
    // We look for such cases here.
    //
    // Caller: Swizzling methods via adjustCustomFlagsForMethodChange()
    static void
    scanChangedMethod(Class cls, const method_t *meth)
    {
        if (fastpath(!Traits::isInterestingSelector(meth->name))) {
            return;
        }

        if (cls) {
            bool isMeta = as_objc_class(cls)->isMetaClass();
            if (isMeta && Domain != Scope::Instances) {
                if (cls == metaclassNSObject() && !isNSObjectSwizzled(isMeta)) {
                    setNSObjectSwizzled(cls, isMeta);
                }
            }
            if (!isMeta && Domain != Scope::Classes) {
                if (cls == classNSObject() && !isNSObjectSwizzled(isMeta)) {
                    setNSObjectSwizzled(cls, isMeta);
                }
            }
        } else {
            // We're called from method_exchangeImplementations, only NSObject
            // class and metaclass may be problematic (exchanging the default
            // builtin IMP of an interesting seleector, is a swizzling that,
            // may flip our scanned property. For other classes, the previous
            // value had already flipped the property).
            //
            // However, as we don't know the class, we need to scan all of
            // NSObject class and metaclass methods (this is SLOW).
            scanChangedMethodForUnknownClass(meth);
        }
    }
};

} // namespace scanner

// AWZ methods: +alloc / +allocWithZone:
struct AWZScanner : scanner::Mixin<AWZScanner, AWZ, PrintCustomAWZ, scanner::Scope::Classes> {
    static bool isCustom(Class cls) {
        return cls->hasCustomAWZ();
    }
    static void setCustom(Class cls) {
        cls->setHasCustomAWZ();
    }
    static void setDefault(Class cls) {
        cls->setHasDefaultAWZ();
    }
    static bool isInterestingSelector(SEL sel) {
        return sel == @selector(alloc) || sel == @selector(allocWithZone:);
    }
    static bool scanMethodLists(method_list_t * const *mlists, method_list_t * const *end) {
        SEL sels[2] = { @selector(alloc), @selector(allocWithZone:), };
        return method_lists_contains_any(mlists, end, sels, 2);
    }
};

// Retain/Release methods that are extremely rarely overridden
//
// retain/release/autorelease/retainCount/
// _tryRetain/_isDeallocating/retainWeakReference/allowsWeakReference
struct RRScanner : scanner::Mixin<RRScanner, RR, PrintCustomRR
#if !SUPPORT_NONPOINTER_ISA
, scanner::Scope::Instances
#endif
> {
    static bool isCustom(Class cls) {
        return cls->hasCustomRR();
    }
    static void setCustom(Class cls) {
        cls->setHasCustomRR();
    }
    static void setDefault(Class cls) {
        cls->setHasDefaultRR();
    }
    static bool isInterestingSelector(SEL sel) {
        return sel == @selector(retain) ||
               sel == @selector(release) ||
               sel == @selector(autorelease) ||
               sel == @selector(_tryRetain) ||
               sel == @selector(_isDeallocating) ||
               sel == @selector(retainCount) ||
               sel == @selector(allowsWeakReference) ||
               sel == @selector(retainWeakReference);
    }
    static bool scanMethodLists(method_list_t * const *mlists, method_list_t * const *end) {
        SEL sels[8] = {
            @selector(retain),
            @selector(release),
            @selector(autorelease),
            @selector(_tryRetain),
            @selector(_isDeallocating),
            @selector(retainCount),
            @selector(allowsWeakReference),
            @selector(retainWeakReference),
        };
        return method_lists_contains_any(mlists, end, sels, 8);
    }
};

// Core NSObject methods that are extremely rarely overridden
//
// +new, ±class, ±self, ±isKindOfClass:, ±respondsToSelector
struct CoreScanner : scanner::Mixin<CoreScanner, Core, PrintCustomCore> {
    static bool isCustom(Class cls) {
        return cls->hasCustomCore();
    }
    static void setCustom(Class cls) {
        cls->setHasCustomCore();
    }
    static void setDefault(Class cls) {
        cls->setHasDefaultCore();
    }
    static bool isInterestingSelector(SEL sel) {
        return sel == @selector(new) ||
               sel == @selector(self) ||
               sel == @selector(class) ||
               sel == @selector(isKindOfClass:) ||
               sel == @selector(respondsToSelector:);
    }
    static bool scanMethodLists(method_list_t * const *mlists, method_list_t * const *end) {
        SEL sels[5] = {
            @selector(new),
            @selector(self),
            @selector(class),
            @selector(isKindOfClass:),
            @selector(respondsToSelector:)
        };
        return method_lists_contains_any(mlists, end, sels, 5);
    }
};

class category_list : nocopy_t {
    union {
        locstamped_category_t lc;
        struct {
            locstamped_category_t *array;
            // this aliases with locstamped_category_t::hi
            // which is an aliased pointer
            uint32_t is_array :  1;
            uint32_t count    : 31;
            uint32_t size     : 32;
        };
    } _u;

public:
    category_list() : _u{{nullptr, nullptr}} { }
    category_list(locstamped_category_t lc) : _u{{lc}} { }
    category_list(category_list &&other) : category_list() {
        std::swap(_u, other._u);
    }
    ~category_list()
    {
        if (_u.is_array) {
            free(_u.array);
        }
    }

    uint32_t count() const
    {
        if (_u.is_array) return _u.count;
        return _u.lc.cat ? 1 : 0;
    }

    uint32_t arrayByteSize(uint32_t size) const
    {
        return sizeof(locstamped_category_t) * size;
    }

    const locstamped_category_t *array() const
    {
        return _u.is_array ? _u.array : &_u.lc;
    }

    void append(locstamped_category_t lc)
    {
        if (_u.is_array) {
            if (_u.count == _u.size) {
                // Have a typical malloc growth:
                // - size <=  8: grow by 2
                // - size <= 16: grow by 4
                // - size <= 32: grow by 8
                // ... etc
                _u.size += _u.size < 8 ? 2 : 1 << (fls(_u.size) - 2);
                _u.array = (locstamped_category_t *)reallocf(_u.array, arrayByteSize(_u.size));
            }
            _u.array[_u.count++] = lc;
        } else if (_u.lc.cat == NULL) {
            _u.lc = lc;
        } else {
            locstamped_category_t *arr = (locstamped_category_t *)malloc(arrayByteSize(2));
            arr[0] = _u.lc;
            arr[1] = lc;

            _u.array = arr;
            _u.is_array = true;
            _u.count = 2;
            _u.size = 2;
        }
    }

    void erase(category_t *cat)
    {
        if (_u.is_array) {
            for (int i = 0; i < _u.count; i++) {
                if (_u.array[i].cat == cat) {
                    // shift entries to preserve list order
                    memmove(&_u.array[i], &_u.array[i+1], arrayByteSize(_u.count - i - 1));
                    return;
                }
            }
        } else if (_u.lc.cat == cat) {
            _u.lc.cat = NULL;
            _u.lc.hi = NULL;
        }
    }
};

class UnattachedCategories : public ExplicitInitDenseMap<Class, category_list>
{
public:
    void addForClass(locstamped_category_t lc, Class cls)
    {
        runtimeLock.assertLocked();

        if (slowpath(PrintConnecting)) {
            _objc_inform("CLASS: found category %c%s(%s)",
                         cls->isMetaClass() ? '+' : '-',
                         cls->nameForLogging(), lc.cat->name);
        }

        auto result = get().try_emplace(cls, lc);
        if (!result.second) {
            result.first->second.append(lc);
        }
    }

    void attachToClass(Class cls, Class previously, int flags)
    {
        runtimeLock.assertLocked();
        ASSERT((flags & ATTACH_CLASS) ||
               (flags & ATTACH_METACLASS) ||
               (flags & ATTACH_CLASS_AND_METACLASS));

        auto &map = get();
        auto it = map.find(previously);

        if (it != map.end()) {
            category_list &list = it->second;
            if (flags & ATTACH_CLASS_AND_METACLASS) {
                int otherFlags = flags & ~ATTACH_CLASS_AND_METACLASS;
                attachCategories(cls, list.array(), list.count(), otherFlags | ATTACH_CLASS);
                attachCategories(cls->ISA(), list.array(), list.count(), otherFlags | ATTACH_METACLASS);
            } else {
                attachCategories(cls, list.array(), list.count(), flags);
            }
            map.erase(it);
        }
    }

    void eraseCategoryForClass(category_t *cat, Class cls)
    {
        runtimeLock.assertLocked();

        auto &map = get();
        auto it = map.find(cls);
        if (it != map.end()) {
            category_list &list = it->second;
            list.erase(cat);
            if (list.count() == 0) {
                map.erase(it);
            }
        }
    }

    void eraseClass(Class cls)
    {
        runtimeLock.assertLocked();

        get().erase(cls);
    }
};

static UnattachedCategories unattachedCategories;

} // namespace objc

static bool isBundleClass(Class cls)
{
    return cls->data()->ro()->flags & RO_FROM_BUNDLE;
}


static void 
fixupMethodList(method_list_t *mlist, bool bundleCopy, bool sort)
{
    runtimeLock.assertLocked();
    ASSERT(!mlist->isFixedUp());

    // fixme lock less in attachMethodLists ?
    // dyld3 may have already uniqued, but not sorted, the list
    if (!mlist->isUniqued()) {
        mutex_locker_t lock(selLock);
    
        // Unique selectors in list.
        for (auto& meth : *mlist) {
            const char *name = sel_cname(meth.name);
            meth.name = sel_registerNameNoLock(name, bundleCopy);
        }
    }

    // Sort by selector address.
    if (sort) {
        method_t::SortBySELAddress sorter;
        std::stable_sort(mlist->begin(), mlist->end(), sorter);
    }
    
    // Mark method list as uniqued and sorted
    mlist->setFixedUp();
}


static void 
prepareMethodLists(Class cls, method_list_t **addedLists, int addedCount,
                   bool baseMethods, bool methodsFromBundle)
{
    runtimeLock.assertLocked();

    if (addedCount == 0) return;

    // There exist RR/AWZ/Core special cases for some class's base methods.
    // But this code should never need to scan base methods for RR/AWZ/Core:
    // default RR/AWZ/Core cannot be set before setInitialized().
    // Therefore we need not handle any special cases here.
    if (baseMethods) {
        ASSERT(cls->hasCustomAWZ() && cls->hasCustomRR() && cls->hasCustomCore());
    }

    // Add method lists to array.
    // Reallocate un-fixed method lists.
    // The new methods are PREPENDED to the method list array.

    for (int i = 0; i < addedCount; i++) {
        method_list_t *mlist = addedLists[i];
        ASSERT(mlist);

        // Fixup selectors if necessary
        if (!mlist->isFixedUp()) {
            fixupMethodList(mlist, methodsFromBundle, true/*sort*/);
        }
    }

    // If the class is initialized, then scan for method implementations
    // tracked by the class's flags. If it's not initialized yet,
    // then objc_class::setInitialized() will take care of it.
    if (cls->isInitialized()) {
        objc::AWZScanner::scanAddedMethodLists(cls, addedLists, addedCount);
        objc::RRScanner::scanAddedMethodLists(cls, addedLists, addedCount);
        objc::CoreScanner::scanAddedMethodLists(cls, addedLists, addedCount);
    }
}

class_rw_ext_t *
class_rw_t::extAlloc(const class_ro_t *ro, bool deepCopy)
{
    runtimeLock.assertLocked();

    auto rwe = objc::zalloc<class_rw_ext_t>();

    rwe->version = (ro->flags & RO_META) ? 7 : 0;

    method_list_t *list = ro->baseMethods();
    if (list) {
        if (deepCopy) list = list->duplicate();
        rwe->methods.attachLists(&list, 1);
    }

    // See comments in objc_duplicateClass
    // property lists and protocol lists historically
    // have not been deep-copied
    //
    // This is probably wrong and ought to be fixed some day
    property_list_t *proplist = ro->baseProperties;
    if (proplist) {
        rwe->properties.attachLists(&proplist, 1);
    }

    protocol_list_t *protolist = ro->baseProtocols;
    if (protolist) {
        rwe->protocols.attachLists(&protolist, 1);
    }

    set_ro_or_rwe(rwe, ro);
    return rwe;
}

// Attach method lists and properties and protocols from categories to a class.
// Assumes the categories in cats are all loaded and sorted by load order, 
// oldest categories first.
static void
attachCategories(Class cls, const locstamped_category_t *cats_list, uint32_t cats_count,
                 int flags)
{
    if (slowpath(PrintReplacedMethods)) {
        printReplacements(cls, cats_list, cats_count);
    }
    if (slowpath(PrintConnecting)) {
        _objc_inform("CLASS: attaching %d categories to%s class '%s'%s",
                     cats_count, (flags & ATTACH_EXISTING) ? " existing" : "",
                     cls->nameForLogging(), (flags & ATTACH_METACLASS) ? " (meta)" : "");
    }

    /*
     * Only a few classes have more than 64 categories during launch.
     * This uses a little stack, and avoids malloc.
     *
     * Categories must be added in the proper order, which is back
     * to front. To do that with the chunking, we iterate cats_list
     * from front to back, build up the local buffers backwards,
     * and call attachLists on the chunks. attachLists prepends the
     * lists, so the final result is in the expected order.
     */
    constexpr uint32_t ATTACH_BUFSIZ = 64;
    method_list_t   *mlists[ATTACH_BUFSIZ];
    property_list_t *proplists[ATTACH_BUFSIZ];
    protocol_list_t *protolists[ATTACH_BUFSIZ];

    uint32_t mcount = 0;
    uint32_t propcount = 0;
    uint32_t protocount = 0;
    bool fromBundle = NO;
    bool isMeta = (flags & ATTACH_METACLASS);
    auto rwe = cls->data()->extAllocIfNeeded();

    for (uint32_t i = 0; i < cats_count; i++) {
        auto& entry = cats_list[i];

        method_list_t *mlist = entry.cat->methodsForMeta(isMeta);
        if (mlist) {
            if (mcount == ATTACH_BUFSIZ) {
                prepareMethodLists(cls, mlists, mcount, NO, fromBundle);
                rwe->methods.attachLists(mlists, mcount);
                mcount = 0;
            }
            mlists[ATTACH_BUFSIZ - ++mcount] = mlist;
            fromBundle |= entry.hi->isBundle();
        }

        property_list_t *proplist =
            entry.cat->propertiesForMeta(isMeta, entry.hi);
        if (proplist) {
            if (propcount == ATTACH_BUFSIZ) {
                rwe->properties.attachLists(proplists, propcount);
                propcount = 0;
            }
            proplists[ATTACH_BUFSIZ - ++propcount] = proplist;
        }

        protocol_list_t *protolist = entry.cat->protocolsForMeta(isMeta);
        if (protolist) {
            if (protocount == ATTACH_BUFSIZ) {
                rwe->protocols.attachLists(protolists, protocount);
                protocount = 0;
            }
            protolists[ATTACH_BUFSIZ - ++protocount] = protolist;
        }
    }

    if (mcount > 0) {
        prepareMethodLists(cls, mlists + ATTACH_BUFSIZ - mcount, mcount, NO, fromBundle);
        rwe->methods.attachLists(mlists + ATTACH_BUFSIZ - mcount, mcount);
        if (flags & ATTACH_EXISTING) flushCaches(cls);
    }

    rwe->properties.attachLists(proplists + ATTACH_BUFSIZ - propcount, propcount);

    rwe->protocols.attachLists(protolists + ATTACH_BUFSIZ - protocount, protocount);
}


/***********************************************************************
* methodizeClass
* Fixes up cls's method list, protocol list, and property list.
* Attaches any outstanding categories.
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static void methodizeClass(Class cls, Class previously)
{
    runtimeLock.assertLocked();

    bool isMeta = cls->isMetaClass();
    auto rw = cls->data();
    auto ro = rw->ro();
    auto rwe = rw->ext();

    // Methodizing for the first time
    if (PrintConnecting) {
        _objc_inform("CLASS: methodizing class '%s' %s", 
                     cls->nameForLogging(), isMeta ? "(meta)" : "");
    }

    // Install methods and properties that the class implements itself.
    method_list_t *list = ro->baseMethods();
    if (list) {
        prepareMethodLists(cls, &list, 1, YES, isBundleClass(cls));
        if (rwe) rwe->methods.attachLists(&list, 1);
    }

    property_list_t *proplist = ro->baseProperties;
    if (rwe && proplist) {
        rwe->properties.attachLists(&proplist, 1);
    }

    protocol_list_t *protolist = ro->baseProtocols;
    if (rwe && protolist) {
        rwe->protocols.attachLists(&protolist, 1);
    }

    // Root classes get bonus method implementations if they don't have 
    // them already. These apply before category replacements.
    if (cls->isRootMetaclass()) {
        // root metaclass
        addMethod(cls, @selector(initialize), (IMP)&objc_noop_imp, "", NO);
    }

    // Attach categories.
    if (previously) {
        if (isMeta) {
            objc::unattachedCategories.attachToClass(cls, previously,
                                                     ATTACH_METACLASS);
        } else {
            // When a class relocates, categories with class methods
            // may be registered on the class itself rather than on
            // the metaclass. Tell attachToClass to look for those.
            objc::unattachedCategories.attachToClass(cls, previously,
                                                     ATTACH_CLASS_AND_METACLASS);
        }
    }
    objc::unattachedCategories.attachToClass(cls, cls,
                                             isMeta ? ATTACH_METACLASS : ATTACH_CLASS);

#if DEBUG
    // Debug: sanity-check all SELs; log method list contents
    for (const auto& meth : rw->methods()) {
        if (PrintConnecting) {
            _objc_inform("METHOD %c[%s %s]", isMeta ? '+' : '-', 
                         cls->nameForLogging(), sel_getName(meth.name));
        }
        ASSERT(sel_registerName(sel_getName(meth.name)) == meth.name); 
    }
#endif
}


/***********************************************************************
* nonMetaClasses
* Returns the secondary metaclass => class map
* Used for some cases of +initialize and +resolveClassMethod:.
* This map does not contain all class and metaclass pairs. It only 
* contains metaclasses whose classes would be in the runtime-allocated 
* named-class table, but are not because some other class with the same name 
* is in that table.
* Classes with no duplicates are not included.
* Classes in the preoptimized named-class table are not included.
* Classes whose duplicates are in the preoptimized table are not included.
* Most code should use getMaybeUnrealizedNonMetaClass() 
* instead of reading this table.
* Locking: runtimeLock must be read- or write-locked by the caller
**********************************************************************/
static NXMapTable *nonmeta_class_map = nil;
static NXMapTable *nonMetaClasses(void)
{
    runtimeLock.assertLocked();

    if (nonmeta_class_map) return nonmeta_class_map;

    // nonmeta_class_map is typically small
    INIT_ONCE_PTR(nonmeta_class_map, 
                  NXCreateMapTable(NXPtrValueMapPrototype, 32), 
                  NXFreeMapTable(v));

    return nonmeta_class_map;
}


/***********************************************************************
* addNonMetaClass
* Adds metacls => cls to the secondary metaclass map
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static void addNonMetaClass(Class cls)
{
    runtimeLock.assertLocked();
    void *old;
    old = NXMapInsert(nonMetaClasses(), cls->ISA(), cls);

    ASSERT(!cls->isMetaClassMaybeUnrealized());
    ASSERT(cls->ISA()->isMetaClassMaybeUnrealized());
    ASSERT(!old);
}


static void removeNonMetaClass(Class cls)
{
    runtimeLock.assertLocked();
    NXMapRemove(nonMetaClasses(), cls->ISA());
}


static bool scanMangledField(const char *&string, const char *end, 
                             const char *&field, int& length)
{
    // Leading zero not allowed.
    if (*string == '0') return false;

    length = 0;
    field = string;
    while (field < end) {
        char c = *field;
        if (!isdigit(c)) break;
        field++;
        if (__builtin_smul_overflow(length, 10, &length)) return false;
        if (__builtin_sadd_overflow(length, c - '0', &length)) return false;
    }

    string = field + length;
    return length > 0  &&  string <= end;
}


/***********************************************************************
* copySwiftV1DemangledName
* Returns the pretty form of the given Swift-v1-mangled class or protocol name. 
* Returns nil if the string doesn't look like a mangled Swift v1 name.
* The result must be freed with free().
**********************************************************************/
static char *copySwiftV1DemangledName(const char *string, bool isProtocol = false)
{
    if (!string) return nil;

    // Swift mangling prefix.
    if (strncmp(string, isProtocol ? "_TtP" : "_TtC", 4) != 0) return nil;
    string += 4;

    const char *end = string + strlen(string);

    // Module name.
    const char *prefix;
    int prefixLength;
    if (string[0] == 's') {
        // "s" is the Swift module.
        prefix = "Swift";
        prefixLength = 5;
        string += 1;
    } else {
        if (! scanMangledField(string, end, prefix, prefixLength)) return nil;
    }

    // Class or protocol name.
    const char *suffix;
    int suffixLength;
    if (! scanMangledField(string, end, suffix, suffixLength)) return nil;

    if (isProtocol) {
        // Remainder must be "_".
        if (strcmp(string, "_") != 0) return nil;
    } else {
        // Remainder must be empty.
        if (string != end) return nil;
    }

    char *result;
    asprintf(&result, "%.*s.%.*s", prefixLength,prefix, suffixLength,suffix);
    return result;
}


/***********************************************************************
* copySwiftV1MangledName
* Returns the Swift 1.0 mangled form of the given class or protocol name. 
* Returns nil if the string doesn't look like an unmangled Swift name.
* The result must be freed with free().
**********************************************************************/
static char *copySwiftV1MangledName(const char *string, bool isProtocol = false)
{
    if (!string) return nil;

    size_t dotCount = 0;
    size_t dotIndex;
    const char *s;
    for (s = string; *s; s++) {
        if (*s == '.') {
            dotCount++;
            dotIndex = s - string;
        }
    }
    size_t stringLength = s - string;

    if (dotCount != 1  ||  dotIndex == 0  ||  dotIndex >= stringLength-1) {
        return nil;
    }
    
    const char *prefix = string;
    size_t prefixLength = dotIndex;
    const char *suffix = string + dotIndex + 1;
    size_t suffixLength = stringLength - (dotIndex + 1);
    
    char *name;

    if (prefixLength == 5  &&  memcmp(prefix, "Swift", 5) == 0) {
        asprintf(&name, "_Tt%cs%zu%.*s%s", 
                 isProtocol ? 'P' : 'C', 
                 suffixLength, (int)suffixLength, suffix, 
                 isProtocol ? "_" : "");
    } else {
        asprintf(&name, "_Tt%c%zu%.*s%zu%.*s%s", 
                 isProtocol ? 'P' : 'C', 
                 prefixLength, (int)prefixLength, prefix, 
                 suffixLength, (int)suffixLength, suffix, 
                 isProtocol ? "_" : "");
    }
    return name;
}


/***********************************************************************
* getClassExceptSomeSwift
* Looks up a class by name. The class MIGHT NOT be realized.
* Demangled Swift names are recognized.
* Classes known to the Swift runtime but not yet used are NOT recognized.
*   (such as subclasses of un-instantiated generics)
* Use look_up_class() to find them as well.
* Locking: runtimeLock must be read- or write-locked by the caller.
**********************************************************************/

// This is a misnomer: gdb_objc_realized_classes is actually a list of 
// named classes not in the dyld shared cache, whether realized or not.
NXMapTable *gdb_objc_realized_classes;  // exported for debuggers in objc-gdb.h
uintptr_t objc_debug_realized_class_generation_count;

static Class getClass_impl(const char *name)
{
    runtimeLock.assertLocked();

    // allocated in _read_images
    ASSERT(gdb_objc_realized_classes);

    // Try runtime-allocated table
    Class result = (Class)NXMapGet(gdb_objc_realized_classes, name);
    if (result) return result;

    // Try table from dyld shared cache.
    // Note we do this last to handle the case where we dlopen'ed a shared cache
    // dylib with duplicates of classes already present in the main executable.
    // In that case, we put the class from the main executable in
    // gdb_objc_realized_classes and want to check that before considering any
    // newly loaded shared cache binaries.
    return getPreoptimizedClass(name);
}

static Class getClassExceptSomeSwift(const char *name)
{
    runtimeLock.assertLocked();

    // Try name as-is
    Class result = getClass_impl(name);
    if (result) return result;

    // Try Swift-mangled equivalent of the given name.
    if (char *swName = copySwiftV1MangledName(name)) {
        result = getClass_impl(swName);
        free(swName);
        return result;
    }

    return nil;
}


/***********************************************************************
* addNamedClass
* Adds name => cls to the named non-meta class map.
* Warns about duplicate class names and keeps the old mapping.
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static void addNamedClass(Class cls, const char *name, Class replacing = nil)
{
    runtimeLock.assertLocked();
    Class old;
    if ((old = getClassExceptSomeSwift(name))  &&  old != replacing) {
        inform_duplicate(name, old, cls);

        // getMaybeUnrealizedNonMetaClass uses name lookups.
        // Classes not found by name lookup must be in the
        // secondary meta->nonmeta table.
        addNonMetaClass(cls);
    } else {
        NXMapInsert(gdb_objc_realized_classes, name, cls);
    }
    ASSERT(!(cls->data()->flags & RO_META));

    // wrong: constructed classes are already realized when they get here
    // ASSERT(!cls->isRealized());
}


/***********************************************************************
* removeNamedClass
* Removes cls from the name => cls map.
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static void removeNamedClass(Class cls, const char *name)
{
    runtimeLock.assertLocked();
    ASSERT(!(cls->data()->flags & RO_META));
    if (cls == NXMapGet(gdb_objc_realized_classes, name)) {
        NXMapRemove(gdb_objc_realized_classes, name);
    } else {
        // cls has a name collision with another class - don't remove the other
        // but do remove cls from the secondary metaclass->class map.
        removeNonMetaClass(cls);
    }
}


/***********************************************************************
* futureNamedClasses
* Returns the classname => future class map for unrealized future classes.
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static NXMapTable *future_named_class_map = nil;
static NXMapTable *futureNamedClasses()
{
    runtimeLock.assertLocked();
    
    if (future_named_class_map) return future_named_class_map;

    // future_named_class_map is big enough for CF's classes and a few others
    future_named_class_map = 
        NXCreateMapTable(NXStrValueMapPrototype, 32);

    return future_named_class_map;
}


static bool haveFutureNamedClasses() {
    return future_named_class_map  &&  NXCountMapTable(future_named_class_map);
}


/***********************************************************************
* addFutureNamedClass
* Installs cls as the class structure to use for the named class if it appears.
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static void addFutureNamedClass(const char *name, Class cls)
{
    void *old;

    runtimeLock.assertLocked();

    if (PrintFuture) {
        _objc_inform("FUTURE: reserving %p for %s", (void*)cls, name);
    }

    class_rw_t *rw = objc::zalloc<class_rw_t>();
    class_ro_t *ro = (class_ro_t *)calloc(sizeof(class_ro_t), 1);
    ro->name = strdupIfMutable(name);
    rw->set_ro(ro);
    cls->setData(rw);
    cls->data()->flags = RO_FUTURE;

    old = NXMapKeyCopyingInsert(futureNamedClasses(), name, cls);
    ASSERT(!old);
}


/***********************************************************************
* popFutureNamedClass
* Removes the named class from the unrealized future class list, 
* because it has been realized.
* Returns nil if the name is not used by a future class.
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static Class popFutureNamedClass(const char *name)
{
    runtimeLock.assertLocked();

    Class cls = nil;

    if (future_named_class_map) {
        cls = (Class)NXMapKeyFreeingRemove(future_named_class_map, name);
        if (cls && NXCountMapTable(future_named_class_map) == 0) {
            NXFreeMapTable(future_named_class_map);
            future_named_class_map = nil;
        }
    }

    return cls;
}


/***********************************************************************
* remappedClasses
* Returns the oldClass => newClass map for realized future classes.
* Returns the oldClass => nil map for ignored weak-linked classes.
* Locking: runtimeLock must be read- or write-locked by the caller
**********************************************************************/
static objc::DenseMap<Class, Class> *remappedClasses(bool create)
{
    static objc::LazyInitDenseMap<Class, Class> remapped_class_map;

    runtimeLock.assertLocked();

    // start big enough to hold CF's classes and a few others
    return remapped_class_map.get(create, 32);
}


/***********************************************************************
* noClassesRemapped
* Returns YES if no classes have been remapped
* Locking: runtimeLock must be read- or write-locked by the caller
**********************************************************************/
static bool noClassesRemapped(void)
{
    runtimeLock.assertLocked();

    bool result = (remappedClasses(NO) == nil);
#if DEBUG
    // Catch construction of an empty table, which defeats optimization.
    auto *map = remappedClasses(NO);
    if (map) ASSERT(map->size() > 0);
#endif
    return result;
}


/***********************************************************************
* addRemappedClass
* newcls is a realized future class, replacing oldcls.
* OR newcls is nil, replacing ignored weak-linked class oldcls.
* Locking: runtimeLock must be write-locked by the caller
**********************************************************************/
static void addRemappedClass(Class oldcls, Class newcls)
{
    runtimeLock.assertLocked();

    if (PrintFuture) {
        _objc_inform("FUTURE: using %p instead of %p for %s", 
                     (void*)newcls, (void*)oldcls, oldcls->nameForLogging());
    }

    auto result = remappedClasses(YES)->insert({ oldcls, newcls });
#if DEBUG
    if (!std::get<1>(result)) {
        // An existing mapping was overwritten. This is not allowed
        // unless it was to nil.
        auto iterator = std::get<0>(result);
        auto value = std::get<1>(*iterator);
        ASSERT(value == nil);
    }
#else
    (void)result;
#endif
}


/***********************************************************************
* remapClass
* Returns the live class pointer for cls, which may be pointing to 
* a class struct that has been reallocated.
* Returns nil if cls is ignored because of weak linking.
* Locking: runtimeLock must be read- or write-locked by the caller
**********************************************************************/
static Class remapClass(Class cls)
{
    runtimeLock.assertLocked();

    if (!cls) return nil;

    auto *map = remappedClasses(NO);
    if (!map)
        return cls;
    
    auto iterator = map->find(cls);
    if (iterator == map->end())
        return cls;
    return std::get<1>(*iterator);
}

static Class remapClass(classref_t cls)
{
    return remapClass((Class)cls);
}

Class _class_remap(Class cls)
{
    mutex_locker_t lock(runtimeLock);
    return remapClass(cls);
}

/***********************************************************************
* remapClassRef
* Fix up a class ref, in case the class referenced has been reallocated 
* or is an ignored weak-linked class.
* Locking: runtimeLock must be read- or write-locked by the caller
**********************************************************************/
static void remapClassRef(Class *clsref)
{
    runtimeLock.assertLocked();

    Class newcls = remapClass(*clsref);    
    if (*clsref != newcls) *clsref = newcls;
}


_Nullable Class
objc_loadClassref(_Nullable Class * _Nonnull clsref)
{
    auto *atomicClsref = explicit_atomic<uintptr_t>::from_pointer((uintptr_t *)clsref);
    
    uintptr_t cls = atomicClsref->load(std::memory_order_relaxed);
    if (fastpath((cls & 1) == 0))
        return (Class)cls;
    
    auto stub = (stub_class_t *)(cls & ~1ULL);
    Class initialized = stub->initializer((Class)stub, nil);
    atomicClsref->store((uintptr_t)initialized, std::memory_order_relaxed);
    return initialized;
}


/***********************************************************************
* getMaybeUnrealizedNonMetaClass
* Return the ordinary class for this class or metaclass. 
* `inst` is an instance of `cls` or a subclass thereof, or nil. 
* Non-nil inst is faster.
* The result may be unrealized.
* Used by +initialize. 
* Locking: runtimeLock must be read- or write-locked by the caller
**********************************************************************/
static Class getMaybeUnrealizedNonMetaClass(Class metacls, id inst)
{
    static int total, named, secondary, sharedcache, dyld3;
    runtimeLock.assertLocked();
    ASSERT(metacls->isRealized());

    total++;

    // return cls itself if it's already a non-meta class
    if (!metacls->isMetaClass()) return metacls;

    // metacls really is a metaclass
    // which means inst (if any) is a class

    // special case for root metaclass
    // where inst == inst->ISA() == metacls is possible
    if (metacls->ISA() == metacls) {
        Class cls = metacls->superclass;
        ASSERT(cls->isRealized());
        ASSERT(!cls->isMetaClass());
        ASSERT(cls->ISA() == metacls);
        if (cls->ISA() == metacls) return cls;
    }

    // use inst if available
    if (inst) {
        Class cls = remapClass((Class)inst);
        // cls may be a subclass - find the real class for metacls
        // fixme this probably stops working once Swift starts
        // reallocating classes if cls is unrealized.
        while (cls) {
            if (cls->ISA() == metacls) {
                ASSERT(!cls->isMetaClassMaybeUnrealized());
                return cls;
            }
            cls = cls->superclass;
        }
#if DEBUG
        _objc_fatal("cls is not an instance of metacls");
#else
        // release build: be forgiving and fall through to slow lookups
#endif
    }

    // try name lookup
    {
        Class cls = getClassExceptSomeSwift(metacls->mangledName());
        if (cls && cls->ISA() == metacls) {
            named++;
            if (PrintInitializing) {
                _objc_inform("INITIALIZE: %d/%d (%g%%) "
                             "successful by-name metaclass lookups",
                             named, total, named*100.0/total);
            }
            return cls;
        }
    }

    // try secondary table
    {
        Class cls = (Class)NXMapGet(nonMetaClasses(), metacls);
        if (cls) {
            secondary++;
            if (PrintInitializing) {
                _objc_inform("INITIALIZE: %d/%d (%g%%) "
                             "successful secondary metaclass lookups",
                             secondary, total, secondary*100.0/total);
            }

            ASSERT(cls->ISA() == metacls);            
            return cls;
        }
    }

    // try the dyld closure table
    if (isPreoptimized())
    {
        // Try table from dyld closure first.  It was built to ignore the dupes it
        // knows will come from the cache, so anything left in here was there when
        // we launched
        Class cls = nil;
        // Note, we have to pass the lambda directly here as otherwise we would try
        // message copy and autorelease.
        _dyld_for_each_objc_class(metacls->mangledName(),
                                  [&cls, metacls](void* classPtr, bool isLoaded, bool* stop) {
          // Skip images which aren't loaded.  This supports the case where dyld
          // might soft link an image from the main binary so its possibly not
          // loaded yet.
          if (!isLoaded)
            return;

          // Found a loaded image with this class name, so check if its the right one
          Class result = (Class)classPtr;
          if (result->ISA() == metacls) {
              cls = result;
              *stop = true;
          }
        });

        if (cls) {
            dyld3++;
            if (PrintInitializing) {
                _objc_inform("INITIALIZE: %d/%d (%g%%) "
                             "successful dyld closure metaclass lookups",
                             dyld3, total, dyld3*100.0/total);
            }

            return cls;
        }
    }

    // try any duplicates in the dyld shared cache
    {
        Class cls = nil;

        int count;
        Class *classes = copyPreoptimizedClasses(metacls->mangledName(),&count);
        if (classes) {
            for (int i = 0; i < count; i++) {
                if (classes[i]->ISA() == metacls) {
                    cls = classes[i];
                    break;
                }
            }
            free(classes);
        }

        if (cls) {
            sharedcache++;
            if (PrintInitializing) {
                _objc_inform("INITIALIZE: %d/%d (%g%%) "
                             "successful shared cache metaclass lookups",
                             sharedcache, total, sharedcache*100.0/total);
            }

            return cls;
        }
    }

    _objc_fatal("no class for metaclass %p", (void*)metacls);
}


/***********************************************************************
* class_initialize.  Send the '+initialize' message on demand to any
* uninitialized class. Force initialization of superclasses first.
* inst is an instance of cls, or nil. Non-nil is better for performance.
* Returns the class pointer. If the class was unrealized then 
* it may be reallocated.
* Locking: 
*   runtimeLock must be held by the caller
*   This function may drop the lock.
*   On exit the lock is re-acquired or dropped as requested by leaveLocked.
**********************************************************************/
static Class initializeAndMaybeRelock(Class cls, id inst,
                                      mutex_t& lock, bool leaveLocked)
{
    lock.assertLocked();
    ASSERT(cls->isRealized());

    if (cls->isInitialized()) {
        if (!leaveLocked) lock.unlock();
        return cls;
    }

    // Find the non-meta class for cls, if it is not already one.
    // The +initialize message is sent to the non-meta class object.
    Class nonmeta = getMaybeUnrealizedNonMetaClass(cls, inst);

    // Realize the non-meta class if necessary.
    if (nonmeta->isRealized()) {
        // nonmeta is cls, which was already realized
        // OR nonmeta is distinct, but is already realized
        // - nothing else to do
        lock.unlock();
    } else {
        nonmeta = realizeClassMaybeSwiftAndUnlock(nonmeta, lock);
        // runtimeLock is now unlocked
        // fixme Swift can't relocate the class today,
        // but someday it will:
        cls = object_getClass(nonmeta);
    }

    // runtimeLock is now unlocked, for +initialize dispatch
    ASSERT(nonmeta->isRealized());
    initializeNonMetaClass(nonmeta);

    if (leaveLocked) runtimeLock.lock();
    return cls;
}

// Locking: acquires runtimeLock
Class class_initialize(Class cls, id obj)
{
    runtimeLock.lock();
    return initializeAndMaybeRelock(cls, obj, runtimeLock, false);
}

// Locking: caller must hold runtimeLock; this may drop and re-acquire it
static Class initializeAndLeaveLocked(Class cls, id obj, mutex_t& lock)
{
    return initializeAndMaybeRelock(cls, obj, lock, true);
}


/***********************************************************************
* addRootClass
* Adds cls as a new realized root class.
* Locking: runtimeLock must be held by the caller.
**********************************************************************/
static void addRootClass(Class cls)
{
    runtimeLock.assertLocked();

    ASSERT(cls->isRealized());

    objc_debug_realized_class_generation_count++;
    
    cls->data()->nextSiblingClass = _firstRealizedClass;
    _firstRealizedClass = cls;
}

static void removeRootClass(Class cls)
{
    runtimeLock.assertLocked();

    objc_debug_realized_class_generation_count++;
    
    Class *classp;
    for (classp = &_firstRealizedClass; 
         *classp != cls; 
         classp = &(*classp)->data()->nextSiblingClass)
    { }
    
    *classp = (*classp)->data()->nextSiblingClass;
}


/***********************************************************************
* addSubclass
* Adds subcls as a subclass of supercls.
* Locking: runtimeLock must be held by the caller.
**********************************************************************/
static void addSubclass(Class supercls, Class subcls)
{
    runtimeLock.assertLocked();

    if (supercls  &&  subcls) {
        ASSERT(supercls->isRealized());
        ASSERT(subcls->isRealized());

        objc_debug_realized_class_generation_count++;
        
        subcls->data()->nextSiblingClass = supercls->data()->firstSubclass;
        supercls->data()->firstSubclass = subcls;

        if (supercls->hasCxxCtor()) {
            subcls->setHasCxxCtor();
        }

        if (supercls->hasCxxDtor()) {
            subcls->setHasCxxDtor();
        }

        objc::AWZScanner::scanAddedSubClass(subcls, supercls);
        objc::RRScanner::scanAddedSubClass(subcls, supercls);
        objc::CoreScanner::scanAddedSubClass(subcls, supercls);

        // Special case: instancesRequireRawIsa does not propagate
        // from root class to root metaclass
        if (supercls->instancesRequireRawIsa()  &&  supercls->superclass) {
            subcls->setInstancesRequireRawIsaRecursively(true);
        }
    }
}


/***********************************************************************
* removeSubclass
* Removes subcls as a subclass of supercls.
* Locking: runtimeLock must be held by the caller.
**********************************************************************/
static void removeSubclass(Class supercls, Class subcls)
{
    runtimeLock.assertLocked();
    ASSERT(supercls->isRealized());
    ASSERT(subcls->isRealized());
    ASSERT(subcls->superclass == supercls);

    objc_debug_realized_class_generation_count++;
    
    Class *cp;
    for (cp = &supercls->data()->firstSubclass; 
         *cp  &&  *cp != subcls; 
         cp = &(*cp)->data()->nextSiblingClass)
        ;
    ASSERT(*cp == subcls);
    *cp = subcls->data()->nextSiblingClass;
}



/***********************************************************************
* protocols
* Returns the protocol name => protocol map for protocols.
* Locking: runtimeLock must read- or write-locked by the caller
**********************************************************************/
static NXMapTable *protocols(void)
{
    static NXMapTable *protocol_map = nil;
    
    runtimeLock.assertLocked();

    INIT_ONCE_PTR(protocol_map, 
                  NXCreateMapTable(NXStrValueMapPrototype, 16), 
                  NXFreeMapTable(v) );

    return protocol_map;
}


/***********************************************************************
* getProtocol
* Looks up a protocol by name. Demangled Swift names are recognized.
* Locking: runtimeLock must be read- or write-locked by the caller.
**********************************************************************/
static NEVER_INLINE Protocol *getProtocol(const char *name)
{
    runtimeLock.assertLocked();

    // Try name as-is.
    Protocol *result = (Protocol *)NXMapGet(protocols(), name);
    if (result) return result;

    // Try Swift-mangled equivalent of the given name.
    if (char *swName = copySwiftV1MangledName(name, true/*isProtocol*/)) {
        result = (Protocol *)NXMapGet(protocols(), swName);
        free(swName);
        if (result) return result;
    }

    // Try table from dyld shared cache
    // Temporarily check that we are using the new table.  Eventually this check
    // will always be true.
    // FIXME: Remove this check when we can
    if (sharedCacheSupportsProtocolRoots()) {
        result = getPreoptimizedProtocol(name);
        if (result) return result;
    }

    return nil;
}


/***********************************************************************
* remapProtocol
* Returns the live protocol pointer for proto, which may be pointing to 
* a protocol struct that has been reallocated.
* Locking: runtimeLock must be read- or write-locked by the caller
**********************************************************************/
static ALWAYS_INLINE protocol_t *remapProtocol(protocol_ref_t proto)
{
    runtimeLock.assertLocked();

    // Protocols in shared cache images have a canonical bit to mark that they
    // are the definition we should use
    if (((protocol_t *)proto)->isCanonical())
        return (protocol_t *)proto;

    protocol_t *newproto = (protocol_t *)
        getProtocol(((protocol_t *)proto)->mangledName);
    return newproto ? newproto : (protocol_t *)proto;
}


/***********************************************************************
* remapProtocolRef
* Fix up a protocol ref, in case the protocol referenced has been reallocated.
* Locking: runtimeLock must be read- or write-locked by the caller
**********************************************************************/
static size_t UnfixedProtocolReferences;
static void remapProtocolRef(protocol_t **protoref)
{
    runtimeLock.assertLocked();

    protocol_t *newproto = remapProtocol((protocol_ref_t)*protoref);
    if (*protoref != newproto) {
        *protoref = newproto;
        UnfixedProtocolReferences++;
    }
}


/***********************************************************************
* moveIvars
* Slides a class's ivars to accommodate the given superclass size.
* Ivars are NOT compacted to compensate for a superclass that shrunk.
* Locking: runtimeLock must be held by the caller.
**********************************************************************/
static void moveIvars(class_ro_t *ro, uint32_t superSize)
{
    runtimeLock.assertLocked();

    uint32_t diff;

    ASSERT(superSize > ro->instanceStart);
    diff = superSize - ro->instanceStart;

    if (ro->ivars) {
        // Find maximum alignment in this class's ivars
        uint32_t maxAlignment = 1;
        for (const auto& ivar : *ro->ivars) {
            if (!ivar.offset) continue;  // anonymous bitfield

            uint32_t alignment = ivar.alignment();
            if (alignment > maxAlignment) maxAlignment = alignment;
        }

        // Compute a slide value that preserves that alignment
        uint32_t alignMask = maxAlignment - 1;
        diff = (diff + alignMask) & ~alignMask;

        // Slide all of this class's ivars en masse
        for (const auto& ivar : *ro->ivars) {
            if (!ivar.offset) continue;  // anonymous bitfield

            uint32_t oldOffset = (uint32_t)*ivar.offset;
            uint32_t newOffset = oldOffset + diff;
            *ivar.offset = newOffset;

            if (PrintIvars) {
                _objc_inform("IVARS:    offset %u -> %u for %s "
                             "(size %u, align %u)", 
                             oldOffset, newOffset, ivar.name, 
                             ivar.size, ivar.alignment());
            }
        }
    }

    *(uint32_t *)&ro->instanceStart += diff;
    *(uint32_t *)&ro->instanceSize += diff;
}


static void reconcileInstanceVariables(Class cls, Class supercls, const class_ro_t*& ro) 
{
    class_rw_t *rw = cls->data();

    ASSERT(supercls);
    ASSERT(!cls->isMetaClass());

    /* debug: print them all before sliding
    if (ro->ivars) {
        for (const auto& ivar : *ro->ivars) {
            if (!ivar.offset) continue;  // anonymous bitfield

            _objc_inform("IVARS: %s.%s (offset %u, size %u, align %u)", 
                         ro->name, ivar.name, 
                         *ivar.offset, ivar.size, ivar.alignment());
        }
    }
    */

    // Non-fragile ivars - reconcile this class with its superclass
    const class_ro_t *super_ro = supercls->data()->ro();
    
    if (DebugNonFragileIvars) {
        // Debugging: Force non-fragile ivars to slide.
        // Intended to find compiler, runtime, and program bugs.
        // If it fails with this and works without, you have a problem.
        
        // Operation: Reset everything to 0 + misalignment. 
        // Then force the normal sliding logic to push everything back.
        
        // Exceptions: root classes, metaclasses, *NSCF* classes, 
        // __CF* classes, NSConstantString, NSSimpleCString
        
        // (already know it's not root because supercls != nil)
        const char *clsname = cls->mangledName();
        if (!strstr(clsname, "NSCF")  &&  
            0 != strncmp(clsname, "__CF", 4)  &&  
            0 != strcmp(clsname, "NSConstantString")  &&  
            0 != strcmp(clsname, "NSSimpleCString")) 
        {
            uint32_t oldStart = ro->instanceStart;
            class_ro_t *ro_w = make_ro_writeable(rw);
            ro = rw->ro();
            
            // Find max ivar alignment in class.
            // default to word size to simplify ivar update
            uint32_t alignment = 1<<WORD_SHIFT;
            if (ro->ivars) {
                for (const auto& ivar : *ro->ivars) {
                    if (ivar.alignment() > alignment) {
                        alignment = ivar.alignment();
                    }
                }
            }
            uint32_t misalignment = ro->instanceStart % alignment;
            uint32_t delta = ro->instanceStart - misalignment;
            ro_w->instanceStart = misalignment;
            ro_w->instanceSize -= delta;
            
            if (PrintIvars) {
                _objc_inform("IVARS: DEBUG: forcing ivars for class '%s' "
                             "to slide (instanceStart %zu -> %zu)", 
                             cls->nameForLogging(), (size_t)oldStart, 
                             (size_t)ro->instanceStart);
            }
            
            if (ro->ivars) {
                for (const auto& ivar : *ro->ivars) {
                    if (!ivar.offset) continue;  // anonymous bitfield
                    *ivar.offset -= delta;
                }
            }
        }
    }

    if (ro->instanceStart >= super_ro->instanceSize) {
        // Superclass has not overgrown its space. We're done here.
        return;
    }
    // fixme can optimize for "class has no new ivars", etc

    if (ro->instanceStart < super_ro->instanceSize) {
        // Superclass has changed size. This class's ivars must move.
        // Also slide layout bits in parallel.
        // This code is incapable of compacting the subclass to 
        //   compensate for a superclass that shrunk, so don't do that.
        if (PrintIvars) {
            _objc_inform("IVARS: sliding ivars for class %s "
                         "(superclass was %u bytes, now %u)", 
                         cls->nameForLogging(), ro->instanceStart, 
                         super_ro->instanceSize);
        }
        class_ro_t *ro_w = make_ro_writeable(rw);
        ro = rw->ro();
        moveIvars(ro_w, super_ro->instanceSize);
        gdb_objc_class_changed(cls, OBJC_CLASS_IVARS_CHANGED, ro->name);
    } 
}


/***********************************************************************
* realizeClassWithoutSwift
* Performs first-time initialization on class cls, 
* including allocating its read-write data.
* Does not perform any Swift-side initialization.
* Returns the real class structure for the class. 
* Locking: runtimeLock must be write-locked by the caller
**********************************************************************/
static Class realizeClassWithoutSwift(Class cls, Class previously)
{
    runtimeLock.assertLocked();

    class_rw_t *rw;
    Class supercls;
    Class metacls;

    if (!cls) return nil;
    if (cls->isRealized()) return cls;
    ASSERT(cls == remapClass(cls));

    // fixme verify class is not in an un-dlopened part of the shared cache?

    auto ro = (const class_ro_t *)cls->data();
    auto isMeta = ro->flags & RO_META;
    if (ro->flags & RO_FUTURE) {
        // This was a future class. rw data is already allocated.
        rw = cls->data();
        ro = cls->data()->ro();
        ASSERT(!isMeta);
        cls->changeInfo(RW_REALIZED|RW_REALIZING, RW_FUTURE);
    } else {
        // Normal class. Allocate writeable class data.
        rw = objc::zalloc<class_rw_t>();
        rw->set_ro(ro);
        rw->flags = RW_REALIZED|RW_REALIZING|isMeta;
        cls->setData(rw);
    }

#if FAST_CACHE_META
    if (isMeta) cls->cache.setBit(FAST_CACHE_META);
#endif

    // Choose an index for this class.
    // Sets cls->instancesRequireRawIsa if indexes no more indexes are available
    cls->chooseClassArrayIndex();

    if (PrintConnecting) {
        _objc_inform("CLASS: realizing class '%s'%s %p %p #%u %s%s",
                     cls->nameForLogging(), isMeta ? " (meta)" : "", 
                     (void*)cls, ro, cls->classArrayIndex(),
                     cls->isSwiftStable() ? "(swift)" : "",
                     cls->isSwiftLegacy() ? "(pre-stable swift)" : "");
    }

    // Realize superclass and metaclass, if they aren't already.
    // This needs to be done after RW_REALIZED is set above, for root classes.
    // This needs to be done after class index is chosen, for root metaclasses.
    // This assumes that none of those classes have Swift contents,
    //   or that Swift's initializers have already been called.
    //   fixme that assumption will be wrong if we add support
    //   for ObjC subclasses of Swift classes.
    supercls = realizeClassWithoutSwift(remapClass(cls->superclass), nil);
    metacls = realizeClassWithoutSwift(remapClass(cls->ISA()), nil);

#if SUPPORT_NONPOINTER_ISA
    if (isMeta) {
        // Metaclasses do not need any features from non pointer ISA
        // This allows for a faspath for classes in objc_retain/objc_release.
        cls->setInstancesRequireRawIsa();
    } else {
        // Disable non-pointer isa for some classes and/or platforms.
        // Set instancesRequireRawIsa.
        bool instancesRequireRawIsa = cls->instancesRequireRawIsa();
        bool rawIsaIsInherited = false;
        static bool hackedDispatch = false;

        if (DisableNonpointerIsa) {
            // Non-pointer isa disabled by environment or app SDK version
            instancesRequireRawIsa = true;
        }
        else if (!hackedDispatch  &&  0 == strcmp(ro->name, "OS_object"))
        {
            // hack for libdispatch et al - isa also acts as vtable pointer
            hackedDispatch = true;
            instancesRequireRawIsa = true;
        }
        else if (supercls  &&  supercls->superclass  &&
                 supercls->instancesRequireRawIsa())
        {
            // This is also propagated by addSubclass()
            // but nonpointer isa setup needs it earlier.
            // Special case: instancesRequireRawIsa does not propagate
            // from root class to root metaclass
            instancesRequireRawIsa = true;
            rawIsaIsInherited = true;
        }

        if (instancesRequireRawIsa) {
            cls->setInstancesRequireRawIsaRecursively(rawIsaIsInherited);
        }
    }
// SUPPORT_NONPOINTER_ISA
#endif

    // Update superclass and metaclass in case of remapping
    cls->superclass = supercls;
    cls->initClassIsa(metacls);

    // Reconcile instance variable offsets / layout.
    // This may reallocate class_ro_t, updating our ro variable.
    if (supercls  &&  !isMeta) reconcileInstanceVariables(cls, supercls, ro);

    // Set fastInstanceSize if it wasn't set already.
    cls->setInstanceSize(ro->instanceSize);

    // Copy some flags from ro to rw
    if (ro->flags & RO_HAS_CXX_STRUCTORS) {
        cls->setHasCxxDtor();
        if (! (ro->flags & RO_HAS_CXX_DTOR_ONLY)) {
            cls->setHasCxxCtor();
        }
    }
    
    // Propagate the associated objects forbidden flag from ro or from
    // the superclass.
    if ((ro->flags & RO_FORBIDS_ASSOCIATED_OBJECTS) ||
        (supercls && supercls->forbidsAssociatedObjects()))
    {
        rw->flags |= RW_FORBIDS_ASSOCIATED_OBJECTS;
    }

    // Connect this class to its superclass's subclass lists
    if (supercls) {
        addSubclass(supercls, cls);
    } else {
        addRootClass(cls);
    }

    // Attach categories
    methodizeClass(cls, previously);

    return cls;
}


/***********************************************************************
* _objc_realizeClassFromSwift
* Called by Swift when it needs the ObjC part of a class to be realized.
* There are four cases:
* 1. cls != nil; previously == cls
*    Class cls is being realized in place
* 2. cls != nil; previously == nil
*    Class cls is being constructed at runtime
* 3. cls != nil; previously != cls
*    The class that was at previously has been reallocated to cls
* 4. cls == nil, previously != nil
*    The class at previously is hereby disavowed
*
* Only variants #1 and #2 are supported today.
*
* Locking: acquires runtimeLock
**********************************************************************/
Class _objc_realizeClassFromSwift(Class cls, void *previously)
{
    if (cls) {
        if (previously && previously != (void*)cls) {
            // #3: relocation
            mutex_locker_t lock(runtimeLock);
            addRemappedClass((Class)previously, cls);
            addClassTableEntry(cls);
            addNamedClass(cls, cls->mangledName(), /*replacing*/nil);
            return realizeClassWithoutSwift(cls, (Class)previously);
        } else {
            // #1 and #2: realization in place, or new class
            mutex_locker_t lock(runtimeLock);

            if (!previously) {
                // #2: new class
                cls = readClass(cls, false/*bundle*/, false/*shared cache*/);
            }

            // #1 and #2: realization in place, or new class
            // We ignore the Swift metadata initializer callback.
            // We assume that's all handled since we're being called from Swift.
            return realizeClassWithoutSwift(cls, nil);
        }
    }
    else {
        // #4: disavowal
        // In the future this will mean remapping the old address to nil
        // and if necessary removing the old address from any other tables.
        _objc_fatal("Swift requested that class %p be ignored, "
                    "but libobjc does not support that.", previously);
    }
}

/***********************************************************************
* realizeSwiftClass
* Performs first-time initialization on class cls, 
* including allocating its read-write data, 
* and any Swift-side initialization.
* Returns the real class structure for the class. 
* Locking: acquires runtimeLock indirectly
**********************************************************************/
static Class realizeSwiftClass(Class cls)
{
    runtimeLock.assertUnlocked();

    // Some assumptions:
    // * Metaclasses never have a Swift initializer.
    // * Root classes never have a Swift initializer.
    //   (These two together avoid initialization order problems at the root.)
    // * Unrealized non-Swift classes have no Swift ancestry.
    // * Unrealized Swift classes with no initializer have no ancestry that
    //   does have the initializer.
    //   (These two together mean we don't need to scan superclasses here
    //   and we don't need to worry about Swift superclasses inside
    //   realizeClassWithoutSwift()).

    // fixme some of these assumptions will be wrong
    // if we add support for ObjC sublasses of Swift classes.

#if DEBUG
    runtimeLock.lock();
    ASSERT(remapClass(cls) == cls);
    ASSERT(cls->isSwiftStable_ButAllowLegacyForNow());
    ASSERT(!cls->isMetaClassMaybeUnrealized());
    ASSERT(cls->superclass);
    runtimeLock.unlock();
#endif

    // Look for a Swift metadata initialization function
    // installed on the class. If it is present we call it.
    // That function in turn initializes the Swift metadata,
    // prepares the "compiler-generated" ObjC metadata if not
    // already present, and calls _objc_realizeSwiftClass() to finish
    // our own initialization.

    if (auto init = cls->swiftMetadataInitializer()) {
        if (PrintConnecting) {
            _objc_inform("CLASS: calling Swift metadata initializer "
                         "for class '%s' (%p)", cls->nameForLogging(), cls);
        }

        Class newcls = init(cls, nil);

        // fixme someday Swift will need to relocate classes at this point,
        // but we don't accept that yet.
        if (cls != newcls) {
            mutex_locker_t lock(runtimeLock);
            addRemappedClass(cls, newcls);
        }

        return newcls;
    }
    else {
        // No Swift-side initialization callback.
        // Perform our own realization directly.
        mutex_locker_t lock(runtimeLock);
        return realizeClassWithoutSwift(cls, nil);
    }
}


/***********************************************************************
* realizeClassMaybeSwift (MaybeRelock / AndUnlock / AndLeaveLocked)
* Realize a class that might be a Swift class.
* Returns the real class structure for the class. 
* Locking: 
*   runtimeLock must be held on entry
*   runtimeLock may be dropped during execution
*   ...AndUnlock function leaves runtimeLock unlocked on exit
*   ...AndLeaveLocked re-acquires runtimeLock if it was dropped
* This complication avoids repeated lock transitions in some cases.
**********************************************************************/
static Class
realizeClassMaybeSwiftMaybeRelock(Class cls, mutex_t& lock, bool leaveLocked)
{
    lock.assertLocked();

    if (!cls->isSwiftStable_ButAllowLegacyForNow()) {
        // Non-Swift class. Realize it now with the lock still held.
        // fixme wrong in the future for objc subclasses of swift classes
        realizeClassWithoutSwift(cls, nil);
        if (!leaveLocked) lock.unlock();
    } else {
        // Swift class. We need to drop locks and call the Swift
        // runtime to initialize it.
        lock.unlock();
        cls = realizeSwiftClass(cls);
        ASSERT(cls->isRealized());    // callback must have provoked realization
        if (leaveLocked) lock.lock();
    }

    return cls;
}

static Class
realizeClassMaybeSwiftAndUnlock(Class cls, mutex_t& lock)
{
    return realizeClassMaybeSwiftMaybeRelock(cls, lock, false);
}

static Class
realizeClassMaybeSwiftAndLeaveLocked(Class cls, mutex_t& lock)
{
    return realizeClassMaybeSwiftMaybeRelock(cls, lock, true);
}


/***********************************************************************
* missingWeakSuperclass
* Return YES if some superclass of cls was weak-linked and is missing.
**********************************************************************/
static bool 
missingWeakSuperclass(Class cls)
{
    ASSERT(!cls->isRealized());

    if (!cls->superclass) {
        // superclass nil. This is normal for root classes only.
        return (!(cls->data()->flags & RO_ROOT));
    } else {
        // superclass not nil. Check if a higher superclass is missing.
        Class supercls = remapClass(cls->superclass);
        ASSERT(cls != cls->superclass);
        ASSERT(cls != supercls);
        if (!supercls) return YES;
        if (supercls->isRealized()) return NO;
        return missingWeakSuperclass(supercls);
    }
}


/***********************************************************************
* realizeAllClassesInImage
* Non-lazily realizes all unrealized classes in the given image.
* Locking: runtimeLock must be held by the caller.
* Locking: this function may drop and re-acquire the lock.
**********************************************************************/
static void realizeAllClassesInImage(header_info *hi)
{
    runtimeLock.assertLocked();

    size_t count, i;
    classref_t const *classlist;

    if (hi->areAllClassesRealized()) return;

    classlist = _getObjc2ClassList(hi, &count);

    for (i = 0; i < count; i++) {
        Class cls = remapClass(classlist[i]);
        if (cls) {
            realizeClassMaybeSwiftAndLeaveLocked(cls, runtimeLock);
        }
    }

    hi->setAllClassesRealized(YES);
}


/***********************************************************************
* realizeAllClasses
* Non-lazily realizes all unrealized classes in all known images.
* Locking: runtimeLock must be held by the caller.
* Locking: this function may drop and re-acquire the lock.
* Dropping the lock makes this function thread-unsafe with respect 
*   to concurrent image unload, but the callers of this function 
*   already ultimately do something that is also thread-unsafe with 
*   respect to image unload (such as using the list of all classes).
**********************************************************************/
static void realizeAllClasses(void)
{
    runtimeLock.assertLocked();

    header_info *hi;
    for (hi = FirstHeader; hi; hi = hi->getNext()) {
        realizeAllClassesInImage(hi);  // may drop and re-acquire runtimeLock
    }
}


/***********************************************************************
* _objc_allocateFutureClass
* Allocate an unresolved future class for the given class name.
* Returns any existing allocation if one was already made.
* Assumes the named class doesn't exist yet.
* Locking: acquires runtimeLock
**********************************************************************/
Class _objc_allocateFutureClass(const char *name)
{
    mutex_locker_t lock(runtimeLock);

    Class cls;
    NXMapTable *map = futureNamedClasses();

    if ((cls = (Class)NXMapGet(map, name))) {
        // Already have a future class for this name.
        return cls;
    }

    cls = _calloc_class(sizeof(objc_class));
    addFutureNamedClass(name, cls);

    return cls;
}


/***********************************************************************
* objc_getFutureClass.  Return the id of the named class.
* If the class does not exist, return an uninitialized class 
* structure that will be used for the class when and if it 
* does get loaded.
* Not thread safe. 
**********************************************************************/
Class objc_getFutureClass(const char *name)
{
    Class cls;

    // YES unconnected, NO class handler
    // (unconnected is OK because it will someday be the real class)
    cls = look_up_class(name, YES, NO);
    if (cls) {
        if (PrintFuture) {
            _objc_inform("FUTURE: found %p already in use for %s", 
                         (void*)cls, name);
        }

        return cls;
    }
    
    // No class or future class with that name yet. Make one.
    // fixme not thread-safe with respect to 
    // simultaneous library load or getFutureClass.
    return _objc_allocateFutureClass(name);
}


BOOL _class_isFutureClass(Class cls)
{
    return cls  &&  cls->isFuture();
}


/***********************************************************************
* _objc_flush_caches
* Flushes all caches.
* (Historical behavior: flush caches for cls, its metaclass, 
* and subclasses thereof. Nil flushes all classes.)
* Locking: acquires runtimeLock
**********************************************************************/
static void flushCaches(Class cls)
{
    runtimeLock.assertLocked();
#if CONFIG_USE_CACHE_LOCK
    mutex_locker_t lock(cacheUpdateLock);
#endif

    if (cls) {
        foreach_realized_class_and_subclass(cls, [](Class c){
            cache_erase_nolock(c);
            return true;
        });
    }
    else {
        foreach_realized_class_and_metaclass([](Class c){
            cache_erase_nolock(c);
            return true;
        });
    }
}


void _objc_flush_caches(Class cls)
{
    {
        mutex_locker_t lock(runtimeLock);
        flushCaches(cls);
        if (cls  &&  cls->superclass  &&  cls != cls->getIsa()) {
            flushCaches(cls->getIsa());
        } else {
            // cls is a root class or root metaclass. Its metaclass is itself
            // or a subclass so the metaclass caches were already flushed.
        }
    }

    if (!cls) {
        // collectALot if cls==nil
#if CONFIG_USE_CACHE_LOCK
        mutex_locker_t lock(cacheUpdateLock);
#else
        mutex_locker_t lock(runtimeLock);
#endif
        cache_collect(true);
    }
}


/***********************************************************************
* map_images
* Process the given images which are being mapped in by dyld.
* Calls ABI-agnostic code after taking ABI-specific locks.
*
* Locking: write-locks runtimeLock
**********************************************************************/
void
map_images(unsigned count, const char * const paths[],
           const struct mach_header * const mhdrs[])
{
    mutex_locker_t lock(runtimeLock);
    return map_images_nolock(count, paths, mhdrs);
}


static void load_categories_nolock(header_info *hi) {
    bool hasClassProperties = hi->info()->hasCategoryClassProperties();

    size_t count;
    auto processCatlist = [&](category_t * const *catlist) {
        for (unsigned i = 0; i < count; i++) {
            category_t *cat = catlist[i];
            Class cls = remapClass(cat->cls);
            locstamped_category_t lc{cat, hi};

            if (!cls) {
                // Category's target class is missing (probably weak-linked).
                // Ignore the category.
                if (PrintConnecting) {
                    _objc_inform("CLASS: IGNORING category \?\?\?(%s) %p with "
                                 "missing weak-linked target class",
                                 cat->name, cat);
                }
                continue;
            }

            // Process this category.
            if (cls->isStubClass()) {
                // Stub classes are never realized. Stub classes
                // don't know their metaclass until they're
                // initialized, so we have to add categories with
                // class methods or properties to the stub itself.
                // methodizeClass() will find them and add them to
                // the metaclass as appropriate.
                if (cat->instanceMethods ||
                    cat->protocols ||
                    cat->instanceProperties ||
                    cat->classMethods ||
                    cat->protocols ||
                    (hasClassProperties && cat->_classProperties))
                {
                    objc::unattachedCategories.addForClass(lc, cls);
                }
            } else {
                // First, register the category with its target class.
                // Then, rebuild the class's method lists (etc) if
                // the class is realized.
                if (cat->instanceMethods ||  cat->protocols
                    ||  cat->instanceProperties)
                {
                    if (cls->isRealized()) {
                        attachCategories(cls, &lc, 1, ATTACH_EXISTING);
                    } else {
                        objc::unattachedCategories.addForClass(lc, cls);
                    }
                }

                if (cat->classMethods  ||  cat->protocols
                    ||  (hasClassProperties && cat->_classProperties))
                {
                    if (cls->ISA()->isRealized()) {
                        attachCategories(cls->ISA(), &lc, 1, ATTACH_EXISTING | ATTACH_METACLASS);
                    } else {
                        objc::unattachedCategories.addForClass(lc, cls->ISA());
                    }
                }
            }
        }
    };

    processCatlist(_getObjc2CategoryList(hi, &count));
    processCatlist(_getObjc2CategoryList2(hi, &count));
}

static void loadAllCategories() {
    mutex_locker_t lock(runtimeLock);

    for (auto *hi = FirstHeader; hi != NULL; hi = hi->getNext()) {
        load_categories_nolock(hi);
    }
}

/***********************************************************************
* load_images
* Process +load in the given images which are being mapped in by dyld.
*
* Locking: write-locks runtimeLock and loadMethodLock
**********************************************************************/
extern bool hasLoadMethods(const headerType *mhdr);
extern void prepare_load_methods(const headerType *mhdr);

void
load_images(const char *path __unused, const struct mach_header *mh)
{
    if (!didInitialAttachCategories && didCallDyldNotifyRegister) {
        didInitialAttachCategories = true;
        loadAllCategories();
    }

    // Return without taking locks if there are no +load methods here.
    if (!hasLoadMethods((const headerType *)mh)) return;

    recursive_mutex_locker_t lock(loadMethodLock);

    // Discover load methods
    {
        mutex_locker_t lock2(runtimeLock);
        prepare_load_methods((const headerType *)mh);
    }

    // Call +load methods (without runtimeLock - re-entrant)
    call_load_methods();
}


/***********************************************************************
* unmap_image
* Process the given image which is about to be unmapped by dyld.
*
* Locking: write-locks runtimeLock and loadMethodLock
**********************************************************************/
void 
unmap_image(const char *path __unused, const struct mach_header *mh)
{
    recursive_mutex_locker_t lock(loadMethodLock);
    mutex_locker_t lock2(runtimeLock);
    unmap_image_nolock(mh);
}


/***********************************************************************
* mustReadClasses
* Preflight check in advance of readClass() from an image.
**********************************************************************/
bool mustReadClasses(header_info *hi, bool hasDyldRoots)
{
    const char *reason;

    // If the image is not preoptimized then we must read classes.
    if (!hi->hasPreoptimizedClasses()) {
        reason = nil; // Don't log this one because it is noisy.
        goto readthem;
    }

    // If iOS simulator then we must read classes.
#if TARGET_OS_SIMULATOR
    reason = "the image is for iOS simulator";
    goto readthem;
#endif

    ASSERT(!hi->isBundle());  // no MH_BUNDLE in shared cache

    // If the image may have missing weak superclasses then we must read classes
    if (!noMissingWeakSuperclasses() || hasDyldRoots) {
        reason = "the image may contain classes with missing weak superclasses";
        goto readthem;
    }

    // If there are unresolved future classes then we must read classes.
    if (haveFutureNamedClasses()) {
        reason = "there are unresolved future classes pending";
        goto readthem;
    }

    // readClass() rewrites bits in backward-deploying Swift stable ABI code.
    // The assumption here is there there are no such classes
    // in the dyld shared cache.
#if DEBUG
    {
        size_t count;
        classref_t const *classlist = _getObjc2ClassList(hi, &count);
        for (size_t i = 0; i < count; i++) {
            Class cls = remapClass(classlist[i]);
            ASSERT(!cls->isUnfixedBackwardDeployingStableSwift());
        }
    }
#endif

    // readClass() does not need to do anything.
    return NO;

 readthem:
    if (PrintPreopt  &&  reason) {
        _objc_inform("PREOPTIMIZATION: reading classes manually from %s "
                     "because %s", hi->fname(), reason);
    }
    return YES;
}


/***********************************************************************
* readClass
* Read a class and metaclass as written by a compiler.
* Returns the new class pointer. This could be: 
* - cls
* - nil  (cls has a missing weak-linked superclass)
* - something else (space for this class was reserved by a future class)
*
* Note that all work performed by this function is preflighted by 
* mustReadClasses(). Do not change this function without updating that one.
*
* Locking: runtimeLock acquired by map_images or objc_readClassPair
**********************************************************************/
Class readClass(Class cls, bool headerIsBundle, bool headerIsPreoptimized)
{
    const char *mangledName = cls->mangledName();
    
    if (missingWeakSuperclass(cls)) {
        // No superclass (probably weak-linked). 
        // Disavow any knowledge of this subclass.
        if (PrintConnecting) {
            _objc_inform("CLASS: IGNORING class '%s' with "
                         "missing weak-linked superclass", 
                         cls->nameForLogging());
        }
        addRemappedClass(cls, nil);
        cls->superclass = nil;
        return nil;
    }
    
    cls->fixupBackwardDeployingStableSwift();

    Class replacing = nil;
    if (Class newCls = popFutureNamedClass(mangledName)) {
        // This name was previously allocated as a future class.
        // Copy objc_class to future class's struct.
        // Preserve future's rw data block.
        
        if (newCls->isAnySwift()) {
            _objc_fatal("Can't complete future class request for '%s' "
                        "because the real class is too big.", 
                        cls->nameForLogging());
        }
        
        class_rw_t *rw = newCls->data();
        const class_ro_t *old_ro = rw->ro();
        memcpy(newCls, cls, sizeof(objc_class));
        rw->set_ro((class_ro_t *)newCls->data());
        newCls->setData(rw);
        freeIfMutable((char *)old_ro->name);
        free((void *)old_ro);
        
        addRemappedClass(cls, newCls);
        
        replacing = cls;
        cls = newCls;
    }
    
    if (headerIsPreoptimized  &&  !replacing) {
        // class list built in shared cache
        // fixme strict assert doesn't work because of duplicates
        // ASSERT(cls == getClass(name));
        ASSERT(getClassExceptSomeSwift(mangledName));
    } else {
        addNamedClass(cls, mangledName, replacing);
        addClassTableEntry(cls);
    }

    // for future reference: shared cache never contains MH_BUNDLEs
    if (headerIsBundle) {
        cls->data()->flags |= RO_FROM_BUNDLE;
        cls->ISA()->data()->flags |= RO_FROM_BUNDLE;
    }
    
    return cls;
}


/***********************************************************************
* readProtocol
* Read a protocol as written by a compiler.
**********************************************************************/
static void
readProtocol(protocol_t *newproto, Class protocol_class,
             NXMapTable *protocol_map, 
             bool headerIsPreoptimized, bool headerIsBundle)
{
    // This is not enough to make protocols in unloaded bundles safe, 
    // but it does prevent crashes when looking up unrelated protocols.
    auto insertFn = headerIsBundle ? NXMapKeyCopyingInsert : NXMapInsert;

    protocol_t *oldproto = (protocol_t *)getProtocol(newproto->mangledName);

    if (oldproto) {
        if (oldproto != newproto) {
            // Some other definition already won.
            if (PrintProtocols) {
                _objc_inform("PROTOCOLS: protocol at %p is %s  "
                             "(duplicate of %p)",
                             newproto, oldproto->nameForLogging(), oldproto);
            }

            // If we are a shared cache binary then we have a definition of this
            // protocol, but if another one was chosen then we need to clear our
            // isCanonical bit so that no-one trusts it.
            // Note, if getProtocol returned a shared cache protocol then the
            // canonical definition is already in the shared cache and we don't
            // need to do anything.
            if (headerIsPreoptimized && !oldproto->isCanonical()) {
                // Note newproto is an entry in our __objc_protolist section which
                // for shared cache binaries points to the original protocol in
                // that binary, not the shared cache uniqued one.
                auto cacheproto = (protocol_t *)
                    getSharedCachePreoptimizedProtocol(newproto->mangledName);
                if (cacheproto && cacheproto->isCanonical())
                    cacheproto->clearIsCanonical();
            }
        }
    }
    else if (headerIsPreoptimized) {
        // Shared cache initialized the protocol object itself, 
        // but in order to allow out-of-cache replacement we need 
        // to add it to the protocol table now.

        protocol_t *cacheproto = (protocol_t *)
            getPreoptimizedProtocol(newproto->mangledName);
        protocol_t *installedproto;
        if (cacheproto  &&  cacheproto != newproto) {
            // Another definition in the shared cache wins (because 
            // everything in the cache was fixed up to point to it).
            installedproto = cacheproto;
        }
        else {
            // This definition wins.
            installedproto = newproto;
        }
        
        ASSERT(installedproto->getIsa() == protocol_class);
        ASSERT(installedproto->size >= sizeof(protocol_t));
        insertFn(protocol_map, installedproto->mangledName, 
                 installedproto);
        
        if (PrintProtocols) {
            _objc_inform("PROTOCOLS: protocol at %p is %s", 
                         installedproto, installedproto->nameForLogging());
            if (newproto != installedproto) {
                _objc_inform("PROTOCOLS: protocol at %p is %s  "
                             "(duplicate of %p)", 
                             newproto, installedproto->nameForLogging(), 
                             installedproto);
            }
        }
    }
    else if (newproto->size >= sizeof(protocol_t)) {
        // New protocol from an un-preoptimized image
        // with sufficient storage. Fix it up in place.
        // fixme duplicate protocols from unloadable bundle
        newproto->initIsa(protocol_class);  // fixme pinned
        insertFn(protocol_map, newproto->mangledName, newproto);
        if (PrintProtocols) {
            _objc_inform("PROTOCOLS: protocol at %p is %s",
                         newproto, newproto->nameForLogging());
        }
    }
    else {
        // New protocol from an un-preoptimized image 
        // with insufficient storage. Reallocate it.
        // fixme duplicate protocols from unloadable bundle
        size_t size = max(sizeof(protocol_t), (size_t)newproto->size);
        protocol_t *installedproto = (protocol_t *)calloc(size, 1);
        memcpy(installedproto, newproto, newproto->size);
        installedproto->size = (typeof(installedproto->size))size;
        
        installedproto->initIsa(protocol_class);  // fixme pinned
        insertFn(protocol_map, installedproto->mangledName, installedproto);
        if (PrintProtocols) {
            _objc_inform("PROTOCOLS: protocol at %p is %s  ", 
                         installedproto, installedproto->nameForLogging());
            _objc_inform("PROTOCOLS: protocol at %p is %s  "
                         "(reallocated to %p)", 
                         newproto, installedproto->nameForLogging(), 
                         installedproto);
        }
    }
}

/***********************************************************************
* _read_images
* Perform initial processing of the headers in the linked 
* list beginning with headerList. 
*
* Called by: map_images_nolock
*
* Locking: runtimeLock acquired by map_images
**********************************************************************/
void _read_images(header_info **hList, uint32_t hCount, int totalClasses, int unoptimizedTotalClasses)
{
    header_info *hi;
    uint32_t hIndex;
    size_t count;
    size_t i;
    Class *resolvedFutureClasses = nil;
    size_t resolvedFutureClassCount = 0;
    static bool doneOnce;
    bool launchTime = NO;
    TimeLogger ts(PrintImageTimes);

    runtimeLock.assertLocked();

#define EACH_HEADER \
    hIndex = 0;         \
    hIndex < hCount && (hi = hList[hIndex]); \
    hIndex++

    if (!doneOnce) {
        doneOnce = YES;
        launchTime = YES;

#if SUPPORT_NONPOINTER_ISA
        // Disable non-pointer isa under some conditions.

# if SUPPORT_INDEXED_ISA
        // Disable nonpointer isa if any image contains old Swift code
        for (EACH_HEADER) {
            if (hi->info()->containsSwift()  &&
                hi->info()->swiftUnstableVersion() < objc_image_info::SwiftVersion3)
            {
                DisableNonpointerIsa = true;
                if (PrintRawIsa) {
                    _objc_inform("RAW ISA: disabling non-pointer isa because "
                                 "the app or a framework contains Swift code "
                                 "older than Swift 3.0");
                }
                break;
            }
        }
# endif

# if TARGET_OS_OSX
        // Disable non-pointer isa if the app is too old
        // (linked before OS X 10.11)
        if (dyld_get_program_sdk_version() < DYLD_MACOSX_VERSION_10_11) {
            DisableNonpointerIsa = true;
            if (PrintRawIsa) {
                _objc_inform("RAW ISA: disabling non-pointer isa because "
                             "the app is too old (SDK version " SDK_FORMAT ")",
                             FORMAT_SDK(dyld_get_program_sdk_version()));
            }
        }

        // Disable non-pointer isa if the app has a __DATA,__objc_rawisa section
        // New apps that load old extensions may need this.
        for (EACH_HEADER) {
            if (hi->mhdr()->filetype != MH_EXECUTE) continue;
            unsigned long size;
            if (getsectiondata(hi->mhdr(), "__DATA", "__objc_rawisa", &size)) {
                DisableNonpointerIsa = true;
                if (PrintRawIsa) {
                    _objc_inform("RAW ISA: disabling non-pointer isa because "
                                 "the app has a __DATA,__objc_rawisa section");
                }
            }
            break;  // assume only one MH_EXECUTE image
        }
# endif

#endif

        if (DisableTaggedPointers) {
            disableTaggedPointers();
        }
        
        initializeTaggedPointerObfuscator();

        if (PrintConnecting) {
            _objc_inform("CLASS: found %d classes during launch", totalClasses);
        }

        // namedClasses
        // Preoptimized classes don't go in this table.
        // 4/3 is NXMapTable's load factor
        int namedClassesSize = 
            (isPreoptimized() ? unoptimizedTotalClasses : totalClasses) * 4 / 3;
        gdb_objc_realized_classes =
            NXCreateMapTable(NXStrValueMapPrototype, namedClassesSize);

        ts.log("IMAGE TIMES: first time tasks");
    }

    // Fix up @selector references
    static size_t UnfixedSelectors;
    {
        mutex_locker_t lock(selLock);
        for (EACH_HEADER) {
            if (hi->hasPreoptimizedSelectors()) continue;

            bool isBundle = hi->isBundle();
            SEL *sels = _getObjc2SelectorRefs(hi, &count);
            UnfixedSelectors += count;
            for (i = 0; i < count; i++) {
                const char *name = sel_cname(sels[i]);
                SEL sel = sel_registerNameNoLock(name, isBundle);
                if (sels[i] != sel) {
                    sels[i] = sel;
                }
            }
        }
    }

    ts.log("IMAGE TIMES: fix up selector references");

    // Discover classes. Fix up unresolved future classes. Mark bundle classes.
    bool hasDyldRoots = dyld_shared_cache_some_image_overridden();

    for (EACH_HEADER) {
        if (! mustReadClasses(hi, hasDyldRoots)) {
            // Image is sufficiently optimized that we need not call readClass()
            continue;
        }

        classref_t const *classlist = _getObjc2ClassList(hi, &count);

        bool headerIsBundle = hi->isBundle();
        bool headerIsPreoptimized = hi->hasPreoptimizedClasses();

        for (i = 0; i < count; i++) {
            Class cls = (Class)classlist[i];
            Class newCls = readClass(cls, headerIsBundle, headerIsPreoptimized);

            if (newCls != cls  &&  newCls) {
                // Class was moved but not deleted. Currently this occurs 
                // only when the new class resolved a future class.
                // Non-lazily realize the class below.
                resolvedFutureClasses = (Class *)
                    realloc(resolvedFutureClasses, 
                            (resolvedFutureClassCount+1) * sizeof(Class));
                resolvedFutureClasses[resolvedFutureClassCount++] = newCls;
            }
        }
    }

    ts.log("IMAGE TIMES: discover classes");

    // Fix up remapped classes
    // Class list and nonlazy class list remain unremapped.
    // Class refs and super refs are remapped for message dispatching.
    
    if (!noClassesRemapped()) {
        for (EACH_HEADER) {
            Class *classrefs = _getObjc2ClassRefs(hi, &count);
            for (i = 0; i < count; i++) {
                remapClassRef(&classrefs[i]);
            }
            // fixme why doesn't test future1 catch the absence of this?
            classrefs = _getObjc2SuperRefs(hi, &count);
            for (i = 0; i < count; i++) {
                remapClassRef(&classrefs[i]);
            }
        }
    }

    ts.log("IMAGE TIMES: remap classes");

#if SUPPORT_FIXUP
    // Fix up old objc_msgSend_fixup call sites
    for (EACH_HEADER) {
        message_ref_t *refs = _getObjc2MessageRefs(hi, &count);
        if (count == 0) continue;

        if (PrintVtables) {
            _objc_inform("VTABLES: repairing %zu unsupported vtable dispatch "
                         "call sites in %s", count, hi->fname());
        }
        for (i = 0; i < count; i++) {
            fixupMessageRef(refs+i);
        }
    }

    ts.log("IMAGE TIMES: fix up objc_msgSend_fixup");
#endif

    bool cacheSupportsProtocolRoots = sharedCacheSupportsProtocolRoots();

    // Discover protocols. Fix up protocol refs.
    for (EACH_HEADER) {
        extern objc_class OBJC_CLASS_$_Protocol;
        Class cls = (Class)&OBJC_CLASS_$_Protocol;
        ASSERT(cls);
        NXMapTable *protocol_map = protocols();
        bool isPreoptimized = hi->hasPreoptimizedProtocols();

        // Skip reading protocols if this is an image from the shared cache
        // and we support roots
        // Note, after launch we do need to walk the protocol as the protocol
        // in the shared cache is marked with isCanonical() and that may not
        // be true if some non-shared cache binary was chosen as the canonical
        // definition
        if (launchTime && isPreoptimized && cacheSupportsProtocolRoots) {
            if (PrintProtocols) {
                _objc_inform("PROTOCOLS: Skipping reading protocols in image: %s",
                             hi->fname());
            }
            continue;
        }

        bool isBundle = hi->isBundle();

        protocol_t * const *protolist = _getObjc2ProtocolList(hi, &count);
        for (i = 0; i < count; i++) {
            readProtocol(protolist[i], cls, protocol_map, 
                         isPreoptimized, isBundle);
        }
    }

    ts.log("IMAGE TIMES: discover protocols");

    // Fix up @protocol references
    // Preoptimized images may have the right 
    // answer already but we don't know for sure.
    for (EACH_HEADER) {
        // At launch time, we know preoptimized image refs are pointing at the
        // shared cache definition of a protocol.  We can skip the check on
        // launch, but have to visit @protocol refs for shared cache images
        // loaded later.
        if (launchTime && cacheSupportsProtocolRoots && hi->isPreoptimized())
            continue;
        protocol_t **protolist = _getObjc2ProtocolRefs(hi, &count);
        for (i = 0; i < count; i++) {
            remapProtocolRef(&protolist[i]);
        }
    }

    ts.log("IMAGE TIMES: fix up @protocol references");

    // Discover categories. Only do this after the initial category
    // attachment has been done. For categories present at startup,
    // discovery is deferred until the first load_images call after
    // the call to _dyld_objc_notify_register completes. rdar://problem/53119145
    if (didInitialAttachCategories) {
        for (EACH_HEADER) {
            load_categories_nolock(hi);
        }
    }

    ts.log("IMAGE TIMES: discover categories");

    // Category discovery MUST BE Late to avoid potential races
    // when other threads call the new category code before
    // this thread finishes its fixups.

    // +load handled by prepare_load_methods()

    // Realize non-lazy classes (for +load methods and static instances)
    for (EACH_HEADER) {
        classref_t const *classlist = 
            _getObjc2NonlazyClassList(hi, &count);
        for (i = 0; i < count; i++) {
            Class cls = remapClass(classlist[i]);
            if (!cls) continue;

            addClassTableEntry(cls);

            if (cls->isSwiftStable()) {
                if (cls->swiftMetadataInitializer()) {
                    _objc_fatal("Swift class %s with a metadata initializer "
                                "is not allowed to be non-lazy",
                                cls->nameForLogging());
                }
                // fixme also disallow relocatable classes
                // We can't disallow all Swift classes because of
                // classes like Swift.__EmptyArrayStorage
            }
            realizeClassWithoutSwift(cls, nil);
        }
    }

    ts.log("IMAGE TIMES: realize non-lazy classes");

    // Realize newly-resolved future classes, in case CF manipulates them
    if (resolvedFutureClasses) {
        for (i = 0; i < resolvedFutureClassCount; i++) {
            Class cls = resolvedFutureClasses[i];
            if (cls->isSwiftStable()) {
                _objc_fatal("Swift class is not allowed to be future");
            }
            realizeClassWithoutSwift(cls, nil);
            cls->setInstancesRequireRawIsaRecursively(false/*inherited*/);
        }
        free(resolvedFutureClasses);
    }

    ts.log("IMAGE TIMES: realize future classes");

    if (DebugNonFragileIvars) {
        realizeAllClasses();
    }


    // Print preoptimization statistics
    if (PrintPreopt) {
        static unsigned int PreoptTotalMethodLists;
        static unsigned int PreoptOptimizedMethodLists;
        static unsigned int PreoptTotalClasses;
        static unsigned int PreoptOptimizedClasses;

        for (EACH_HEADER) {
            if (hi->hasPreoptimizedSelectors()) {
                _objc_inform("PREOPTIMIZATION: honoring preoptimized selectors "
                             "in %s", hi->fname());
            }
            else if (hi->info()->optimizedByDyld()) {
                _objc_inform("PREOPTIMIZATION: IGNORING preoptimized selectors "
                             "in %s", hi->fname());
            }

            classref_t const *classlist = _getObjc2ClassList(hi, &count);
            for (i = 0; i < count; i++) {
                Class cls = remapClass(classlist[i]);
                if (!cls) continue;

                PreoptTotalClasses++;
                if (hi->hasPreoptimizedClasses()) {
                    PreoptOptimizedClasses++;
                }
                
                const method_list_t *mlist;
                if ((mlist = ((class_ro_t *)cls->data())->baseMethods())) {
                    PreoptTotalMethodLists++;
                    if (mlist->isFixedUp()) {
                        PreoptOptimizedMethodLists++;
                    }
                }
                if ((mlist=((class_ro_t *)cls->ISA()->data())->baseMethods())) {
                    PreoptTotalMethodLists++;
                    if (mlist->isFixedUp()) {
                        PreoptOptimizedMethodLists++;
                    }
                }
            }
        }

        _objc_inform("PREOPTIMIZATION: %zu selector references not "
                     "pre-optimized", UnfixedSelectors);
        _objc_inform("PREOPTIMIZATION: %u/%u (%.3g%%) method lists pre-sorted",
                     PreoptOptimizedMethodLists, PreoptTotalMethodLists, 
                     PreoptTotalMethodLists
                     ? 100.0*PreoptOptimizedMethodLists/PreoptTotalMethodLists 
                     : 0.0);
        _objc_inform("PREOPTIMIZATION: %u/%u (%.3g%%) classes pre-registered",
                     PreoptOptimizedClasses, PreoptTotalClasses, 
                     PreoptTotalClasses 
                     ? 100.0*PreoptOptimizedClasses/PreoptTotalClasses
                     : 0.0);
        _objc_inform("PREOPTIMIZATION: %zu protocol references not "
                     "pre-optimized", UnfixedProtocolReferences);
    }

#undef EACH_HEADER
}


/***********************************************************************
* prepare_load_methods
* Schedule +load for classes in this image, any un-+load-ed 
* superclasses in other images, and any categories in this image.
**********************************************************************/
// Recursively schedule +load for cls and any un-+load-ed superclasses.
// cls must already be connected.
static void schedule_class_load(Class cls)
{
    if (!cls) return;
    ASSERT(cls->isRealized());  // _read_images should realize

    if (cls->data()->flags & RW_LOADED) return;

    // Ensure superclass-first ordering
    schedule_class_load(cls->superclass);

    add_class_to_loadable_list(cls);
    cls->setInfo(RW_LOADED); 
}

// Quick scan for +load methods that doesn't take a lock.
bool hasLoadMethods(const headerType *mhdr)
{
    size_t count;
    if (_getObjc2NonlazyClassList(mhdr, &count)  &&  count > 0) return true;
    if (_getObjc2NonlazyCategoryList(mhdr, &count)  &&  count > 0) return true;
    return false;
}

void prepare_load_methods(const headerType *mhdr)
{
    size_t count, i;

    runtimeLock.assertLocked();

    classref_t const *classlist = 
        _getObjc2NonlazyClassList(mhdr, &count);
    for (i = 0; i < count; i++) {
        schedule_class_load(remapClass(classlist[i]));
    }

    category_t * const *categorylist = _getObjc2NonlazyCategoryList(mhdr, &count);
    for (i = 0; i < count; i++) {
        category_t *cat = categorylist[i];
        Class cls = remapClass(cat->cls);
        if (!cls) continue;  // category for ignored weak-linked class
        if (cls->isSwiftStable()) {
            _objc_fatal("Swift class extensions and categories on Swift "
                        "classes are not allowed to have +load methods");
        }
        realizeClassWithoutSwift(cls, nil);
        ASSERT(cls->ISA()->isRealized());
        add_category_to_loadable_list(cat);
    }
}


/***********************************************************************
* _unload_image
* Only handles MH_BUNDLE for now.
* Locking: write-lock and loadMethodLock acquired by unmap_image
**********************************************************************/
void _unload_image(header_info *hi)
{
    size_t count, i;

    loadMethodLock.assertLocked();
    runtimeLock.assertLocked();

    // Unload unattached categories and categories waiting for +load.

    // Ignore __objc_catlist2. We don't support unloading Swift
    // and we never will.
    category_t * const *catlist = _getObjc2CategoryList(hi, &count);
    for (i = 0; i < count; i++) {
        category_t *cat = catlist[i];
        Class cls = remapClass(cat->cls);
        if (!cls) continue;  // category for ignored weak-linked class

        // fixme for MH_DYLIB cat's class may have been unloaded already

        // unattached list
        objc::unattachedCategories.eraseCategoryForClass(cat, cls);

        // +load queue
        remove_category_from_loadable_list(cat);
    }

    // Unload classes.

    // Gather classes from both __DATA,__objc_clslist 
    // and __DATA,__objc_nlclslist. arclite's hack puts a class in the latter
    // only, and we need to unload that class if we unload an arclite image.

    objc::DenseSet<Class> classes{};
    classref_t const *classlist;

    classlist = _getObjc2ClassList(hi, &count);
    for (i = 0; i < count; i++) {
        Class cls = remapClass(classlist[i]);
        if (cls) classes.insert(cls);
    }

    classlist = _getObjc2NonlazyClassList(hi, &count);
    for (i = 0; i < count; i++) {
        Class cls = remapClass(classlist[i]);
        if (cls) classes.insert(cls);
    }

    // First detach classes from each other. Then free each class.
    // This avoid bugs where this loop unloads a subclass before its superclass

    for (Class cls: classes) {
        remove_class_from_loadable_list(cls);
        detach_class(cls->ISA(), YES);
        detach_class(cls, NO);
    }
    for (Class cls: classes) {
        free_class(cls->ISA());
        free_class(cls);
    }

    // XXX FIXME -- Clean up protocols:
    // <rdar://problem/9033191> Support unloading protocols at dylib/image unload time

    // fixme DebugUnload
}


/***********************************************************************
* method_getDescription
* Returns a pointer to this method's objc_method_description.
* Locking: none
**********************************************************************/
struct objc_method_description *
method_getDescription(Method m)
{
    if (!m) return nil;
    return (struct objc_method_description *)m;
}


IMP 
method_getImplementation(Method m)
{
    return m ? m->imp : nil;
}


/***********************************************************************
* method_getName
* Returns this method's selector.
* The method must not be nil.
* The method must already have been fixed-up.
* Locking: none
**********************************************************************/
SEL 
method_getName(Method m)
{
    if (!m) return nil;

    ASSERT(m->name == sel_registerName(sel_getName(m->name)));
    return m->name;
}


/***********************************************************************
* method_getTypeEncoding
* Returns this method's old-style type encoding string.
* The method must not be nil.
* Locking: none
**********************************************************************/
const char *
method_getTypeEncoding(Method m)
{
    if (!m) return nil;
    return m->types;
}


/***********************************************************************
* method_setImplementation
* Sets this method's implementation to imp.
* The previous implementation is returned.
**********************************************************************/
static IMP 
_method_setImplementation(Class cls, method_t *m, IMP imp)
{
    runtimeLock.assertLocked();

    if (!m) return nil;
    if (!imp) return nil;

    IMP old = m->imp;
    m->imp = imp;

    // Cache updates are slow if cls is nil (i.e. unknown)
    // RR/AWZ updates are slow if cls is nil (i.e. unknown)
    // fixme build list of classes whose Methods are known externally?

    flushCaches(cls);

    adjustCustomFlagsForMethodChange(cls, m);

    return old;
}

IMP 
method_setImplementation(Method m, IMP imp)
{
    // Don't know the class - will be slow if RR/AWZ are affected
    // fixme build list of classes whose Methods are known externally?
    mutex_locker_t lock(runtimeLock);
    return _method_setImplementation(Nil, m, imp);
}


void method_exchangeImplementations(Method m1, Method m2)
{
    if (!m1  ||  !m2) return;

    mutex_locker_t lock(runtimeLock);

    IMP m1_imp = m1->imp;
    m1->imp = m2->imp;
    m2->imp = m1_imp;


    // RR/AWZ updates are slow because class is unknown
    // Cache updates are slow because class is unknown
    // fixme build list of classes whose Methods are known externally?

    flushCaches(nil);

    adjustCustomFlagsForMethodChange(nil, m1);
    adjustCustomFlagsForMethodChange(nil, m2);
}


/***********************************************************************
* ivar_getOffset
* fixme
* Locking: none
**********************************************************************/
ptrdiff_t
ivar_getOffset(Ivar ivar)
{
    if (!ivar) return 0;
    return *ivar->offset;
}


/***********************************************************************
* ivar_getName
* fixme
* Locking: none
**********************************************************************/
const char *
ivar_getName(Ivar ivar)
{
    if (!ivar) return nil;
    return ivar->name;
}


/***********************************************************************
* ivar_getTypeEncoding
* fixme
* Locking: none
**********************************************************************/
const char *
ivar_getTypeEncoding(Ivar ivar)
{
    if (!ivar) return nil;
    return ivar->type;
}



const char *property_getName(objc_property_t prop)
{
    return prop->name;
}

const char *property_getAttributes(objc_property_t prop)
{
    return prop->attributes;
}

objc_property_attribute_t *property_copyAttributeList(objc_property_t prop, 
                                                      unsigned int *outCount)
{
    if (!prop) {
        if (outCount) *outCount = 0;
        return nil;
    }

    mutex_locker_t lock(runtimeLock);
    return copyPropertyAttributeList(prop->attributes,outCount);
}

char * property_copyAttributeValue(objc_property_t prop, const char *name)
{
    if (!prop  ||  !name  ||  *name == '\0') return nil;
    
    mutex_locker_t lock(runtimeLock);
    return copyPropertyAttributeValue(prop->attributes, name);
}


/***********************************************************************
* getExtendedTypesIndexesForMethod
* Returns:
* a is the count of methods in all method lists before m's method list
* b is the index of m in m's method list
* a+b is the index of m's extended types in the extended types array
**********************************************************************/
static void getExtendedTypesIndexesForMethod(protocol_t *proto, const method_t *m, bool isRequiredMethod, bool isInstanceMethod, uint32_t& a, uint32_t &b)
{
    a = 0;

    if (proto->instanceMethods) {
        if (isRequiredMethod && isInstanceMethod) {
            b = proto->instanceMethods->indexOfMethod(m);
            return;
        }
        a += proto->instanceMethods->count;
    }

    if (proto->classMethods) {
        if (isRequiredMethod && !isInstanceMethod) {
            b = proto->classMethods->indexOfMethod(m);
            return;
        }
        a += proto->classMethods->count;
    }

    if (proto->optionalInstanceMethods) {
        if (!isRequiredMethod && isInstanceMethod) {
            b = proto->optionalInstanceMethods->indexOfMethod(m);
            return;
        }
        a += proto->optionalInstanceMethods->count;
    }

    if (proto->optionalClassMethods) {
        if (!isRequiredMethod && !isInstanceMethod) {
            b = proto->optionalClassMethods->indexOfMethod(m);
            return;
        }
        a += proto->optionalClassMethods->count;
    }
}


/***********************************************************************
* getExtendedTypesIndexForMethod
* Returns the index of m's extended types in proto's extended types array.
**********************************************************************/
static uint32_t getExtendedTypesIndexForMethod(protocol_t *proto, const method_t *m, bool isRequiredMethod, bool isInstanceMethod)
{
    uint32_t a;
    uint32_t b;
    getExtendedTypesIndexesForMethod(proto, m, isRequiredMethod, 
                                     isInstanceMethod, a, b);
    return a + b;
}


/***********************************************************************
* fixupProtocolMethodList
* Fixes up a single method list in a protocol.
**********************************************************************/
static void
fixupProtocolMethodList(protocol_t *proto, method_list_t *mlist,  
                        bool required, bool instance)
{
    runtimeLock.assertLocked();

    if (!mlist) return;
    if (mlist->isFixedUp()) return;

    const char **extTypes = proto->extendedMethodTypes();
    fixupMethodList(mlist, true/*always copy for simplicity*/,
                    !extTypes/*sort if no extended method types*/);
    
    if (extTypes) {
        // Sort method list and extended method types together.
        // fixupMethodList() can't do this.
        // fixme COW stomp
        uint32_t count = mlist->count;
        uint32_t prefix;
        uint32_t junk;
        getExtendedTypesIndexesForMethod(proto, &mlist->get(0), 
                                         required, instance, prefix, junk);
        for (uint32_t i = 0; i < count; i++) {
            for (uint32_t j = i+1; j < count; j++) {
                method_t& mi = mlist->get(i);
                method_t& mj = mlist->get(j);
                if (mi.name > mj.name) {
                    std::swap(mi, mj);
                    std::swap(extTypes[prefix+i], extTypes[prefix+j]);
                }
            }
        }
    }
}


/***********************************************************************
* fixupProtocol
* Fixes up all of a protocol's method lists.
**********************************************************************/
static void 
fixupProtocol(protocol_t *proto)
{
    runtimeLock.assertLocked();

    if (proto->protocols) {
        for (uintptr_t i = 0; i < proto->protocols->count; i++) {
            protocol_t *sub = remapProtocol(proto->protocols->list[i]);
            if (!sub->isFixedUp()) fixupProtocol(sub);
        }
    }

    fixupProtocolMethodList(proto, proto->instanceMethods, YES, YES);
    fixupProtocolMethodList(proto, proto->classMethods, YES, NO);
    fixupProtocolMethodList(proto, proto->optionalInstanceMethods, NO, YES);
    fixupProtocolMethodList(proto, proto->optionalClassMethods, NO, NO);

    // fixme memory barrier so we can check this with no lock
    proto->setFixedUp();
}


/***********************************************************************
* fixupProtocolIfNeeded
* Fixes up all of a protocol's method lists if they aren't fixed up already.
* Locking: write-locks runtimeLock.
**********************************************************************/
static void 
fixupProtocolIfNeeded(protocol_t *proto)
{
    runtimeLock.assertUnlocked();
    ASSERT(proto);

    if (!proto->isFixedUp()) {
        mutex_locker_t lock(runtimeLock);
        fixupProtocol(proto);
    }
}


static method_list_t *
getProtocolMethodList(protocol_t *proto, bool required, bool instance)
{
    method_list_t **mlistp = nil;
    if (required) {
        if (instance) {
            mlistp = &proto->instanceMethods;
        } else {
            mlistp = &proto->classMethods;
        }
    } else {
        if (instance) {
            mlistp = &proto->optionalInstanceMethods;
        } else {
            mlistp = &proto->optionalClassMethods;
        }
    }

    return *mlistp;
}


/***********************************************************************
* protocol_getMethod_nolock
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static method_t *
protocol_getMethod_nolock(protocol_t *proto, SEL sel, 
                          bool isRequiredMethod, bool isInstanceMethod, 
                          bool recursive)
{
    runtimeLock.assertLocked();

    if (!proto  ||  !sel) return nil;

    ASSERT(proto->isFixedUp());

    method_list_t *mlist = 
        getProtocolMethodList(proto, isRequiredMethod, isInstanceMethod);
    if (mlist) {
        method_t *m = search_method_list(mlist, sel);
        if (m) return m;
    }

    if (recursive  &&  proto->protocols) {
        method_t *m;
        for (uint32_t i = 0; i < proto->protocols->count; i++) {
            protocol_t *realProto = remapProtocol(proto->protocols->list[i]);
            m = protocol_getMethod_nolock(realProto, sel, 
                                          isRequiredMethod, isInstanceMethod, 
                                          true);
            if (m) return m;
        }
    }

    return nil;
}


/***********************************************************************
* protocol_getMethod
* fixme
* Locking: acquires runtimeLock
**********************************************************************/
Method 
protocol_getMethod(protocol_t *proto, SEL sel, bool isRequiredMethod, bool isInstanceMethod, bool recursive)
{
    if (!proto) return nil;
    fixupProtocolIfNeeded(proto);

    mutex_locker_t lock(runtimeLock);
    return protocol_getMethod_nolock(proto, sel, isRequiredMethod, 
                                     isInstanceMethod, recursive);
}


/***********************************************************************
* protocol_getMethodTypeEncoding_nolock
* Return the @encode string for the requested protocol method.
* Returns nil if the compiler did not emit any extended @encode data.
* Locking: runtimeLock must be held by the caller
**********************************************************************/
const char * 
protocol_getMethodTypeEncoding_nolock(protocol_t *proto, SEL sel, 
                                      bool isRequiredMethod, 
                                      bool isInstanceMethod)
{
    runtimeLock.assertLocked();

    if (!proto) return nil;
    if (!proto->extendedMethodTypes()) return nil;

    ASSERT(proto->isFixedUp());

    method_t *m = 
        protocol_getMethod_nolock(proto, sel, 
                                  isRequiredMethod, isInstanceMethod, false);
    if (m) {
        uint32_t i = getExtendedTypesIndexForMethod(proto, m, 
                                                    isRequiredMethod, 
                                                    isInstanceMethod);
        return proto->extendedMethodTypes()[i];
    }

    // No method with that name. Search incorporated protocols.
    if (proto->protocols) {
        for (uintptr_t i = 0; i < proto->protocols->count; i++) {
            const char *enc = 
                protocol_getMethodTypeEncoding_nolock(remapProtocol(proto->protocols->list[i]), sel, isRequiredMethod, isInstanceMethod);
            if (enc) return enc;
        }
    }

    return nil;
}

/***********************************************************************
* _protocol_getMethodTypeEncoding
* Return the @encode string for the requested protocol method.
* Returns nil if the compiler did not emit any extended @encode data.
* Locking: acquires runtimeLock
**********************************************************************/
const char * 
_protocol_getMethodTypeEncoding(Protocol *proto_gen, SEL sel, 
                                BOOL isRequiredMethod, BOOL isInstanceMethod)
{
    protocol_t *proto = newprotocol(proto_gen);

    if (!proto) return nil;
    fixupProtocolIfNeeded(proto);

    mutex_locker_t lock(runtimeLock);
    return protocol_getMethodTypeEncoding_nolock(proto, sel, 
                                                 isRequiredMethod, 
                                                 isInstanceMethod);
}


/***********************************************************************
* protocol_t::demangledName
* Returns the (Swift-demangled) name of the given protocol.
* Locking: none
**********************************************************************/
const char *
protocol_t::demangledName() 
{
    ASSERT(hasDemangledNameField());
    
    if (! _demangledName) {
        char *de = copySwiftV1DemangledName(mangledName, true/*isProtocol*/);
        if (! OSAtomicCompareAndSwapPtrBarrier(nil, (void*)(de ?: mangledName), 
                                               (void**)&_demangledName)) 
        {
            if (de) free(de);
        }
    }
    return _demangledName;
}

/***********************************************************************
* protocol_getName
* Returns the (Swift-demangled) name of the given protocol.
* Locking: runtimeLock must not be held by the caller
**********************************************************************/
const char *
protocol_getName(Protocol *proto)
{
    if (!proto) return "nil";
    else return newprotocol(proto)->demangledName();
}


/***********************************************************************
* protocol_getInstanceMethodDescription
* Returns the description of a named instance method.
* Locking: runtimeLock must not be held by the caller
**********************************************************************/
struct objc_method_description 
protocol_getMethodDescription(Protocol *p, SEL aSel, 
                              BOOL isRequiredMethod, BOOL isInstanceMethod)
{
    Method m = 
        protocol_getMethod(newprotocol(p), aSel, 
                           isRequiredMethod, isInstanceMethod, true);
    if (m) return *method_getDescription(m);
    else return (struct objc_method_description){nil, nil};
}


/***********************************************************************
* protocol_conformsToProtocol_nolock
* Returns YES if self conforms to other.
* Locking: runtimeLock must be held by the caller.
**********************************************************************/
static bool 
protocol_conformsToProtocol_nolock(protocol_t *self, protocol_t *other)
{
    runtimeLock.assertLocked();

    if (!self  ||  !other) {
        return NO;
    }

    // protocols need not be fixed up

    if (0 == strcmp(self->mangledName, other->mangledName)) {
        return YES;
    }

    if (self->protocols) {
        uintptr_t i;
        for (i = 0; i < self->protocols->count; i++) {
            protocol_t *proto = remapProtocol(self->protocols->list[i]);
            if (other == proto) {
              return YES;
            }
            if (0 == strcmp(other->mangledName, proto->mangledName)) {
                return YES;
            }
            if (protocol_conformsToProtocol_nolock(proto, other)) {
                return YES;
            }
        }
    }

    return NO;
}


/***********************************************************************
* protocol_conformsToProtocol
* Returns YES if self conforms to other.
* Locking: acquires runtimeLock
**********************************************************************/
BOOL protocol_conformsToProtocol(Protocol *self, Protocol *other)
{
    mutex_locker_t lock(runtimeLock);
    return protocol_conformsToProtocol_nolock(newprotocol(self), 
                                              newprotocol(other));
}


/***********************************************************************
* protocol_isEqual
* Return YES if two protocols are equal (i.e. conform to each other)
* Locking: acquires runtimeLock
**********************************************************************/
BOOL protocol_isEqual(Protocol *self, Protocol *other)
{
    if (self == other) return YES;
    if (!self  ||  !other) return NO;

    if (!protocol_conformsToProtocol(self, other)) return NO;
    if (!protocol_conformsToProtocol(other, self)) return NO;

    return YES;
}


/***********************************************************************
* protocol_copyMethodDescriptionList
* Returns descriptions of a protocol's methods.
* Locking: acquires runtimeLock
**********************************************************************/
struct objc_method_description *
protocol_copyMethodDescriptionList(Protocol *p, 
                                   BOOL isRequiredMethod,BOOL isInstanceMethod,
                                   unsigned int *outCount)
{
    protocol_t *proto = newprotocol(p);
    struct objc_method_description *result = nil;
    unsigned int count = 0;

    if (!proto) {
        if (outCount) *outCount = 0;
        return nil;
    }

    fixupProtocolIfNeeded(proto);

    mutex_locker_t lock(runtimeLock);

    method_list_t *mlist = 
        getProtocolMethodList(proto, isRequiredMethod, isInstanceMethod);

    if (mlist) {
        result = (struct objc_method_description *)
            calloc(mlist->count + 1, sizeof(struct objc_method_description));
        for (const auto& meth : *mlist) {
            result[count].name = meth.name;
            result[count].types = (char *)meth.types;
            count++;
        }
    }

    if (outCount) *outCount = count;
    return result;
}


/***********************************************************************
* protocol_getProperty
* fixme
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static property_t * 
protocol_getProperty_nolock(protocol_t *proto, const char *name, 
                            bool isRequiredProperty, bool isInstanceProperty)
{
    runtimeLock.assertLocked();

    if (!isRequiredProperty) {
        // Only required properties are currently supported.
        return nil;
    }

    property_list_t *plist = isInstanceProperty ? 
        proto->instanceProperties : proto->classProperties();
    if (plist) {
        for (auto& prop : *plist) {
            if (0 == strcmp(name, prop.name)) {
                return &prop;
            }
        }
    }

    if (proto->protocols) {
        uintptr_t i;
        for (i = 0; i < proto->protocols->count; i++) {
            protocol_t *p = remapProtocol(proto->protocols->list[i]);
            property_t *prop = 
                protocol_getProperty_nolock(p, name, 
                                            isRequiredProperty, 
                                            isInstanceProperty);
            if (prop) return prop;
        }
    }

    return nil;
}

objc_property_t protocol_getProperty(Protocol *p, const char *name, 
                              BOOL isRequiredProperty, BOOL isInstanceProperty)
{
    if (!p  ||  !name) return nil;

    mutex_locker_t lock(runtimeLock);
    return (objc_property_t)
        protocol_getProperty_nolock(newprotocol(p), name, 
                                    isRequiredProperty, isInstanceProperty);
}


/***********************************************************************
* protocol_copyPropertyList
* protocol_copyPropertyList2
* fixme
* Locking: acquires runtimeLock
**********************************************************************/
static property_t **
copyPropertyList(property_list_t *plist, unsigned int *outCount)
{
    property_t **result = nil;
    unsigned int count = 0;

    if (plist) {
        count = plist->count;
    }

    if (count > 0) {
        result = (property_t **)malloc((count+1) * sizeof(property_t *));

        count = 0;
        for (auto& prop : *plist) {
            result[count++] = &prop;
        }
        result[count] = nil;
    }

    if (outCount) *outCount = count;
    return result;
}

objc_property_t *
protocol_copyPropertyList2(Protocol *proto, unsigned int *outCount, 
                           BOOL isRequiredProperty, BOOL isInstanceProperty)
{
    if (!proto  ||  !isRequiredProperty) {
        // Optional properties are not currently supported.
        if (outCount) *outCount = 0;
        return nil;
    }

    mutex_locker_t lock(runtimeLock);

    property_list_t *plist = isInstanceProperty
        ? newprotocol(proto)->instanceProperties
        : newprotocol(proto)->classProperties();
    return (objc_property_t *)copyPropertyList(plist, outCount);
}

objc_property_t *
protocol_copyPropertyList(Protocol *proto, unsigned int *outCount)
{
    return protocol_copyPropertyList2(proto, outCount, 
                                      YES/*required*/, YES/*instance*/);
}


/***********************************************************************
* protocol_copyProtocolList
* Copies this protocol's incorporated protocols. 
* Does not copy those protocol's incorporated protocols in turn.
* Locking: acquires runtimeLock
**********************************************************************/
Protocol * __unsafe_unretained * 
protocol_copyProtocolList(Protocol *p, unsigned int *outCount)
{
    unsigned int count = 0;
    Protocol **result = nil;
    protocol_t *proto = newprotocol(p);
    
    if (!proto) {
        if (outCount) *outCount = 0;
        return nil;
    }

    mutex_locker_t lock(runtimeLock);

    if (proto->protocols) {
        count = (unsigned int)proto->protocols->count;
    }
    if (count > 0) {
        result = (Protocol **)malloc((count+1) * sizeof(Protocol *));

        unsigned int i;
        for (i = 0; i < count; i++) {
            result[i] = (Protocol *)remapProtocol(proto->protocols->list[i]);
        }
        result[i] = nil;
    }

    if (outCount) *outCount = count;
    return result;
}


/***********************************************************************
* objc_allocateProtocol
* Creates a new protocol. The protocol may not be used until 
* objc_registerProtocol() is called.
* Returns nil if a protocol with the same name already exists.
* Locking: acquires runtimeLock
**********************************************************************/
Protocol *
objc_allocateProtocol(const char *name)
{
    mutex_locker_t lock(runtimeLock);

    if (getProtocol(name)) {
        return nil;
    }

    protocol_t *result = (protocol_t *)calloc(sizeof(protocol_t), 1);

    extern objc_class OBJC_CLASS_$___IncompleteProtocol;
    Class cls = (Class)&OBJC_CLASS_$___IncompleteProtocol;
    result->initProtocolIsa(cls);
    result->size = sizeof(protocol_t);
    // fixme mangle the name if it looks swift-y?
    result->mangledName = strdupIfMutable(name);

    // fixme reserve name without installing

    return (Protocol *)result;
}


/***********************************************************************
* objc_registerProtocol
* Registers a newly-constructed protocol. The protocol is now 
* ready for use and immutable.
* Locking: acquires runtimeLock
**********************************************************************/
void objc_registerProtocol(Protocol *proto_gen) 
{
    protocol_t *proto = newprotocol(proto_gen);

    mutex_locker_t lock(runtimeLock);

    extern objc_class OBJC_CLASS_$___IncompleteProtocol;
    Class oldcls = (Class)&OBJC_CLASS_$___IncompleteProtocol;
    extern objc_class OBJC_CLASS_$_Protocol;
    Class cls = (Class)&OBJC_CLASS_$_Protocol;

    if (proto->ISA() == cls) {
        _objc_inform("objc_registerProtocol: protocol '%s' was already "
                     "registered!", proto->nameForLogging());
        return;
    }
    if (proto->ISA() != oldcls) {
        _objc_inform("objc_registerProtocol: protocol '%s' was not allocated "
                     "with objc_allocateProtocol!", proto->nameForLogging());
        return;
    }

    // NOT initProtocolIsa(). The protocol object may already 
    // have been retained and we must preserve that count.
    proto->changeIsa(cls);

    // Don't add this protocol if we already have it.
    // Should we warn on duplicates?
    if (getProtocol(proto->mangledName) == nil) {
        NXMapKeyCopyingInsert(protocols(), proto->mangledName, proto);
    }
}


/***********************************************************************
* protocol_addProtocol
* Adds an incorporated protocol to another protocol.
* No method enforcement is performed.
* `proto` must be under construction. `addition` must not.
* Locking: acquires runtimeLock
**********************************************************************/
void 
protocol_addProtocol(Protocol *proto_gen, Protocol *addition_gen) 
{
    protocol_t *proto = newprotocol(proto_gen);
    protocol_t *addition = newprotocol(addition_gen);

    extern objc_class OBJC_CLASS_$___IncompleteProtocol;
    Class cls = (Class)&OBJC_CLASS_$___IncompleteProtocol;

    if (!proto_gen) return;
    if (!addition_gen) return;

    mutex_locker_t lock(runtimeLock);

    if (proto->ISA() != cls) {
        _objc_inform("protocol_addProtocol: modified protocol '%s' is not "
                     "under construction!", proto->nameForLogging());
        return;
    }
    if (addition->ISA() == cls) {
        _objc_inform("protocol_addProtocol: added protocol '%s' is still "
                     "under construction!", addition->nameForLogging());
        return;        
    }
    
    protocol_list_t *protolist = proto->protocols;
    if (!protolist) {
        protolist = (protocol_list_t *)
            calloc(1, sizeof(protocol_list_t) 
                             + sizeof(protolist->list[0]));
    } else {
        protolist = (protocol_list_t *)
            realloc(protolist, protocol_list_size(protolist) 
                              + sizeof(protolist->list[0]));
    }

    protolist->list[protolist->count++] = (protocol_ref_t)addition;
    proto->protocols = protolist;
}


/***********************************************************************
* protocol_addMethodDescription
* Adds a method to a protocol. The protocol must be under construction.
* Locking: acquires runtimeLock
**********************************************************************/
static void
protocol_addMethod_nolock(method_list_t*& list, SEL name, const char *types)
{
    if (!list) {
        list = (method_list_t *)calloc(sizeof(method_list_t), 1);
        list->entsizeAndFlags = sizeof(list->first);
        list->setFixedUp();
    } else {
        size_t size = list->byteSize() + list->entsize();
        list = (method_list_t *)realloc(list, size);
    }

    method_t& meth = list->get(list->count++);
    meth.name = name;
    meth.types = types ? strdupIfMutable(types) : "";
    meth.imp = nil;
}

void 
protocol_addMethodDescription(Protocol *proto_gen, SEL name, const char *types,
                              BOOL isRequiredMethod, BOOL isInstanceMethod) 
{
    protocol_t *proto = newprotocol(proto_gen);

    extern objc_class OBJC_CLASS_$___IncompleteProtocol;
    Class cls = (Class)&OBJC_CLASS_$___IncompleteProtocol;

    if (!proto_gen) return;

    mutex_locker_t lock(runtimeLock);

    if (proto->ISA() != cls) {
        _objc_inform("protocol_addMethodDescription: protocol '%s' is not "
                     "under construction!", proto->nameForLogging());
        return;
    }

    if (isRequiredMethod  &&  isInstanceMethod) {
        protocol_addMethod_nolock(proto->instanceMethods, name, types);
    } else if (isRequiredMethod  &&  !isInstanceMethod) {
        protocol_addMethod_nolock(proto->classMethods, name, types);
    } else if (!isRequiredMethod  &&  isInstanceMethod) {
        protocol_addMethod_nolock(proto->optionalInstanceMethods, name,types);
    } else /*  !isRequiredMethod  &&  !isInstanceMethod) */ {
        protocol_addMethod_nolock(proto->optionalClassMethods, name, types);
    }
}


/***********************************************************************
* protocol_addProperty
* Adds a property to a protocol. The protocol must be under construction.
* Locking: acquires runtimeLock
**********************************************************************/
static void 
protocol_addProperty_nolock(property_list_t *&plist, const char *name, 
                            const objc_property_attribute_t *attrs, 
                            unsigned int count)
{
    if (!plist) {
        plist = (property_list_t *)calloc(sizeof(property_list_t), 1);
        plist->entsizeAndFlags = sizeof(property_t);
    } else {
        plist = (property_list_t *)
            realloc(plist, sizeof(property_list_t) 
                    + plist->count * plist->entsize());
    }

    property_t& prop = plist->get(plist->count++);
    prop.name = strdupIfMutable(name);
    prop.attributes = copyPropertyAttributeString(attrs, count);
}

void 
protocol_addProperty(Protocol *proto_gen, const char *name, 
                     const objc_property_attribute_t *attrs, 
                     unsigned int count,
                     BOOL isRequiredProperty, BOOL isInstanceProperty)
{
    protocol_t *proto = newprotocol(proto_gen);

    extern objc_class OBJC_CLASS_$___IncompleteProtocol;
    Class cls = (Class)&OBJC_CLASS_$___IncompleteProtocol;

    if (!proto) return;
    if (!name) return;

    mutex_locker_t lock(runtimeLock);

    if (proto->ISA() != cls) {
        _objc_inform("protocol_addProperty: protocol '%s' is not "
                     "under construction!", proto->nameForLogging());
        return;
    }

    if (isRequiredProperty  &&  isInstanceProperty) {
        protocol_addProperty_nolock(proto->instanceProperties, name, attrs, count);
    }
    else if (isRequiredProperty  &&  !isInstanceProperty) {
        protocol_addProperty_nolock(proto->_classProperties, name, attrs, count);
    }
    //else if (!isRequiredProperty  &&  isInstanceProperty) {
    //    protocol_addProperty_nolock(proto->optionalInstanceProperties, name, attrs, count);
    //}
    //else /*  !isRequiredProperty  &&  !isInstanceProperty) */ {
    //    protocol_addProperty_nolock(proto->optionalClassProperties, name, attrs, count);
    //}
}

static int
objc_getRealizedClassList_nolock(Class *buffer, int bufferLen)
{
    int count = 0;

    if (buffer) {
        int c = 0;
        foreach_realized_class([=, &count, &c](Class cls) {
            count++;
            if (c < bufferLen) {
                buffer[c++] = cls;
            }
            return true;
        });
    } else {
        foreach_realized_class([&count](Class cls) {
            count++;
            return true;
        });
    }

    return count;
}

static Class *
objc_copyRealizedClassList_nolock(unsigned int *outCount)
{
    Class *result = nil;
    unsigned int count = 0;

    foreach_realized_class([&count](Class cls) {
        count++;
        return true;
    });

    if (count > 0) {
        unsigned int c = 0;

        result = (Class *)malloc((1+count) * sizeof(Class));
        foreach_realized_class([=, &c](Class cls) {
            result[c++] = cls;
            return true;
        });
        result[c] = nil;
    }

    if (outCount) *outCount = count;
    return result;
}

static void
class_getImpCache_nolock(Class cls, cache_t &cache, objc_imp_cache_entry *buffer, int len)
{
    bucket_t *buckets = cache.buckets();

    uintptr_t count = cache.capacity();
    uintptr_t index;
    int wpos = 0;

    for (index = 0; index < count && wpos < len; index += 1) {
        if (buckets[index].sel()) {
            buffer[wpos].imp = buckets[index].imp(cls);
            buffer[wpos].sel = buckets[index].sel();
            wpos++;
        }
    }
}

/***********************************************************************
 * objc_getClassList
 * Returns pointers to all classes.
 * This requires all classes be realized, which is regretfully non-lazy.
 * Locking: acquires runtimeLock
 **********************************************************************/
int
objc_getClassList(Class *buffer, int bufferLen)
{
    mutex_locker_t lock(runtimeLock);

    realizeAllClasses();

    return objc_getRealizedClassList_nolock(buffer, bufferLen);
}

/***********************************************************************
 * objc_copyClassList
 * Returns pointers to Realized classes.
 *
 * outCount may be nil. *outCount is the number of classes returned.
 * If the returned array is not nil, it is nil-terminated and must be
 * freed with free().
 * Locking: write-locks runtimeLock
 **********************************************************************/
Class *
objc_copyRealizedClassList(unsigned int *outCount)
{
    mutex_locker_t lock(runtimeLock);

    return objc_copyRealizedClassList_nolock(outCount);
}


/***********************************************************************
* objc_copyClassList
* Returns pointers to all classes.
* This requires all classes be realized, which is regretfully non-lazy.
* 
* outCount may be nil. *outCount is the number of classes returned. 
* If the returned array is not nil, it is nil-terminated and must be 
* freed with free().
* Locking: write-locks runtimeLock
**********************************************************************/
Class *
objc_copyClassList(unsigned int *outCount)
{
    mutex_locker_t lock(runtimeLock);

    realizeAllClasses();

    return objc_copyRealizedClassList_nolock(outCount);
}

/***********************************************************************
 * class_copyImpCache
 * Returns the current content of the Class IMP Cache
 *
 * outCount may be nil. *outCount is the number of entries returned.
 * If the returned array is not nil, it is nil-terminated and must be
 * freed with free().
 * Locking: write-locks cacheUpdateLock
 **********************************************************************/
objc_imp_cache_entry *
class_copyImpCache(Class cls, int *outCount)
{
    objc_imp_cache_entry *buffer = nullptr;

#if CONFIG_USE_CACHE_LOCK
    mutex_locker_t lock(cacheUpdateLock);
#else
    mutex_locker_t lock(runtimeLock);
#endif

    cache_t &cache = cls->cache;
    int count = (int)cache.occupied();

    if (count) {
        buffer = (objc_imp_cache_entry *)calloc(1+count, sizeof(objc_imp_cache_entry));
        class_getImpCache_nolock(cls, cache, buffer, count);
    }

    if (outCount) *outCount = count;
    return buffer;
}


/***********************************************************************
* objc_copyProtocolList
* Returns pointers to all protocols.
* Locking: read-locks runtimeLock
**********************************************************************/
Protocol * __unsafe_unretained * 
objc_copyProtocolList(unsigned int *outCount) 
{
    mutex_locker_t lock(runtimeLock);

    NXMapTable *protocol_map = protocols();

    // Find all the protocols from the pre-optimized images.  These protocols
    // won't be in the protocol map.
    objc::DenseMap<const char*, Protocol*> preoptimizedProtocols;
    if (sharedCacheSupportsProtocolRoots()) {
        header_info *hi;
        for (hi = FirstHeader; hi; hi = hi->getNext()) {
            if (!hi->hasPreoptimizedProtocols())
                continue;

            size_t count, i;
            const protocol_t * const *protolist = _getObjc2ProtocolList(hi, &count);
            for (i = 0; i < count; i++) {
                const protocol_t* protocol = protolist[i];

                // Skip protocols we have in the run time map.  These likely
                // correspond to protocols added dynamically which have the same
                // name as a protocol found later in a dlopen'ed shared cache image.
                if (NXMapGet(protocol_map, protocol->mangledName) != nil)
                    continue;

                // The protocols in the shared cache protolist point to their
                // original on-disk object, not the optimized one.  We can use the name
                // to find the optimized one.
                Protocol* optimizedProto = getPreoptimizedProtocol(protocol->mangledName);
                preoptimizedProtocols.insert({ protocol->mangledName, optimizedProto });
            }
        }
    }

    unsigned int count = NXCountMapTable(protocol_map) + (unsigned int)preoptimizedProtocols.size();
    if (count == 0) {
        if (outCount) *outCount = 0;
        return nil;
    }

    Protocol **result = (Protocol **)malloc((count+1) * sizeof(Protocol*));

    unsigned int i = 0;
    Protocol *proto;
    const char *name;
    NXMapState state = NXInitMapState(protocol_map);
    while (NXNextMapState(protocol_map, &state, 
                          (const void **)&name, (const void **)&proto))
    {
        result[i++] = proto;
    }

    // Add any protocols found in the pre-optimized table
    for (auto it : preoptimizedProtocols) {
        result[i++] = it.second;
    }
    
    result[i++] = nil;
    ASSERT(i == count+1);

    if (outCount) *outCount = count;
    return result;
}


/***********************************************************************
* objc_getProtocol
* Get a protocol by name, or return nil
* Locking: read-locks runtimeLock
**********************************************************************/
Protocol *objc_getProtocol(const char *name)
{
    mutex_locker_t lock(runtimeLock); 
    return getProtocol(name);
}


/***********************************************************************
* class_copyMethodList
* fixme
* Locking: read-locks runtimeLock
**********************************************************************/
Method *
class_copyMethodList(Class cls, unsigned int *outCount)
{
    unsigned int count = 0;
    Method *result = nil;

    if (!cls) {
        if (outCount) *outCount = 0;
        return nil;
    }

    mutex_locker_t lock(runtimeLock);
    const auto methods = cls->data()->methods();
    
    ASSERT(cls->isRealized());

    count = methods.count();

    if (count > 0) {
        result = (Method *)malloc((count + 1) * sizeof(Method));
        
        count = 0;
        for (auto& meth : methods) {
            result[count++] = &meth;
        }
        result[count] = nil;
    }

    if (outCount) *outCount = count;
    return result;
}


/***********************************************************************
* class_copyIvarList
* fixme
* Locking: read-locks runtimeLock
**********************************************************************/
Ivar *
class_copyIvarList(Class cls, unsigned int *outCount)
{
    const ivar_list_t *ivars;
    Ivar *result = nil;
    unsigned int count = 0;

    if (!cls) {
        if (outCount) *outCount = 0;
        return nil;
    }

    mutex_locker_t lock(runtimeLock);

    ASSERT(cls->isRealized());
    
    if ((ivars = cls->data()->ro()->ivars)  &&  ivars->count) {
        result = (Ivar *)malloc((ivars->count+1) * sizeof(Ivar));
        
        for (auto& ivar : *ivars) {
            if (!ivar.offset) continue;  // anonymous bitfield
            result[count++] = &ivar;
        }
        result[count] = nil;
    }
    
    if (outCount) *outCount = count;
    return result;
}


/***********************************************************************
* class_copyPropertyList. Returns a heap block containing the 
* properties declared in the class, or nil if the class 
* declares no properties. Caller must free the block.
* Does not copy any superclass's properties.
* Locking: read-locks runtimeLock
**********************************************************************/
objc_property_t *
class_copyPropertyList(Class cls, unsigned int *outCount)
{
    if (!cls) {
        if (outCount) *outCount = 0;
        return nil;
    }

    mutex_locker_t lock(runtimeLock);

    checkIsKnownClass(cls);
    ASSERT(cls->isRealized());
    
    auto rw = cls->data();

    property_t **result = nil;
    auto const properties = rw->properties();
    unsigned int count = properties.count();
    if (count > 0) {
        result = (property_t **)malloc((count + 1) * sizeof(property_t *));

        count = 0;
        for (auto& prop : properties) {
            result[count++] = &prop;
        }
        result[count] = nil;
    }

    if (outCount) *outCount = count;
    return (objc_property_t *)result;
}


/***********************************************************************
* objc_class::getLoadMethod
* fixme
* Called only from add_class_to_loadable_list.
* Locking: runtimeLock must be read- or write-locked by the caller.
**********************************************************************/
IMP 
objc_class::getLoadMethod()
{
    runtimeLock.assertLocked();

    const method_list_t *mlist;

    ASSERT(isRealized());
    ASSERT(ISA()->isRealized());
    ASSERT(!isMetaClass());
    ASSERT(ISA()->isMetaClass());

    mlist = ISA()->data()->ro()->baseMethods();
    if (mlist) {
        for (const auto& meth : *mlist) {
            const char *name = sel_cname(meth.name);
            if (0 == strcmp(name, "load")) {
                return meth.imp;
            }
        }
    }

    return nil;
}


/***********************************************************************
* _category_getName
* Returns a category's name.
* Locking: none
**********************************************************************/
const char *
_category_getName(Category cat)
{
    return cat->name;
}


/***********************************************************************
* _category_getClassName
* Returns a category's class's name
* Called only from add_category_to_loadable_list and 
* remove_category_from_loadable_list for logging purposes.
* Locking: runtimeLock must be read- or write-locked by the caller
**********************************************************************/
const char *
_category_getClassName(Category cat)
{
    runtimeLock.assertLocked();
    return remapClass(cat->cls)->nameForLogging();
}


/***********************************************************************
* _category_getClass
* Returns a category's class
* Called only by call_category_loads.
* Locking: read-locks runtimeLock
**********************************************************************/
Class 
_category_getClass(Category cat)
{
    mutex_locker_t lock(runtimeLock);
    Class result = remapClass(cat->cls);
    ASSERT(result->isRealized());  // ok for call_category_loads' usage
    return result;
}


/***********************************************************************
* _category_getLoadMethod
* fixme
* Called only from add_category_to_loadable_list
* Locking: runtimeLock must be read- or write-locked by the caller
**********************************************************************/
IMP 
_category_getLoadMethod(Category cat)
{
    runtimeLock.assertLocked();

    const method_list_t *mlist;

    mlist = cat->classMethods;
    if (mlist) {
        for (const auto& meth : *mlist) {
            const char *name = sel_cname(meth.name);
            if (0 == strcmp(name, "load")) {
                return meth.imp;
            }
        }
    }

    return nil;
}


/***********************************************************************
* category_t::propertiesForMeta
* Return a category's instance or class properties.
* hi is the image containing the category.
**********************************************************************/
property_list_t *
category_t::propertiesForMeta(bool isMeta, struct header_info *hi)
{
    if (!isMeta) return instanceProperties;
    else if (hi->info()->hasCategoryClassProperties()) return _classProperties;
    else return nil;
}


/***********************************************************************
* class_copyProtocolList
* fixme
* Locking: read-locks runtimeLock
**********************************************************************/
Protocol * __unsafe_unretained * 
class_copyProtocolList(Class cls, unsigned int *outCount)
{
    unsigned int count = 0;
    Protocol **result = nil;
    
    if (!cls) {
        if (outCount) *outCount = 0;
        return nil;
    }

    mutex_locker_t lock(runtimeLock);
    const auto protocols = cls->data()->protocols();

    checkIsKnownClass(cls);

    ASSERT(cls->isRealized());
    
    count = protocols.count();

    if (count > 0) {
        result = (Protocol **)malloc((count+1) * sizeof(Protocol *));

        count = 0;
        for (const auto& proto : protocols) {
            result[count++] = (Protocol *)remapProtocol(proto);
        }
        result[count] = nil;
    }

    if (outCount) *outCount = count;
    return result;
}


/***********************************************************************
* objc_copyImageNames
* Copies names of loaded images with ObjC contents.
*
* Locking: acquires runtimeLock
**********************************************************************/
const char **objc_copyImageNames(unsigned int *outCount)
{
    mutex_locker_t lock(runtimeLock);

    int HeaderCount = 0;
    for (header_info *hi = FirstHeader; hi != nil; hi = hi->getNext()) {
        HeaderCount++;
    }

#if TARGET_OS_WIN32
    const TCHAR **names = (const TCHAR **)
        malloc((HeaderCount+1) * sizeof(TCHAR *));
#else
    const char **names = (const char **)
        malloc((HeaderCount+1) * sizeof(char *));
#endif

    unsigned int count = 0;
    for (header_info *hi = FirstHeader; hi != nil; hi = hi->getNext()) {
#if TARGET_OS_WIN32
        if (hi->moduleName) {
            names[count++] = hi->moduleName;
        }
#else
        const char *fname = hi->fname();
        if (fname) {
            names[count++] = fname;
        }
#endif
    }
    names[count] = nil;
    
    if (count == 0) {
        // Return nil instead of empty list if there are no images
        free((void *)names);
        names = nil;
    }

    if (outCount) *outCount = count;
    return names;
}


/***********************************************************************
* copyClassNamesForImage_nolock
* Copies class names from the given image.
* Missing weak-import classes are omitted.
* Swift class names are demangled.
*
* Locking: runtimeLock must be held by the caller
**********************************************************************/
const char **
copyClassNamesForImage_nolock(header_info *hi, unsigned int *outCount)
{
    runtimeLock.assertLocked();
    ASSERT(hi);

    size_t count;
    classref_t const *classlist = _getObjc2ClassList(hi, &count);
    const char **names = (const char **)
        malloc((count+1) * sizeof(const char *));

    size_t shift = 0;
    for (size_t i = 0; i < count; i++) {
        Class cls = remapClass(classlist[i]);
        if (cls) {
            names[i-shift] = cls->demangledName(/* needs lock */false);
        } else {
            shift++;  // ignored weak-linked class
        }
    }
    count -= shift;
    names[count] = nil;

    if (outCount) *outCount = (unsigned int)count;
    return names;
}



/***********************************************************************
* objc_copyClassNamesForImage
* Copies class names from the named image.
* The image name must be identical to dladdr's dli_fname value.
* Missing weak-import classes are omitted.
* Swift class names are demangled.
*
* Locking: acquires runtimeLock
**********************************************************************/
const char **
objc_copyClassNamesForImage(const char *image, unsigned int *outCount)
{
    if (!image) {
        if (outCount) *outCount = 0;
        return nil;
    }

    mutex_locker_t lock(runtimeLock);

    // Find the image.
    header_info *hi;
    for (hi = FirstHeader; hi != nil; hi = hi->getNext()) {
#if TARGET_OS_WIN32
        if (0 == wcscmp((TCHAR *)image, hi->moduleName)) break;
#else
        if (0 == strcmp(image, hi->fname())) break;
#endif
    }

    if (!hi) {
        if (outCount) *outCount = 0;
        return nil;
    }

    return copyClassNamesForImage_nolock(hi, outCount);
}


/***********************************************************************
* objc_copyClassNamesForImageHeader
* Copies class names from the given image.
* Missing weak-import classes are omitted.
* Swift class names are demangled.
*
* Locking: acquires runtimeLock
**********************************************************************/
const char **
objc_copyClassNamesForImageHeader(const struct mach_header *mh, unsigned int *outCount)
{
    if (!mh) {
        if (outCount) *outCount = 0;
        return nil;
    }

    mutex_locker_t lock(runtimeLock);

    // Find the image.
    header_info *hi;
    for (hi = FirstHeader; hi != nil; hi = hi->getNext()) {
        if (hi->mhdr() == (const headerType *)mh) break;
    }

    if (!hi) {
        if (outCount) *outCount = 0;
        return nil;
    }

    return copyClassNamesForImage_nolock(hi, outCount);
}


/***********************************************************************
* saveTemporaryString
* Save a string in a thread-local FIFO buffer. 
* This is suitable for temporary strings generated for logging purposes.
**********************************************************************/
static void
saveTemporaryString(char *str)
{
    // Fixed-size FIFO. We free the first string, shift 
    // the rest, and add the new string to the end.
    _objc_pthread_data *data = _objc_fetch_pthread_data(true);
    if (data->printableNames[0]) {
        free(data->printableNames[0]);
    }
    int last = countof(data->printableNames) - 1;
    for (int i = 0; i < last; i++) {
        data->printableNames[i] = data->printableNames[i+1];
    }
    data->printableNames[last] = str;
}


/***********************************************************************
* objc_class::nameForLogging
* Returns the class's name, suitable for display.
* The returned memory is TEMPORARY. Print it or copy it immediately.
* Locking: none
**********************************************************************/
const char *
objc_class::nameForLogging()
{
    // Handle the easy case directly.
    if (isRealized()  ||  isFuture()) {
        if (!isAnySwift()) {
            return data()->ro()->name;
        }
        auto rwe = data()->ext();
        if (rwe && rwe->demangledName) {
            return rwe->demangledName;
        }
    }

    char *result;

    const char *name = mangledName();
    char *de = copySwiftV1DemangledName(name);
    if (de) result = de;
    else result = strdup(name);

    saveTemporaryString(result);
    return result;
}


/***********************************************************************
* objc_class::demangledName
* If realize=false, the class must already be realized or future.
* Locking: runtimeLock may or may not be held by the caller.
**********************************************************************/
mutex_t DemangleCacheLock;
static objc::DenseSet<const char *> *DemangleCache;
const char *
objc_class::demangledName(bool needsLock)
{
    if (!needsLock) {
        runtimeLock.assertLocked();
    }

    // Return previously demangled name if available.
    if (isRealized()  ||  isFuture()) {
        if (!isAnySwift()) {
            return data()->ro()->name;
        }
        auto rwe = data()->ext();
        if (rwe && rwe->demangledName) {
            return rwe->demangledName;
        }
    }

    // Try demangling the mangled name.
    const char *mangled = mangledName();
    char *de = copySwiftV1DemangledName(mangled);
    class_rw_ext_t *rwe;

    if (isRealized()  ||  isFuture()) {
        if (needsLock) {
            mutex_locker_t lock(runtimeLock);
            rwe = data()->extAllocIfNeeded();
        } else {
            rwe = data()->extAllocIfNeeded();
        }
        // Class is already realized or future.
        // Save demangling result in rw data.
        // We may not own runtimeLock so use an atomic operation instead.
        if (! OSAtomicCompareAndSwapPtrBarrier(nil, (void*)(de ?: mangled), 
                                               (void**)&rwe->demangledName))
        {
            if (de) free(de);
        }
        return rwe->demangledName;
    }

    // Class is not yet realized.
    if (!de) {
        // Name is not mangled. Return it without caching.
        return mangled;
    }

    // Class is not yet realized and name is mangled.
    // Allocate the name but don't save it in the class.
    // Save the name in a side cache instead to prevent leaks.
    // When the class is actually realized we may allocate a second
    // copy of the name, but we don't care.
    // (Previously we would try to realize the class now and save the
    // name there, but realization is more complicated for Swift classes.)

    // Only objc_copyClassNamesForImage() should get here.
    // fixme lldb's calls to class_getName() can also get here when
    // interrogating the dyld shared cache. (rdar://27258517)
    // fixme ASSERT(realize);

    const char *cached;
    {
        mutex_locker_t lock(DemangleCacheLock);
        if (!DemangleCache) {
            DemangleCache = new objc::DenseSet<const char *>{};
        }
        cached = *DemangleCache->insert(de).first;
    }
    if (cached != de) free(de);
    return cached;
}


/***********************************************************************
* class_getName
* fixme
* Locking: may acquire DemangleCacheLock
**********************************************************************/
const char *class_getName(Class cls)
{
    if (!cls) return "nil";
    // fixme lldb calls class_getName() on unrealized classes (rdar://27258517)
    // ASSERT(cls->isRealized()  ||  cls->isFuture());
    return cls->demangledName(/* needs lock */true);
}

/***********************************************************************
* objc_debug_class_getNameRaw
* fixme
* Locking: none
**********************************************************************/
const char *objc_debug_class_getNameRaw(Class cls)
{
    if (!cls) return "nil";
    return cls->mangledName();
}


/***********************************************************************
* class_getVersion
* fixme
* Locking: none
**********************************************************************/
int 
class_getVersion(Class cls)
{
    if (!cls) return 0;
    ASSERT(cls->isRealized());
    auto rwe = cls->data()->ext();
    if (rwe) {
        return rwe->version;
    }
    return cls->isMetaClass() ? 7 : 0;
}


/***********************************************************************
* class_setVersion
* fixme
* Locking: none
**********************************************************************/
void 
class_setVersion(Class cls, int version)
{
    if (!cls) return;
    ASSERT(cls->isRealized());
    auto rwe = cls->data()->ext();
    if (!rwe) {
        mutex_locker_t lock(runtimeLock);
        rwe = cls->data()->extAllocIfNeeded();
    }

    rwe->version = version;
}

/***********************************************************************
 * search_method_list_inline
 **********************************************************************/
ALWAYS_INLINE static method_t *
findMethodInSortedMethodList(SEL key, const method_list_t *list)
{
    ASSERT(list);

    const method_t * const first = &list->first;
    const method_t *base = first;
    const method_t *probe;
    uintptr_t keyValue = (uintptr_t)key;
    uint32_t count;
    
    for (count = list->count; count != 0; count >>= 1) {
        probe = base + (count >> 1);
        
        uintptr_t probeValue = (uintptr_t)probe->name;
        
        if (keyValue == probeValue) {
            // `probe` is a match.
            // Rewind looking for the *first* occurrence of this value.
            // This is required for correct category overrides.
            while (probe > first && keyValue == (uintptr_t)probe[-1].name) {
                probe--;
            }
            return (method_t *)probe;
        }
        
        if (keyValue > probeValue) {
            base = probe + 1;
            count--;
        }
    }
    
    return nil;
}

ALWAYS_INLINE static method_t *
search_method_list_inline(const method_list_t *mlist, SEL sel)
{
    int methodListIsFixedUp = mlist->isFixedUp();
    int methodListHasExpectedSize = mlist->entsize() == sizeof(method_t);
    
    if (fastpath(methodListIsFixedUp && methodListHasExpectedSize)) {
        return findMethodInSortedMethodList(sel, mlist);
    } else {
        // Linear search of unsorted method list
        for (auto& meth : *mlist) {
            if (meth.name == sel) return &meth;
        }
    }

#if DEBUG
    // sanity-check negative results
    if (mlist->isFixedUp()) {
        for (auto& meth : *mlist) {
            if (meth.name == sel) {
                _objc_fatal("linear search worked when binary search did not");
            }
        }
    }
#endif

    return nil;
}

NEVER_INLINE static method_t *
search_method_list(const method_list_t *mlist, SEL sel)
{
    return search_method_list_inline(mlist, sel);
}

/***********************************************************************
 * method_lists_contains_any
 **********************************************************************/
static NEVER_INLINE bool
method_lists_contains_any(method_list_t * const *mlists, method_list_t * const *end,
                          SEL sels[], size_t selcount)
{
    while (mlists < end) {
        const method_list_t *mlist = *mlists++;
        int methodListIsFixedUp = mlist->isFixedUp();
        int methodListHasExpectedSize = mlist->entsize() == sizeof(method_t);

        if (fastpath(methodListIsFixedUp && methodListHasExpectedSize)) {
            for (size_t i = 0; i < selcount; i++) {
                if (findMethodInSortedMethodList(sels[i], mlist)) {
                    return true;
                }
            }
        } else {
            for (auto& meth : *mlist) {
                for (size_t i = 0; i < selcount; i++) {
                    if (meth.name == sels[i]) {
                        return true;
                    }
                }
            }
        }
    }
    return false;
}

/***********************************************************************
 * getMethodNoSuper_nolock
 * fixme
 * Locking: runtimeLock must be read- or write-locked by the caller
 **********************************************************************/
static method_t *
getMethodNoSuper_nolock(Class cls, SEL sel)
{
    runtimeLock.assertLocked();

    ASSERT(cls->isRealized());
    // fixme nil cls? 
    // fixme nil sel?

    auto const methods = cls->data()->methods();
    for (auto mlists = methods.beginLists(),
              end = methods.endLists();
         mlists != end;
         ++mlists)
    {
        // <rdar://problem/46904873> getMethodNoSuper_nolock is the hottest
        // caller of search_method_list, inlining it turns
        // getMethodNoSuper_nolock into a frame-less function and eliminates
        // any store from this codepath.
        method_t *m = search_method_list_inline(*mlists, sel);
        if (m) return m;
    }

    return nil;
}


/***********************************************************************
* getMethod_nolock
* fixme
* Locking: runtimeLock must be read- or write-locked by the caller
**********************************************************************/
static method_t *
getMethod_nolock(Class cls, SEL sel)
{
    method_t *m = nil;

    runtimeLock.assertLocked();

    // fixme nil cls?
    // fixme nil sel?

    ASSERT(cls->isRealized());

    while (cls  &&  ((m = getMethodNoSuper_nolock(cls, sel))) == nil) {
        cls = cls->superclass;
    }

    return m;
}


/***********************************************************************
* _class_getMethod
* fixme
* Locking: read-locks runtimeLock
**********************************************************************/
static Method _class_getMethod(Class cls, SEL sel)
{
    mutex_locker_t lock(runtimeLock);
    return getMethod_nolock(cls, sel);
}


/***********************************************************************
* class_getInstanceMethod.  Return the instance method for the
* specified class and selector.
**********************************************************************/
Method class_getInstanceMethod(Class cls, SEL sel)
{
    if (!cls  ||  !sel) return nil;

    // This deliberately avoids +initialize because it historically did so.

    // This implementation is a bit weird because it's the only place that 
    // wants a Method instead of an IMP.

#warning fixme build and search caches
        
    // Search method lists, try method resolver, etc.
    lookUpImpOrForward(nil, sel, cls, LOOKUP_RESOLVER);

#warning fixme build and search caches

    return _class_getMethod(cls, sel);
}


/***********************************************************************
* resolveClassMethod
* Call +resolveClassMethod, looking for a method to be added to class cls.
* cls should be a metaclass.
* Does not check if the method already exists.
**********************************************************************/
static void resolveClassMethod(id inst, SEL sel, Class cls)
{
    runtimeLock.assertUnlocked();
    ASSERT(cls->isRealized());
    ASSERT(cls->isMetaClass());

    if (!lookUpImpOrNil(inst, @selector(resolveClassMethod:), cls)) {
        // Resolver not implemented.
        return;
    }

    Class nonmeta;
    {
        mutex_locker_t lock(runtimeLock);
        nonmeta = getMaybeUnrealizedNonMetaClass(cls, inst);
        // +initialize path should have realized nonmeta already
        if (!nonmeta->isRealized()) {
            _objc_fatal("nonmeta class %s (%p) unexpectedly not realized",
                        nonmeta->nameForLogging(), nonmeta);
        }
    }
    BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;
    bool resolved = msg(nonmeta, @selector(resolveClassMethod:), sel);

    // Cache the result (good or bad) so the resolver doesn't fire next time.
    // +resolveClassMethod adds to self->ISA() a.k.a. cls
    IMP imp = lookUpImpOrNil(inst, sel, cls);

    if (resolved  &&  PrintResolving) {
        if (imp) {
            _objc_inform("RESOLVE: method %c[%s %s] "
                         "dynamically resolved to %p", 
                         cls->isMetaClass() ? '+' : '-', 
                         cls->nameForLogging(), sel_getName(sel), imp);
        }
        else {
            // Method resolver didn't add anything?
            _objc_inform("RESOLVE: +[%s resolveClassMethod:%s] returned YES"
                         ", but no new implementation of %c[%s %s] was found",
                         cls->nameForLogging(), sel_getName(sel), 
                         cls->isMetaClass() ? '+' : '-', 
                         cls->nameForLogging(), sel_getName(sel));
        }
    }
}


/***********************************************************************
* resolveInstanceMethod
* Call +resolveInstanceMethod, looking for a method to be added to class cls.
* cls may be a metaclass or a non-meta class.
* Does not check if the method already exists.
**********************************************************************/
static void resolveInstanceMethod(id inst, SEL sel, Class cls)
{
    runtimeLock.assertUnlocked();
    ASSERT(cls->isRealized());
    SEL resolve_sel = @selector(resolveInstanceMethod:);

    if (!lookUpImpOrNil(cls, resolve_sel, cls->ISA())) {
        // Resolver not implemented.
        return;
    }

    BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;
    bool resolved = msg(cls, resolve_sel, sel);

    // Cache the result (good or bad) so the resolver doesn't fire next time.
    // +resolveInstanceMethod adds to self a.k.a. cls
    IMP imp = lookUpImpOrNil(inst, sel, cls);

    if (resolved  &&  PrintResolving) {
        if (imp) {
            _objc_inform("RESOLVE: method %c[%s %s] "
                         "dynamically resolved to %p", 
                         cls->isMetaClass() ? '+' : '-', 
                         cls->nameForLogging(), sel_getName(sel), imp);
        }
        else {
            // Method resolver didn't add anything?
            _objc_inform("RESOLVE: +[%s resolveInstanceMethod:%s] returned YES"
                         ", but no new implementation of %c[%s %s] was found",
                         cls->nameForLogging(), sel_getName(sel), 
                         cls->isMetaClass() ? '+' : '-', 
                         cls->nameForLogging(), sel_getName(sel));
        }
    }
}


/***********************************************************************
* resolveMethod_locked
* Call +resolveClassMethod or +resolveInstanceMethod.
*
* Called with the runtimeLock held to avoid pressure in the caller
* Tail calls into lookUpImpOrForward, also to avoid pressure in the callerb
**********************************************************************/
static NEVER_INLINE IMP
resolveMethod_locked(id inst, SEL sel, Class cls, int behavior)
{
    runtimeLock.assertLocked();
    ASSERT(cls->isRealized());

    runtimeLock.unlock();

    if (! cls->isMetaClass()) {
        // try [cls resolveInstanceMethod:sel]
        resolveInstanceMethod(inst, sel, cls);
    } 
    else {
        // try [nonMetaClass resolveClassMethod:sel]
        // and [cls resolveInstanceMethod:sel]
        resolveClassMethod(inst, sel, cls);
        if (!lookUpImpOrNil(inst, sel, cls)) {
            resolveInstanceMethod(inst, sel, cls);
        }
    }

    // chances are that calling the resolver have populated the cache
    // so attempt using it
    return lookUpImpOrForward(inst, sel, cls, behavior | LOOKUP_CACHE);
}


/***********************************************************************
* log_and_fill_cache
* Log this method call. If the logger permits it, fill the method cache.
* cls is the method whose cache should be filled. 
* implementer is the class that owns the implementation in question.
**********************************************************************/
static void
log_and_fill_cache(Class cls, IMP imp, SEL sel, id receiver, Class implementer)
{
#if SUPPORT_MESSAGE_LOGGING
    if (slowpath(objcMsgLogEnabled && implementer)) {
        bool cacheIt = logMessageSend(implementer->isMetaClass(), 
                                      cls->nameForLogging(),
                                      implementer->nameForLogging(), 
                                      sel);
        if (!cacheIt) return;
    }
#endif
    cache_fill(cls, sel, imp, receiver);
}


/***********************************************************************
* lookUpImpOrForward.
* The standard IMP lookup. 
* Without LOOKUP_INITIALIZE: tries to avoid +initialize (but sometimes fails)
* Without LOOKUP_CACHE: skips optimistic unlocked lookup (but uses cache elsewhere)
* Most callers should use LOOKUP_INITIALIZE and LOOKUP_CACHE
* inst is an instance of cls or a subclass thereof, or nil if none is known. 
*   If cls is an un-initialized metaclass then a non-nil inst is faster.
* May return _objc_msgForward_impcache. IMPs destined for external use 
*   must be converted to _objc_msgForward or _objc_msgForward_stret.
*   If you don't want forwarding at all, use LOOKUP_NIL.
**********************************************************************/
IMP lookUpImpOrForward(id inst, SEL sel, Class cls, int behavior)
{
    const IMP forward_imp = (IMP)_objc_msgForward_impcache;
    IMP imp = nil;
    Class curClass;

    runtimeLock.assertUnlocked();

    // Optimistic cache lookup
    if (fastpath(behavior & LOOKUP_CACHE)) {
        imp = cache_getImp(cls, sel);
        if (imp) goto done_nolock;
    }

    // runtimeLock is held during isRealized and isInitialized checking
    // to prevent races against concurrent realization.

    // runtimeLock is held during method search to make
    // method-lookup + cache-fill atomic with respect to method addition.
    // Otherwise, a category could be added but ignored indefinitely because
    // the cache was re-filled with the old value after the cache flush on
    // behalf of the category.

    runtimeLock.lock();

    // We don't want people to be able to craft a binary blob that looks like
    // a class but really isn't one and do a CFI attack.
    //
    // To make these harder we want to make sure this is a class that was
    // either built into the binary or legitimately registered through
    // objc_duplicateClass, objc_initializeClassPair or objc_allocateClassPair.
    //
    // TODO: this check is quite costly during process startup.
    checkIsKnownClass(cls);

    if (slowpath(!cls->isRealized())) {
        cls = realizeClassMaybeSwiftAndLeaveLocked(cls, runtimeLock);
        // runtimeLock may have been dropped but is now locked again
    }

    if (slowpath((behavior & LOOKUP_INITIALIZE) && !cls->isInitialized())) {
        cls = initializeAndLeaveLocked(cls, inst, runtimeLock);
        // runtimeLock may have been dropped but is now locked again

        // If sel == initialize, class_initialize will send +initialize and 
        // then the messenger will send +initialize again after this 
        // procedure finishes. Of course, if this is not being called 
        // from the messenger then it won't happen. 2778172
    }

    runtimeLock.assertLocked();
    curClass = cls;

    // The code used to lookpu the class's cache again right after
    // we take the lock but for the vast majority of the cases
    // evidence shows this is a miss most of the time, hence a time loss.
    //
    // The only codepath calling into this without having performed some
    // kind of cache lookup is class_getInstanceMethod().

    for (unsigned attempts = unreasonableClassCount();;) {
        // curClass method list.
        Method meth = getMethodNoSuper_nolock(curClass, sel);
        if (meth) {
            imp = meth->imp;
            goto done;
        }

        if (slowpath((curClass = curClass->superclass) == nil)) {
            // No implementation found, and method resolver didn't help.
            // Use forwarding.
            imp = forward_imp;
            break;
        }

        // Halt if there is a cycle in the superclass chain.
        if (slowpath(--attempts == 0)) {
            _objc_fatal("Memory corruption in class list.");
        }

        // Superclass cache.
        imp = cache_getImp(curClass, sel);
        if (slowpath(imp == forward_imp)) {
            // Found a forward:: entry in a superclass.
            // Stop searching, but don't cache yet; call method
            // resolver for this class first.
            break;
        }
        if (fastpath(imp)) {
            // Found the method in a superclass. Cache it in this class.
            goto done;
        }
    }

    // No implementation found. Try method resolver once.

    if (slowpath(behavior & LOOKUP_RESOLVER)) {
        behavior ^= LOOKUP_RESOLVER;
        return resolveMethod_locked(inst, sel, cls, behavior);
    }

 done:
    log_and_fill_cache(cls, imp, sel, inst, curClass);
    runtimeLock.unlock();
 done_nolock:
    if (slowpath((behavior & LOOKUP_NIL) && imp == forward_imp)) {
        return nil;
    }
    return imp;
}

/***********************************************************************
* lookupMethodInClassAndLoadCache.
* Like lookUpImpOrForward, but does not search superclasses.
* Caches and returns objc_msgForward if the method is not found in the class.
**********************************************************************/
IMP lookupMethodInClassAndLoadCache(Class cls, SEL sel)
{
    Method meth;
    IMP imp;

    // fixme this is incomplete - no resolver, +initialize - 
    // but it's only used for .cxx_construct/destruct so we don't care
    ASSERT(sel == SEL_cxx_construct  ||  sel == SEL_cxx_destruct);

    // Search cache first.
    imp = cache_getImp(cls, sel);
    if (imp) return imp;

    // Cache miss. Search method list.

    mutex_locker_t lock(runtimeLock);

    meth = getMethodNoSuper_nolock(cls, sel);

    if (meth) {
        // Hit in method list. Cache it.
        cache_fill(cls, sel, meth->imp, nil);
        return meth->imp;
    } else {
        // Miss in method list. Cache objc_msgForward.
        cache_fill(cls, sel, _objc_msgForward_impcache, nil);
        return _objc_msgForward_impcache;
    }
}


/***********************************************************************
* class_getProperty
* fixme
* Locking: read-locks runtimeLock
**********************************************************************/
objc_property_t class_getProperty(Class cls, const char *name)
{
    if (!cls  ||  !name) return nil;

    mutex_locker_t lock(runtimeLock);

    checkIsKnownClass(cls);
    
    ASSERT(cls->isRealized());

    for ( ; cls; cls = cls->superclass) {
        for (auto& prop : cls->data()->properties()) {
            if (0 == strcmp(name, prop.name)) {
                return (objc_property_t)&prop;
            }
        }
    }
    
    return nil;
}


/***********************************************************************
* Locking: fixme
**********************************************************************/

Class gdb_class_getClass(Class cls)
{
    const char *className = cls->mangledName();
    if(!className || !strlen(className)) return Nil;
    Class rCls = look_up_class(className, NO, NO);
    return rCls;
}

Class gdb_object_getClass(id obj)
{
    if (!obj) return nil;
    return gdb_class_getClass(obj->getIsa());
}


/***********************************************************************
* Locking: write-locks runtimeLock
**********************************************************************/
void 
objc_class::setInitialized()
{
    Class metacls;
    Class cls;

    ASSERT(!isMetaClass());

    cls = (Class)this;
    metacls = cls->ISA();

    mutex_locker_t lock(runtimeLock);

    // Special cases:
    // - NSObject AWZ  class methods are default.
    // - NSObject RR   class and instance methods are default.
    // - NSObject Core class and instance methods are default.
    // adjustCustomFlagsForMethodChange() also knows these special cases.
    // attachMethodLists() also knows these special cases.

    objc::AWZScanner::scanInitializedClass(cls, metacls);
    objc::RRScanner::scanInitializedClass(cls, metacls);
    objc::CoreScanner::scanInitializedClass(cls, metacls);

    // Update the +initialize flags.
    // Do this last.
    metacls->changeInfo(RW_INITIALIZED, RW_INITIALIZING);
}


void
objc_class::printInstancesRequireRawIsa(bool inherited)
{
    ASSERT(PrintRawIsa);
    ASSERT(instancesRequireRawIsa());
    _objc_inform("RAW ISA:  %s%s%s", nameForLogging(), 
                 isMetaClass() ? " (meta)" : "", 
                 inherited ? " (inherited)" : "");
}

/***********************************************************************
* Mark this class and all of its subclasses as requiring raw isa pointers
**********************************************************************/
void objc_class::setInstancesRequireRawIsaRecursively(bool inherited)
{
    Class cls = (Class)this;
    runtimeLock.assertLocked();

    if (instancesRequireRawIsa()) return;
    
    foreach_realized_class_and_subclass(cls, [=](Class c){
        if (c->instancesRequireRawIsa()) {
            return false;
        }

        c->setInstancesRequireRawIsa();

        if (PrintRawIsa) c->printInstancesRequireRawIsa(inherited || c != cls);
        return true;
    });
}


/***********************************************************************
* Choose a class index. 
* Set instancesRequireRawIsa if no more class indexes are available.
**********************************************************************/
void objc_class::chooseClassArrayIndex()
{
#if SUPPORT_INDEXED_ISA
    Class cls = (Class)this;
    runtimeLock.assertLocked();

    if (objc_indexed_classes_count >= ISA_INDEX_COUNT) {
        // No more indexes available.
        ASSERT(cls->classArrayIndex() == 0);
        cls->setInstancesRequireRawIsaRecursively(false/*not inherited*/);
        return;
    }

    unsigned index = objc_indexed_classes_count++;
    if (index == 0) index = objc_indexed_classes_count++;  // index 0 is unused
    classForIndex(index) = cls;
    cls->setClassArrayIndex(index);
#endif
}


/***********************************************************************
* Update custom RR and AWZ when a method changes its IMP
**********************************************************************/
static void
adjustCustomFlagsForMethodChange(Class cls, method_t *meth)
{
    objc::AWZScanner::scanChangedMethod(cls, meth);
    objc::RRScanner::scanChangedMethod(cls, meth);
    objc::CoreScanner::scanChangedMethod(cls, meth);
}


/***********************************************************************
* class_getIvarLayout
* Called by the garbage collector. 
* The class must be nil or already realized. 
* Locking: none
**********************************************************************/
const uint8_t *
class_getIvarLayout(Class cls)
{
    if (cls) return cls->data()->ro()->ivarLayout;
    else return nil;
}


/***********************************************************************
* class_getWeakIvarLayout
* Called by the garbage collector. 
* The class must be nil or already realized. 
* Locking: none
**********************************************************************/
const uint8_t *
class_getWeakIvarLayout(Class cls)
{
    if (cls) return cls->data()->ro()->weakIvarLayout;
    else return nil;
}


/***********************************************************************
* class_setIvarLayout
* Changes the class's ivar layout.
* nil layout means no unscanned ivars
* The class must be under construction.
* fixme: sanity-check layout vs instance size?
* fixme: sanity-check layout vs superclass?
* Locking: acquires runtimeLock
**********************************************************************/
void
class_setIvarLayout(Class cls, const uint8_t *layout)
{
    if (!cls) return;

    mutex_locker_t lock(runtimeLock);
    
    checkIsKnownClass(cls);

    // Can only change layout of in-construction classes.
    // note: if modifications to post-construction classes were 
    //   allowed, there would be a race below (us vs. concurrent object_setIvar)
    if (!(cls->data()->flags & RW_CONSTRUCTING)) {
        _objc_inform("*** Can't set ivar layout for already-registered "
                     "class '%s'", cls->nameForLogging());
        return;
    }

    class_ro_t *ro_w = make_ro_writeable(cls->data());

    try_free(ro_w->ivarLayout);
    ro_w->ivarLayout = ustrdupMaybeNil(layout);
}


/***********************************************************************
* class_setWeakIvarLayout
* Changes the class's weak ivar layout.
* nil layout means no weak ivars
* The class must be under construction.
* fixme: sanity-check layout vs instance size?
* fixme: sanity-check layout vs superclass?
* Locking: acquires runtimeLock
**********************************************************************/
void
class_setWeakIvarLayout(Class cls, const uint8_t *layout)
{
    if (!cls) return;

    mutex_locker_t lock(runtimeLock);
    
    checkIsKnownClass(cls);

    // Can only change layout of in-construction classes.
    // note: if modifications to post-construction classes were 
    //   allowed, there would be a race below (us vs. concurrent object_setIvar)
    if (!(cls->data()->flags & RW_CONSTRUCTING)) {
        _objc_inform("*** Can't set weak ivar layout for already-registered "
                     "class '%s'", cls->nameForLogging());
        return;
    }

    class_ro_t *ro_w = make_ro_writeable(cls->data());

    try_free(ro_w->weakIvarLayout);
    ro_w->weakIvarLayout = ustrdupMaybeNil(layout);
}


/***********************************************************************
* getIvar
* Look up an ivar by name.
* Locking: runtimeLock must be read- or write-locked by the caller.
**********************************************************************/
static ivar_t *getIvar(Class cls, const char *name)
{
    runtimeLock.assertLocked();

    const ivar_list_t *ivars;
    ASSERT(cls->isRealized());
    if ((ivars = cls->data()->ro()->ivars)) {
        for (auto& ivar : *ivars) {
            if (!ivar.offset) continue;  // anonymous bitfield

            // ivar.name may be nil for anonymous bitfields etc.
            if (ivar.name  &&  0 == strcmp(name, ivar.name)) {
                return &ivar;
            }
        }
    }

    return nil;
}


/***********************************************************************
* _class_getClassForIvar
* Given a class and an ivar that is in it or one of its superclasses, 
* find the actual class that defined the ivar.
**********************************************************************/
Class _class_getClassForIvar(Class cls, Ivar ivar)
{
    mutex_locker_t lock(runtimeLock);

    for ( ; cls; cls = cls->superclass) {
        if (auto ivars = cls->data()->ro()->ivars) {
            if (ivars->containsIvar(ivar)) {
                return cls;
            }
        }
    }

    return nil;
}


/***********************************************************************
* _class_getVariable
* fixme
* Locking: read-locks runtimeLock
**********************************************************************/
Ivar 
_class_getVariable(Class cls, const char *name)
{
    mutex_locker_t lock(runtimeLock);

    for ( ; cls; cls = cls->superclass) {
        ivar_t *ivar = getIvar(cls, name);
        if (ivar) {
            return ivar;
        }
    }

    return nil;
}


/***********************************************************************
* class_conformsToProtocol
* fixme
* Locking: read-locks runtimeLock
**********************************************************************/
BOOL class_conformsToProtocol(Class cls, Protocol *proto_gen)
{
    protocol_t *proto = newprotocol(proto_gen);
    
    if (!cls) return NO;
    if (!proto_gen) return NO;

    mutex_locker_t lock(runtimeLock);

    checkIsKnownClass(cls);
    
    ASSERT(cls->isRealized());
    
    for (const auto& proto_ref : cls->data()->protocols()) {
        protocol_t *p = remapProtocol(proto_ref);
        if (p == proto || protocol_conformsToProtocol_nolock(p, proto)) {
            return YES;
        }
    }

    return NO;
}


/**********************************************************************
* addMethod
* fixme
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static IMP 
addMethod(Class cls, SEL name, IMP imp, const char *types, bool replace)
{
    IMP result = nil;

    runtimeLock.assertLocked();

    checkIsKnownClass(cls);
    
    ASSERT(types);
    ASSERT(cls->isRealized());

    method_t *m;
    if ((m = getMethodNoSuper_nolock(cls, name))) {
        // already exists
        if (!replace) {
            result = m->imp;
        } else {
            result = _method_setImplementation(cls, m, imp);
        }
    } else {
        auto rwe = cls->data()->extAllocIfNeeded();

        // fixme optimize
        method_list_t *newlist;
        newlist = (method_list_t *)calloc(sizeof(*newlist), 1);
        newlist->entsizeAndFlags = 
            (uint32_t)sizeof(method_t) | fixed_up_method_list;
        newlist->count = 1;
        newlist->first.name = name;
        newlist->first.types = strdupIfMutable(types);
        newlist->first.imp = imp;

        prepareMethodLists(cls, &newlist, 1, NO, NO);
        rwe->methods.attachLists(&newlist, 1);
        flushCaches(cls);

        result = nil;
    }

    return result;
}

/**********************************************************************
* addMethods
* Add the given methods to a class in bulk.
* Returns the selectors which could not be added, when replace == NO and a
* method already exists. The returned selectors are NULL terminated and must be
* freed by the caller. They are NULL if no failures occurred.
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static SEL *
addMethods(Class cls, const SEL *names, const IMP *imps, const char **types,
           uint32_t count, bool replace, uint32_t *outFailedCount)
{
    runtimeLock.assertLocked();
    
    ASSERT(names);
    ASSERT(imps);
    ASSERT(types);
    ASSERT(cls->isRealized());
    
    method_list_t *newlist;
    size_t newlistSize = method_list_t::byteSize(sizeof(method_t), count);
    newlist = (method_list_t *)calloc(newlistSize, 1);
    newlist->entsizeAndFlags =
        (uint32_t)sizeof(method_t) | fixed_up_method_list;
    newlist->count = 0;
    
    method_t *newlistMethods = &newlist->first;
    
    SEL *failedNames = nil;
    uint32_t failedCount = 0;
    
    for (uint32_t i = 0; i < count; i++) {
        method_t *m;
        if ((m = getMethodNoSuper_nolock(cls, names[i]))) {
            // already exists
            if (!replace) {
                // report failure
                if (failedNames == nil) {
                    // allocate an extra entry for a trailing NULL in case
                    // every method fails
                    failedNames = (SEL *)calloc(sizeof(*failedNames),
                                                count + 1);
                }
                failedNames[failedCount] = m->name;
                failedCount++;
            } else {
                _method_setImplementation(cls, m, imps[i]);
            }
        } else {
            method_t *newmethod = &newlistMethods[newlist->count];
            newmethod->name = names[i];
            newmethod->types = strdupIfMutable(types[i]);
            newmethod->imp = imps[i];
            newlist->count++;
        }
    }
    
    if (newlist->count > 0) {
        auto rwe = cls->data()->extAllocIfNeeded();

        // fixme resize newlist because it may have been over-allocated above.
        // Note that realloc() alone doesn't work due to ptrauth.
        
        method_t::SortBySELAddress sorter;
        std::stable_sort(newlist->begin(), newlist->end(), sorter);
        
        prepareMethodLists(cls, &newlist, 1, NO, NO);
        rwe->methods.attachLists(&newlist, 1);
        flushCaches(cls);
    } else {
        // Attaching the method list to the class consumes it. If we don't
        // do that, we have to free the memory ourselves.
        free(newlist);
    }
    
    if (outFailedCount) *outFailedCount = failedCount;
    
    return failedNames;
}


BOOL 
class_addMethod(Class cls, SEL name, IMP imp, const char *types)
{
    if (!cls) return NO;

    mutex_locker_t lock(runtimeLock);
    return ! addMethod(cls, name, imp, types ?: "", NO);
}


IMP 
class_replaceMethod(Class cls, SEL name, IMP imp, const char *types)
{
    if (!cls) return nil;

    mutex_locker_t lock(runtimeLock);
    return addMethod(cls, name, imp, types ?: "", YES);
}


SEL *
class_addMethodsBulk(Class cls, const SEL *names, const IMP *imps,
                     const char **types, uint32_t count,
                     uint32_t *outFailedCount)
{
    if (!cls) {
        if (outFailedCount) *outFailedCount = count;
        return (SEL *)memdup(names, count * sizeof(*names));
    }
    
    mutex_locker_t lock(runtimeLock);
    return addMethods(cls, names, imps, types, count, NO, outFailedCount);
}

void
class_replaceMethodsBulk(Class cls, const SEL *names, const IMP *imps,
                         const char **types, uint32_t count)
{
    if (!cls) return;
    
    mutex_locker_t lock(runtimeLock);
    addMethods(cls, names, imps, types, count, YES, nil);
}


/***********************************************************************
* class_addIvar
* Adds an ivar to a class.
* Locking: acquires runtimeLock
**********************************************************************/
BOOL 
class_addIvar(Class cls, const char *name, size_t size, 
              uint8_t alignment, const char *type)
{
    if (!cls) return NO;

    if (!type) type = "";
    if (name  &&  0 == strcmp(name, "")) name = nil;

    mutex_locker_t lock(runtimeLock);

    checkIsKnownClass(cls);
    ASSERT(cls->isRealized());

    // No class variables
    if (cls->isMetaClass()) {
        return NO;
    }

    // Can only add ivars to in-construction classes.
    if (!(cls->data()->flags & RW_CONSTRUCTING)) {
        return NO;
    }

    // Check for existing ivar with this name, unless it's anonymous.
    // Check for too-big ivar.
    // fixme check for superclass ivar too?
    if ((name  &&  getIvar(cls, name))  ||  size > UINT32_MAX) {
        return NO;
    }

    class_ro_t *ro_w = make_ro_writeable(cls->data());

    // fixme allocate less memory here
    
    ivar_list_t *oldlist, *newlist;
    if ((oldlist = (ivar_list_t *)cls->data()->ro()->ivars)) {
        size_t oldsize = oldlist->byteSize();
        newlist = (ivar_list_t *)calloc(oldsize + oldlist->entsize(), 1);
        memcpy(newlist, oldlist, oldsize);
        free(oldlist);
    } else {
        newlist = (ivar_list_t *)calloc(sizeof(ivar_list_t), 1);
        newlist->entsizeAndFlags = (uint32_t)sizeof(ivar_t);
    }

    uint32_t offset = cls->unalignedInstanceSize();
    uint32_t alignMask = (1<<alignment)-1;
    offset = (offset + alignMask) & ~alignMask;

    ivar_t& ivar = newlist->get(newlist->count++);
#if __x86_64__
    // Deliberately over-allocate the ivar offset variable. 
    // Use calloc() to clear all 64 bits. See the note in struct ivar_t.
    ivar.offset = (int32_t *)(int64_t *)calloc(sizeof(int64_t), 1);
#else
    ivar.offset = (int32_t *)malloc(sizeof(int32_t));
#endif
    *ivar.offset = offset;
    ivar.name = name ? strdupIfMutable(name) : nil;
    ivar.type = strdupIfMutable(type);
    ivar.alignment_raw = alignment;
    ivar.size = (uint32_t)size;

    ro_w->ivars = newlist;
    cls->setInstanceSize((uint32_t)(offset + size));

    // Ivar layout updated in registerClass.

    return YES;
}


/***********************************************************************
* class_addProtocol
* Adds a protocol to a class.
* Locking: acquires runtimeLock
**********************************************************************/
BOOL class_addProtocol(Class cls, Protocol *protocol_gen)
{
    protocol_t *protocol = newprotocol(protocol_gen);

    if (!cls) return NO;
    if (class_conformsToProtocol(cls, protocol_gen)) return NO;

    mutex_locker_t lock(runtimeLock);
    auto rwe = cls->data()->extAllocIfNeeded();

    ASSERT(cls->isRealized());
    
    // fixme optimize
    protocol_list_t *protolist = (protocol_list_t *)
        malloc(sizeof(protocol_list_t) + sizeof(protocol_t *));
    protolist->count = 1;
    protolist->list[0] = (protocol_ref_t)protocol;

    rwe->protocols.attachLists(&protolist, 1);

    // fixme metaclass?

    return YES;
}


/***********************************************************************
* class_addProperty
* Adds a property to a class.
* Locking: acquires runtimeLock
**********************************************************************/
static bool 
_class_addProperty(Class cls, const char *name, 
                   const objc_property_attribute_t *attrs, unsigned int count, 
                   bool replace)
{
    if (!cls) return NO;
    if (!name) return NO;

    property_t *prop = class_getProperty(cls, name);
    if (prop  &&  !replace) {
        // already exists, refuse to replace
        return NO;
    } 
    else if (prop) {
        // replace existing
        mutex_locker_t lock(runtimeLock);
        try_free(prop->attributes);
        prop->attributes = copyPropertyAttributeString(attrs, count);
        return YES;
    }
    else {
        mutex_locker_t lock(runtimeLock);
        auto rwe = cls->data()->extAllocIfNeeded();
        
        ASSERT(cls->isRealized());
        
        property_list_t *proplist = (property_list_t *)
            malloc(sizeof(*proplist));
        proplist->count = 1;
        proplist->entsizeAndFlags = sizeof(proplist->first);
        proplist->first.name = strdupIfMutable(name);
        proplist->first.attributes = copyPropertyAttributeString(attrs, count);
        
        rwe->properties.attachLists(&proplist, 1);
        
        return YES;
    }
}

BOOL 
class_addProperty(Class cls, const char *name, 
                  const objc_property_attribute_t *attrs, unsigned int n)
{
    return _class_addProperty(cls, name, attrs, n, NO);
}

void 
class_replaceProperty(Class cls, const char *name, 
                      const objc_property_attribute_t *attrs, unsigned int n)
{
    _class_addProperty(cls, name, attrs, n, YES);
}


/***********************************************************************
* look_up_class
* Look up a class by name, and realize it.
* Locking: acquires runtimeLock
**********************************************************************/
static BOOL empty_getClass(const char *name, Class *outClass)
{
    *outClass = nil;
    return NO;
}

static ChainedHookFunction<objc_hook_getClass> GetClassHook{empty_getClass};

void objc_setHook_getClass(objc_hook_getClass newValue,
                           objc_hook_getClass *outOldValue)
{
    GetClassHook.set(newValue, outOldValue);
}

Class 
look_up_class(const char *name, 
              bool includeUnconnected __attribute__((unused)), 
              bool includeClassHandler __attribute__((unused)))
{
    if (!name) return nil;

    Class result;
    bool unrealized;
    {
        runtimeLock.lock();
        result = getClassExceptSomeSwift(name);
        unrealized = result  &&  !result->isRealized();
        if (unrealized) {
            result = realizeClassMaybeSwiftAndUnlock(result, runtimeLock);
            // runtimeLock is now unlocked
        } else {
            runtimeLock.unlock();
        }
    }

    if (!result) {
        // Ask Swift about its un-instantiated classes.

        // We use thread-local storage to prevent infinite recursion
        // if the hook function provokes another lookup of the same name
        // (for example, if the hook calls objc_allocateClassPair)

        auto *tls = _objc_fetch_pthread_data(true);

        // Stop if this thread is already looking up this name.
        for (unsigned i = 0; i < tls->classNameLookupsUsed; i++) {
            if (0 == strcmp(name, tls->classNameLookups[i])) {
                return nil;
            }
        }

        // Save this lookup in tls.
        if (tls->classNameLookupsUsed == tls->classNameLookupsAllocated) {
            tls->classNameLookupsAllocated =
                (tls->classNameLookupsAllocated * 2 ?: 1);
            size_t size = tls->classNameLookupsAllocated *
                sizeof(tls->classNameLookups[0]);
            tls->classNameLookups = (const char **)
                realloc(tls->classNameLookups, size);
        }
        tls->classNameLookups[tls->classNameLookupsUsed++] = name;

        // Call the hook.
        Class swiftcls = nil;
        if (GetClassHook.get()(name, &swiftcls)) {
            ASSERT(swiftcls->isRealized());
            result = swiftcls;
        }

        // Erase the name from tls.
        unsigned slot = --tls->classNameLookupsUsed;
        ASSERT(slot >= 0  &&  slot < tls->classNameLookupsAllocated);
        ASSERT(name == tls->classNameLookups[slot]);
        tls->classNameLookups[slot] = nil;
    }

    return result;
}


/***********************************************************************
* objc_duplicateClass
* fixme
* Locking: acquires runtimeLock
**********************************************************************/
Class 
objc_duplicateClass(Class original, const char *name, 
                    size_t extraBytes)
{
    Class duplicate;

    mutex_locker_t lock(runtimeLock);

    checkIsKnownClass(original);

    auto orig_rw  = original->data();
    auto orig_rwe = orig_rw->ext();
    auto orig_ro  = orig_rw->ro();

    ASSERT(original->isRealized());
    ASSERT(!original->isMetaClass());

    duplicate = alloc_class_for_subclass(original, extraBytes);

    duplicate->initClassIsa(original->ISA());
    duplicate->superclass = original->superclass;

    duplicate->cache.initializeToEmpty();

    class_rw_t *rw = objc::zalloc<class_rw_t>();
    rw->flags = (orig_rw->flags | RW_COPIED_RO | RW_REALIZING);
    rw->firstSubclass = nil;
    rw->nextSiblingClass = nil;

    duplicate->bits = original->bits;
    duplicate->setData(rw);

    auto ro = orig_ro->duplicate();
    *(char **)&ro->name = strdupIfMutable(name);
    rw->set_ro(ro);

    if (orig_rwe) {
        auto rwe = rw->extAllocIfNeeded();
        rwe->version = orig_rwe->version;
        rwe->methods = orig_rwe->methods.duplicate();

        // fixme dies when categories are added to the base
        rwe->properties = orig_rwe->properties;
        rwe->protocols = orig_rwe->protocols;
    } else if (ro->baseMethods()) {
        // if we have base methods, we need to make a deep copy
        // which requires a class_rw_ext_t to be allocated
        rw->deepCopy(ro);
    }

    duplicate->chooseClassArrayIndex();

    if (duplicate->superclass) {
        addSubclass(duplicate->superclass, duplicate);
        // duplicate->isa == original->isa so don't addSubclass() for it
    } else {
        addRootClass(duplicate);
    }

    // Don't methodize class - construction above is correct

    addNamedClass(duplicate, ro->name);
    addClassTableEntry(duplicate, /*addMeta=*/false);
    
    if (PrintConnecting) {
        _objc_inform("CLASS: realizing class '%s' (duplicate of %s) %p %p", 
                     name, original->nameForLogging(), (void*)duplicate, ro);
    }

    duplicate->clearInfo(RW_REALIZING);

    return duplicate;
}

/***********************************************************************
* objc_initializeClassPair
* Locking: runtimeLock must be write-locked by the caller
**********************************************************************/

// &UnsetLayout is the default ivar layout during class construction
static const uint8_t UnsetLayout = 0;

static void objc_initializeClassPair_internal(Class superclass, const char *name, Class cls, Class meta)
{
    runtimeLock.assertLocked();

    class_ro_t *cls_ro_w, *meta_ro_w;
    class_rw_t *cls_rw_w, *meta_rw_w;
    
    cls_rw_w   = objc::zalloc<class_rw_t>();
    meta_rw_w  = objc::zalloc<class_rw_t>();
    cls_ro_w   = (class_ro_t *)calloc(sizeof(class_ro_t), 1);
    meta_ro_w  = (class_ro_t *)calloc(sizeof(class_ro_t), 1);

    cls->setData(cls_rw_w);
    cls_rw_w->set_ro(cls_ro_w);
    meta->setData(meta_rw_w);
    meta_rw_w->set_ro(meta_ro_w);

    // Set basic info

    cls_rw_w->flags = RW_CONSTRUCTING | RW_COPIED_RO | RW_REALIZED | RW_REALIZING;
    meta_rw_w->flags = RW_CONSTRUCTING | RW_COPIED_RO | RW_REALIZED | RW_REALIZING | RW_META;

    cls_ro_w->flags = 0;
    meta_ro_w->flags = RO_META;
    if (superclass) {
        uint32_t flagsToCopy = RW_FORBIDS_ASSOCIATED_OBJECTS;
        cls_rw_w->flags |= superclass->data()->flags & flagsToCopy;
        cls_ro_w->instanceStart = superclass->unalignedInstanceSize();
        meta_ro_w->instanceStart = superclass->ISA()->unalignedInstanceSize();
        cls->setInstanceSize(cls_ro_w->instanceStart);
        meta->setInstanceSize(meta_ro_w->instanceStart);
    } else {
        cls_ro_w->flags |= RO_ROOT;
        meta_ro_w->flags |= RO_ROOT;
        cls_ro_w->instanceStart = 0;
        meta_ro_w->instanceStart = (uint32_t)sizeof(objc_class);
        cls->setInstanceSize((uint32_t)sizeof(id));  // just an isa
        meta->setInstanceSize(meta_ro_w->instanceStart);
    }

    cls_ro_w->name = strdupIfMutable(name);
    meta_ro_w->name = strdupIfMutable(name);

    cls_ro_w->ivarLayout = &UnsetLayout;
    cls_ro_w->weakIvarLayout = &UnsetLayout;

    meta->chooseClassArrayIndex();
    cls->chooseClassArrayIndex();

    // This absolutely needs to be done before addSubclass
    // as initializeToEmpty() clobbers the FAST_CACHE bits
    cls->cache.initializeToEmpty();
    meta->cache.initializeToEmpty();

#if FAST_CACHE_META
    meta->cache.setBit(FAST_CACHE_META);
#endif
    meta->setInstancesRequireRawIsa();

    // Connect to superclasses and metaclasses
    cls->initClassIsa(meta);

    if (superclass) {
        meta->initClassIsa(superclass->ISA()->ISA());
        cls->superclass = superclass;
        meta->superclass = superclass->ISA();
        addSubclass(superclass, cls);
        addSubclass(superclass->ISA(), meta);
    } else {
        meta->initClassIsa(meta);
        cls->superclass = Nil;
        meta->superclass = cls;
        addRootClass(cls);
        addSubclass(cls, meta);
    }

    addClassTableEntry(cls);
}


/***********************************************************************
* verifySuperclass
* Sanity-check the superclass provided to 
* objc_allocateClassPair, objc_initializeClassPair, or objc_readClassPair.
**********************************************************************/
bool
verifySuperclass(Class superclass, bool rootOK)
{
    if (!superclass) {
        // Superclass does not exist.
        // If subclass may be a root class, this is OK.
        // If subclass must not be a root class, this is bad.
        return rootOK;
    }

    // Superclass must be realized.
    if (! superclass->isRealized()) return false;

    // Superclass must not be under construction.
    if (superclass->data()->flags & RW_CONSTRUCTING) return false;

    return true;
}


/***********************************************************************
* objc_initializeClassPair
**********************************************************************/
Class objc_initializeClassPair(Class superclass, const char *name, Class cls, Class meta)
{
    // Fail if the class name is in use.
    if (look_up_class(name, NO, NO)) return nil;

    mutex_locker_t lock(runtimeLock);

    // Fail if the class name is in use.
    // Fail if the superclass isn't kosher.
    if (getClassExceptSomeSwift(name)  ||
        !verifySuperclass(superclass, true/*rootOK*/))
    {
        return nil;
    }

    objc_initializeClassPair_internal(superclass, name, cls, meta);

    return cls;
}


/***********************************************************************
* objc_allocateClassPair
* fixme
* Locking: acquires runtimeLock
**********************************************************************/
Class objc_allocateClassPair(Class superclass, const char *name, 
                             size_t extraBytes)
{
    Class cls, meta;

    // Fail if the class name is in use.
    if (look_up_class(name, NO, NO)) return nil;

    mutex_locker_t lock(runtimeLock);

    // Fail if the class name is in use.
    // Fail if the superclass isn't kosher.
    if (getClassExceptSomeSwift(name)  ||
        !verifySuperclass(superclass, true/*rootOK*/))
    {
        return nil;
    }

    // Allocate new classes.
    cls  = alloc_class_for_subclass(superclass, extraBytes);
    meta = alloc_class_for_subclass(superclass, extraBytes);

    // fixme mangle the name if it looks swift-y?
    objc_initializeClassPair_internal(superclass, name, cls, meta);

    return cls;
}


/***********************************************************************
* objc_registerClassPair
* fixme
* Locking: acquires runtimeLock
**********************************************************************/
void objc_registerClassPair(Class cls)
{
    mutex_locker_t lock(runtimeLock);

    checkIsKnownClass(cls);

    if ((cls->data()->flags & RW_CONSTRUCTED)  ||
        (cls->ISA()->data()->flags & RW_CONSTRUCTED)) 
    {
        _objc_inform("objc_registerClassPair: class '%s' was already "
                     "registered!", cls->data()->ro()->name);
        return;
    }

    if (!(cls->data()->flags & RW_CONSTRUCTING)  ||  
        !(cls->ISA()->data()->flags & RW_CONSTRUCTING))
    {
        _objc_inform("objc_registerClassPair: class '%s' was not "
                     "allocated with objc_allocateClassPair!", 
                     cls->data()->ro()->name);
        return;
    }

    // Clear "under construction" bit, set "done constructing" bit
    cls->ISA()->changeInfo(RW_CONSTRUCTED, RW_CONSTRUCTING | RW_REALIZING);
    cls->changeInfo(RW_CONSTRUCTED, RW_CONSTRUCTING | RW_REALIZING);

    // Add to named class table.
    addNamedClass(cls, cls->data()->ro()->name);
}


/***********************************************************************
* objc_readClassPair()
* Read a class and metaclass as written by a compiler.
* Assumes the class and metaclass are not referenced by other things 
* that might need to be fixed up (such as categories and subclasses).
* Does not call +load.
* Returns the class pointer, or nil.
*
* Locking: runtimeLock acquired by map_images
**********************************************************************/
Class objc_readClassPair(Class bits, const struct objc_image_info *info)
{
    mutex_locker_t lock(runtimeLock);

    // No info bits are significant yet.
    (void)info;

    // Fail if the superclass isn't kosher.
    bool rootOK = bits->data()->flags & RO_ROOT;
    if (!verifySuperclass(bits->superclass, rootOK)){
        return nil;
    }

    // Duplicate classes are allowed, just like they are for image loading.
    // readClass will complain about the duplicate.

    Class cls = readClass(bits, false/*bundle*/, false/*shared cache*/);
    if (cls != bits) {
        // This function isn't allowed to remap anything.
        _objc_fatal("objc_readClassPair for class %s changed %p to %p", 
                    cls->nameForLogging(), bits, cls);
    }

    // The only client of this function is old Swift.
    // Stable Swift won't use it.
    // fixme once Swift in the OS settles we can assert(!cls->isSwiftStable()).
    cls = realizeClassWithoutSwift(cls, nil);

    return cls;
}


/***********************************************************************
* detach_class
* Disconnect a class from other data structures.
* Exception: does not remove the class from the +load list
* Call this before free_class.
* Locking: runtimeLock must be held by the caller.
**********************************************************************/
static void detach_class(Class cls, bool isMeta)
{
    runtimeLock.assertLocked();

    // categories not yet attached to this class
    objc::unattachedCategories.eraseClass(cls);

    // superclass's subclass list
    if (cls->isRealized()) {
        Class supercls = cls->superclass;
        if (supercls) {
            removeSubclass(supercls, cls);
        } else {
            removeRootClass(cls);
        }
    }

    // class tables and +load queue
    if (!isMeta) {
        removeNamedClass(cls, cls->mangledName());
    }
    objc::allocatedClasses.get().erase(cls);
}


/***********************************************************************
* free_class
* Frees a class's data structures.
* Call this after detach_class.
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static void free_class(Class cls)
{
    runtimeLock.assertLocked();

    if (! cls->isRealized()) return;

    auto rw = cls->data();
    auto rwe = rw->ext();
    auto ro = rw->ro();

    cache_delete(cls);

    if (rwe) {
        for (auto& meth : rwe->methods) {
            try_free(meth.types);
        }
        rwe->methods.tryFree();
    }
    
    const ivar_list_t *ivars = ro->ivars;
    if (ivars) {
        for (auto& ivar : *ivars) {
            try_free(ivar.offset);
            try_free(ivar.name);
            try_free(ivar.type);
        }
        try_free(ivars);
    }

    if (rwe) {
        for (auto& prop : rwe->properties) {
            try_free(prop.name);
            try_free(prop.attributes);
        }
        rwe->properties.tryFree();

        rwe->protocols.tryFree();
    }
    
    try_free(ro->ivarLayout);
    try_free(ro->weakIvarLayout);
    try_free(ro->name);
    try_free(ro);
    objc::zfree(rwe);
    objc::zfree(rw);
    try_free(cls);
}


void objc_disposeClassPair(Class cls)
{
    mutex_locker_t lock(runtimeLock);

    checkIsKnownClass(cls);

    if (!(cls->data()->flags & (RW_CONSTRUCTED|RW_CONSTRUCTING))  ||
        !(cls->ISA()->data()->flags & (RW_CONSTRUCTED|RW_CONSTRUCTING))) 
    {
        // class not allocated with objc_allocateClassPair
        // disposing still-unregistered class is OK!
        _objc_inform("objc_disposeClassPair: class '%s' was not "
                     "allocated with objc_allocateClassPair!", 
                     cls->data()->ro()->name);
        return;
    }

    if (cls->isMetaClass()) {
        _objc_inform("objc_disposeClassPair: class '%s' is a metaclass, "
                     "not a class!", cls->data()->ro()->name);
        return;
    }

    // Shouldn't have any live subclasses.
    if (cls->data()->firstSubclass) {
        _objc_inform("objc_disposeClassPair: class '%s' still has subclasses, "
                     "including '%s'!", cls->data()->ro()->name,
                     cls->data()->firstSubclass->nameForLogging());
    }
    if (cls->ISA()->data()->firstSubclass) {
        _objc_inform("objc_disposeClassPair: class '%s' still has subclasses, "
                     "including '%s'!", cls->data()->ro()->name,
                     cls->ISA()->data()->firstSubclass->nameForLogging());
    }

    // don't remove_class_from_loadable_list() 
    // - it's not there and we don't have the lock
    detach_class(cls->ISA(), YES);
    detach_class(cls, NO);
    free_class(cls->ISA());
    free_class(cls);
}


/***********************************************************************
* objc_constructInstance
* Creates an instance of `cls` at the location pointed to by `bytes`. 
* `bytes` must point to at least class_getInstanceSize(cls) bytes of 
*   well-aligned zero-filled memory.
* The new object's isa is set. Any C++ constructors are called.
* Returns `bytes` if successful. Returns nil if `cls` or `bytes` is 
*   nil, or if C++ constructors fail.
* Note: class_createInstance() and class_createInstances() preflight this.
**********************************************************************/
id 
objc_constructInstance(Class cls, void *bytes)
{
    if (!cls  ||  !bytes) return nil;

    id obj = (id)bytes;

    // Read class's info bits all at once for performance
    bool hasCxxCtor = cls->hasCxxCtor();
    bool hasCxxDtor = cls->hasCxxDtor();
    bool fast = cls->canAllocNonpointer();
    
    if (fast) {
        obj->initInstanceIsa(cls, hasCxxDtor);
    } else {
        obj->initIsa(cls);
    }

    if (hasCxxCtor) {
        return object_cxxConstructFromClass(obj, cls, OBJECT_CONSTRUCT_NONE);
    } else {
        return obj;
    }
}


/***********************************************************************
* class_createInstance
* fixme
* Locking: none
*
* Note: this function has been carefully written so that the fastpath
* takes no branch.
**********************************************************************/
static ALWAYS_INLINE id
_class_createInstanceFromZone(Class cls, size_t extraBytes, void *zone,
                              int construct_flags = OBJECT_CONSTRUCT_NONE,
                              bool cxxConstruct = true,
                              size_t *outAllocatedSize = nil)
{
    ASSERT(cls->isRealized());

    // Read class's info bits all at once for performance
    bool hasCxxCtor = cxxConstruct && cls->hasCxxCtor();
    bool hasCxxDtor = cls->hasCxxDtor();
    bool fast = cls->canAllocNonpointer();
    size_t size;

    size = cls->instanceSize(extraBytes);
    if (outAllocatedSize) *outAllocatedSize = size;

    id obj;
    if (zone) {
        obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);
    } else {
        obj = (id)calloc(1, size);
    }
    if (slowpath(!obj)) {
        if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
            return _objc_callBadAllocHandler(cls);
        }
        return nil;
    }

    if (!zone && fast) {
        obj->initInstanceIsa(cls, hasCxxDtor);
    } else {
        // Use raw pointer isa on the assumption that they might be
        // doing something weird with the zone or RR.
        obj->initIsa(cls);
    }

    if (fastpath(!hasCxxCtor)) {
        return obj;
    }

    construct_flags |= OBJECT_CONSTRUCT_FREE_ONFAILURE;
    return object_cxxConstructFromClass(obj, cls, construct_flags);
}

id
class_createInstance(Class cls, size_t extraBytes)
{
    if (!cls) return nil;
    return _class_createInstanceFromZone(cls, extraBytes, nil);
}

NEVER_INLINE
id
_objc_rootAllocWithZone(Class cls, malloc_zone_t *zone __unused)
{
    // allocWithZone under __OBJC2__ ignores the zone parameter
    return _class_createInstanceFromZone(cls, 0, nil,
                                         OBJECT_CONSTRUCT_CALL_BADALLOC);
}

/***********************************************************************
* class_createInstances
* fixme
* Locking: none
**********************************************************************/
#if SUPPORT_NONPOINTER_ISA
#warning fixme optimize class_createInstances
#endif
unsigned 
class_createInstances(Class cls, size_t extraBytes, 
                      id *results, unsigned num_requested)
{
    return _class_createInstancesFromZone(cls, extraBytes, nil, 
                                          results, num_requested);
}

/***********************************************************************
* object_copyFromZone
* fixme
* Locking: none
**********************************************************************/
static id 
_object_copyFromZone(id oldObj, size_t extraBytes, void *zone)
{
    if (!oldObj) return nil;
    if (oldObj->isTaggedPointer()) return oldObj;

    // fixme this doesn't handle C++ ivars correctly (#4619414)

    Class cls = oldObj->ISA();
    size_t size;
    id obj = _class_createInstanceFromZone(cls, extraBytes, zone,
                                           OBJECT_CONSTRUCT_NONE, false, &size);
    if (!obj) return nil;

    // Copy everything except the isa, which was already set above.
    uint8_t *copyDst = (uint8_t *)obj + sizeof(Class);
    uint8_t *copySrc = (uint8_t *)oldObj + sizeof(Class);
    size_t copySize = size - sizeof(Class);
    memmove(copyDst, copySrc, copySize);

    fixupCopiedIvars(obj, oldObj);

    return obj;
}


/***********************************************************************
* object_copy
* fixme
* Locking: none
**********************************************************************/
id 
object_copy(id oldObj, size_t extraBytes)
{
    return _object_copyFromZone(oldObj, extraBytes, malloc_default_zone());
}


#if SUPPORT_ZONES

/***********************************************************************
* class_createInstanceFromZone
* fixme
* Locking: none
**********************************************************************/
id
class_createInstanceFromZone(Class cls, size_t extraBytes, void *zone)
{
    if (!cls) return nil;
    return _class_createInstanceFromZone(cls, extraBytes, zone);
}

/***********************************************************************
* object_copyFromZone
* fixme
* Locking: none
**********************************************************************/
id 
object_copyFromZone(id oldObj, size_t extraBytes, void *zone)
{
    return _object_copyFromZone(oldObj, extraBytes, zone);
}

#endif


/***********************************************************************
* objc_destructInstance
* Destroys an instance without freeing memory. 
* Calls C++ destructors.
* Calls ARC ivar cleanup.
* Removes associative references.
* Returns `obj`. Does nothing if `obj` is nil.
**********************************************************************/
void *objc_destructInstance(id obj) 
{
    if (obj) {
        // Read all of the flags at once for performance.
        bool cxx = obj->hasCxxDtor();
        bool assoc = obj->hasAssociatedObjects();

        // This order is important.
        if (cxx) object_cxxDestruct(obj);
        if (assoc) _object_remove_assocations(obj);
        obj->clearDeallocating();
    }

    return obj;
}


/***********************************************************************
* object_dispose
* fixme
* Locking: none
**********************************************************************/
id 
object_dispose(id obj)
{
    if (!obj) return nil;

    objc_destructInstance(obj);    
    free(obj);

    return nil;
}


/***********************************************************************
* _objc_getFreedObjectClass
* fixme
* Locking: none
**********************************************************************/
Class _objc_getFreedObjectClass (void)
{
    return nil;
}



/***********************************************************************
* Tagged pointer objects.
*
* Tagged pointer objects store the class and the object value in the 
* object pointer; the "pointer" does not actually point to anything.
* 
* Tagged pointer objects currently use this representation:
* (LSB)
*  1 bit   set if tagged, clear if ordinary object pointer
*  3 bits  tag index
* 60 bits  payload
* (MSB)
* The tag index defines the object's class. 
* The payload format is defined by the object's class.
*
* If the tag index is 0b111, the tagged pointer object uses an 
* "extended" representation, allowing more classes but with smaller payloads:
* (LSB)
*  1 bit   set if tagged, clear if ordinary object pointer
*  3 bits  0b111
*  8 bits  extended tag index
* 52 bits  payload
* (MSB)
*
* Some architectures reverse the MSB and LSB in these representations.
*
* This representation is subject to change. Representation-agnostic SPI is:
* objc-internal.h for class implementers.
* objc-gdb.h for debuggers.
**********************************************************************/
#if !SUPPORT_TAGGED_POINTERS

// These variables are always provided for debuggers.
uintptr_t objc_debug_taggedpointer_obfuscator = 0;
uintptr_t objc_debug_taggedpointer_mask = 0;
unsigned  objc_debug_taggedpointer_slot_shift = 0;
uintptr_t objc_debug_taggedpointer_slot_mask = 0;
unsigned  objc_debug_taggedpointer_payload_lshift = 0;
unsigned  objc_debug_taggedpointer_payload_rshift = 0;
Class objc_debug_taggedpointer_classes[1] = { nil };

uintptr_t objc_debug_taggedpointer_ext_mask = 0;
unsigned  objc_debug_taggedpointer_ext_slot_shift = 0;
uintptr_t objc_debug_taggedpointer_ext_slot_mask = 0;
unsigned  objc_debug_taggedpointer_ext_payload_lshift = 0;
unsigned  objc_debug_taggedpointer_ext_payload_rshift = 0;
Class objc_debug_taggedpointer_ext_classes[1] = { nil };

static void
disableTaggedPointers() { }

static void
initializeTaggedPointerObfuscator(void) { }

#else

// The "slot" used in the class table and given to the debugger 
// includes the is-tagged bit. This makes objc_msgSend faster.
// The "ext" representation doesn't do that.

uintptr_t objc_debug_taggedpointer_obfuscator;
uintptr_t objc_debug_taggedpointer_mask = _OBJC_TAG_MASK;
unsigned  objc_debug_taggedpointer_slot_shift = _OBJC_TAG_SLOT_SHIFT;
uintptr_t objc_debug_taggedpointer_slot_mask = _OBJC_TAG_SLOT_MASK;
unsigned  objc_debug_taggedpointer_payload_lshift = _OBJC_TAG_PAYLOAD_LSHIFT;
unsigned  objc_debug_taggedpointer_payload_rshift = _OBJC_TAG_PAYLOAD_RSHIFT;
// objc_debug_taggedpointer_classes is defined in objc-msg-*.s

uintptr_t objc_debug_taggedpointer_ext_mask = _OBJC_TAG_EXT_MASK;
unsigned  objc_debug_taggedpointer_ext_slot_shift = _OBJC_TAG_EXT_SLOT_SHIFT;
uintptr_t objc_debug_taggedpointer_ext_slot_mask = _OBJC_TAG_EXT_SLOT_MASK;
unsigned  objc_debug_taggedpointer_ext_payload_lshift = _OBJC_TAG_EXT_PAYLOAD_LSHIFT;
unsigned  objc_debug_taggedpointer_ext_payload_rshift = _OBJC_TAG_EXT_PAYLOAD_RSHIFT;
// objc_debug_taggedpointer_ext_classes is defined in objc-msg-*.s

static void
disableTaggedPointers()
{
    objc_debug_taggedpointer_mask = 0;
    objc_debug_taggedpointer_slot_shift = 0;
    objc_debug_taggedpointer_slot_mask = 0;
    objc_debug_taggedpointer_payload_lshift = 0;
    objc_debug_taggedpointer_payload_rshift = 0;

    objc_debug_taggedpointer_ext_mask = 0;
    objc_debug_taggedpointer_ext_slot_shift = 0;
    objc_debug_taggedpointer_ext_slot_mask = 0;
    objc_debug_taggedpointer_ext_payload_lshift = 0;
    objc_debug_taggedpointer_ext_payload_rshift = 0;
}


// Returns a pointer to the class's storage in the tagged class arrays.
// Assumes the tag is a valid basic tag.
static Class *
classSlotForBasicTagIndex(objc_tag_index_t tag)
{
    uintptr_t tagObfuscator = ((objc_debug_taggedpointer_obfuscator
                                >> _OBJC_TAG_INDEX_SHIFT)
                               & _OBJC_TAG_INDEX_MASK);
    uintptr_t obfuscatedTag = tag ^ tagObfuscator;
    // Array index in objc_tag_classes includes the tagged bit itself
#if SUPPORT_MSB_TAGGED_POINTERS
    return &objc_tag_classes[0x8 | obfuscatedTag];
#else
    return &objc_tag_classes[(obfuscatedTag << 1) | 1];
#endif
}


// Returns a pointer to the class's storage in the tagged class arrays, 
// or nil if the tag is out of range.
static Class *  
classSlotForTagIndex(objc_tag_index_t tag)
{
    if (tag >= OBJC_TAG_First60BitPayload && tag <= OBJC_TAG_Last60BitPayload) {
        return classSlotForBasicTagIndex(tag);
    }

    if (tag >= OBJC_TAG_First52BitPayload && tag <= OBJC_TAG_Last52BitPayload) {
        int index = tag - OBJC_TAG_First52BitPayload;
        uintptr_t tagObfuscator = ((objc_debug_taggedpointer_obfuscator
                                    >> _OBJC_TAG_EXT_INDEX_SHIFT)
                                   & _OBJC_TAG_EXT_INDEX_MASK);
        return &objc_tag_ext_classes[index ^ tagObfuscator];
    }

    return nil;
}

/***********************************************************************
* initializeTaggedPointerObfuscator
* Initialize objc_debug_taggedpointer_obfuscator with randomness.
*
* The tagged pointer obfuscator is intended to make it more difficult
* for an attacker to construct a particular object as a tagged pointer,
* in the presence of a buffer overflow or other write control over some
* memory. The obfuscator is XORed with the tagged pointers when setting
* or retrieving payload values. They are filled with randomness on first
* use.
**********************************************************************/
static void
initializeTaggedPointerObfuscator(void)
{
    if (sdkIsOlderThan(10_14, 12_0, 12_0, 5_0, 3_0) ||
        // Set the obfuscator to zero for apps linked against older SDKs,
        // in case they're relying on the tagged pointer representation.
        DisableTaggedPointerObfuscation) {
        objc_debug_taggedpointer_obfuscator = 0;
    } else {
        // Pull random data into the variable, then shift away all non-payload bits.
        arc4random_buf(&objc_debug_taggedpointer_obfuscator,
                       sizeof(objc_debug_taggedpointer_obfuscator));
        objc_debug_taggedpointer_obfuscator &= ~_OBJC_TAG_MASK;
    }
}


/***********************************************************************
* _objc_registerTaggedPointerClass
* Set the class to use for the given tagged pointer index.
* Aborts if the tag is out of range, or if the tag is already 
* used by some other class.
**********************************************************************/
void
_objc_registerTaggedPointerClass(objc_tag_index_t tag, Class cls)
{
    if (objc_debug_taggedpointer_mask == 0) {
        _objc_fatal("tagged pointers are disabled");
    }

    Class *slot = classSlotForTagIndex(tag);
    if (!slot) {
        _objc_fatal("tag index %u is invalid", (unsigned int)tag);
    }

    Class oldCls = *slot;
    
    if (cls  &&  oldCls  &&  cls != oldCls) {
        _objc_fatal("tag index %u used for two different classes "
                    "(was %p %s, now %p %s)", tag, 
                    oldCls, oldCls->nameForLogging(), 
                    cls, cls->nameForLogging());
    }

    *slot = cls;

    // Store a placeholder class in the basic tag slot that is 
    // reserved for the extended tag space, if it isn't set already.
    // Do this lazily when the first extended tag is registered so 
    // that old debuggers characterize bogus pointers correctly more often.
    if (tag < OBJC_TAG_First60BitPayload || tag > OBJC_TAG_Last60BitPayload) {
        Class *extSlot = classSlotForBasicTagIndex(OBJC_TAG_RESERVED_7);
        if (*extSlot == nil) {
            extern objc_class OBJC_CLASS_$___NSUnrecognizedTaggedPointer;
            *extSlot = (Class)&OBJC_CLASS_$___NSUnrecognizedTaggedPointer;
        }
    }
}


/***********************************************************************
* _objc_getClassForTag
* Returns the class that is using the given tagged pointer tag.
* Returns nil if no class is using that tag or the tag is out of range.
**********************************************************************/
Class
_objc_getClassForTag(objc_tag_index_t tag)
{
    Class *slot = classSlotForTagIndex(tag);
    if (slot) return *slot;
    else return nil;
}

#endif


#if SUPPORT_FIXUP

OBJC_EXTERN void objc_msgSend_fixup(void);
OBJC_EXTERN void objc_msgSendSuper2_fixup(void);
OBJC_EXTERN void objc_msgSend_stret_fixup(void);
OBJC_EXTERN void objc_msgSendSuper2_stret_fixup(void);
#if defined(__i386__)  ||  defined(__x86_64__)
OBJC_EXTERN void objc_msgSend_fpret_fixup(void);
#endif
#if defined(__x86_64__)
OBJC_EXTERN void objc_msgSend_fp2ret_fixup(void);
#endif

OBJC_EXTERN void objc_msgSend_fixedup(void);
OBJC_EXTERN void objc_msgSendSuper2_fixedup(void);
OBJC_EXTERN void objc_msgSend_stret_fixedup(void);
OBJC_EXTERN void objc_msgSendSuper2_stret_fixedup(void);
#if defined(__i386__)  ||  defined(__x86_64__)
OBJC_EXTERN void objc_msgSend_fpret_fixedup(void);
#endif
#if defined(__x86_64__)
OBJC_EXTERN void objc_msgSend_fp2ret_fixedup(void);
#endif

/***********************************************************************
* fixupMessageRef
* Repairs an old vtable dispatch call site. 
* vtable dispatch itself is not supported.
**********************************************************************/
static void 
fixupMessageRef(message_ref_t *msg)
{    
    msg->sel = sel_registerName((const char *)msg->sel);

    if (msg->imp == &objc_msgSend_fixup) { 
        if (msg->sel == @selector(alloc)) {
            msg->imp = (IMP)&objc_alloc;
        } else if (msg->sel == @selector(allocWithZone:)) {
            msg->imp = (IMP)&objc_allocWithZone;
        } else if (msg->sel == @selector(retain)) {
            msg->imp = (IMP)&objc_retain;
        } else if (msg->sel == @selector(release)) {
            msg->imp = (IMP)&objc_release;
        } else if (msg->sel == @selector(autorelease)) {
            msg->imp = (IMP)&objc_autorelease;
        } else {
            msg->imp = &objc_msgSend_fixedup;
        }
    } 
    else if (msg->imp == &objc_msgSendSuper2_fixup) { 
        msg->imp = &objc_msgSendSuper2_fixedup;
    } 
    else if (msg->imp == &objc_msgSend_stret_fixup) { 
        msg->imp = &objc_msgSend_stret_fixedup;
    } 
    else if (msg->imp == &objc_msgSendSuper2_stret_fixup) { 
        msg->imp = &objc_msgSendSuper2_stret_fixedup;
    } 
#if defined(__i386__)  ||  defined(__x86_64__)
    else if (msg->imp == &objc_msgSend_fpret_fixup) { 
        msg->imp = &objc_msgSend_fpret_fixedup;
    } 
#endif
#if defined(__x86_64__)
    else if (msg->imp == &objc_msgSend_fp2ret_fixup) { 
        msg->imp = &objc_msgSend_fp2ret_fixedup;
    } 
#endif
}

// SUPPORT_FIXUP
#endif


// ProKit SPI
static Class setSuperclass(Class cls, Class newSuper)
{
    Class oldSuper;

    runtimeLock.assertLocked();

    ASSERT(cls->isRealized());
    ASSERT(newSuper->isRealized());

    oldSuper = cls->superclass;
    removeSubclass(oldSuper, cls);
    removeSubclass(oldSuper->ISA(), cls->ISA());

    cls->superclass = newSuper;
    cls->ISA()->superclass = newSuper->ISA();
    addSubclass(newSuper, cls);
    addSubclass(newSuper->ISA(), cls->ISA());

    // Flush subclass's method caches.
    flushCaches(cls);
    flushCaches(cls->ISA());
    
    return oldSuper;
}


Class class_setSuperclass(Class cls, Class newSuper)
{
    mutex_locker_t lock(runtimeLock);
    return setSuperclass(cls, newSuper);
}

void runtime_init(void)
{
    objc::unattachedCategories.init(32);
    objc::allocatedClasses.init();
}

// __OBJC2__
#endif