objc4-646.tar.gz

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

/*
 * Copyright (c) 2002-2007 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@
 */

#if !__OBJC2__

/***********************************************************************
* 32-bit implementation
**********************************************************************/

#include "objc-private.h"
#include <stdlib.h>
#include <setjmp.h>
#include <execinfo.h>

#include "objc-exception.h"

static objc_exception_functions_t xtab;

// forward declaration
static void set_default_handlers();


/*
 * Exported functions
 */

// get table; version tells how many
void objc_exception_get_functions(objc_exception_functions_t *table) {
    // only version 0 supported at this point
    if (table && table->version == 0)
        *table = xtab;
}

// set table
void objc_exception_set_functions(objc_exception_functions_t *table) {
    // only version 0 supported at this point
    if (table && table->version == 0)
        xtab = *table;
}

/*
 * The following functions are
 * synthesized by the compiler upon encountering language constructs
 */
 
void objc_exception_throw(id exception) {
    if (!xtab.throw_exc) {
        set_default_handlers();
    }
    
    if (PrintExceptionThrow) {
        _objc_inform("EXCEPTIONS: throwing %p (%s)", 
                     (void*)exception, object_getClassName(exception));
        void* callstack[500];
        int frameCount = backtrace(callstack, 500);
        backtrace_symbols_fd(callstack, frameCount, fileno(stderr));
    }

    OBJC_RUNTIME_OBJC_EXCEPTION_THROW(exception);  // dtrace probe to log throw activity.
    xtab.throw_exc(exception);
    _objc_fatal("objc_exception_throw failed");
}

void objc_exception_try_enter(void *localExceptionData) {
    if (!xtab.throw_exc) {
        set_default_handlers();
    }
    xtab.try_enter(localExceptionData);
}


void objc_exception_try_exit(void *localExceptionData) {
    if (!xtab.throw_exc) {
        set_default_handlers();
    }
    xtab.try_exit(localExceptionData);
}


id objc_exception_extract(void *localExceptionData) {
    if (!xtab.throw_exc) {
        set_default_handlers();
    }
    return xtab.extract(localExceptionData);
}


int objc_exception_match(Class exceptionClass, id exception) {
    if (!xtab.throw_exc) {
        set_default_handlers();
    }
    return xtab.match(exceptionClass, exception);
}


// quick and dirty exception handling code
// default implementation - mostly a toy for use outside/before Foundation
// provides its implementation
// Perhaps the default implementation should just complain loudly and quit


extern void _objc_inform(const char *fmt, ...);

typedef struct { jmp_buf buf; void *pointers[4]; } LocalData_t;

typedef struct _threadChain {
    LocalData_t *topHandler;
    objc_thread_t perThreadID;
    struct _threadChain *next;
}
    ThreadChainLink_t;

static ThreadChainLink_t ThreadChainLink;

static ThreadChainLink_t *getChainLink() {
    // follow links until thread_self() found (someday) XXX
    objc_thread_t self = thread_self();
    ThreadChainLink_t *walker = &ThreadChainLink;
    while (walker->perThreadID != self) {
        if (walker->next != nil) {
            walker = walker->next;
            continue;
        }
        // create a new one
        // XXX not thread safe (!)
        // XXX Also, we don't register to deallocate on thread death
        walker->next = (ThreadChainLink_t *)malloc(sizeof(ThreadChainLink_t));
        walker = walker->next;
        walker->next = nil;
        walker->topHandler = nil;
        walker->perThreadID = self;
    }
    return walker;
}

static void default_try_enter(void *localExceptionData) {
    LocalData_t *data = (LocalData_t *)localExceptionData;
    ThreadChainLink_t *chainLink = getChainLink();
    data->pointers[1] = chainLink->topHandler;
    chainLink->topHandler = data;
    if (PrintExceptions) _objc_inform("EXCEPTIONS: entered try block %p\n", chainLink->topHandler);
}

static void default_throw(id value) {
    ThreadChainLink_t *chainLink = getChainLink();
    LocalData_t *led;
    if (value == nil) {
        if (PrintExceptions) _objc_inform("EXCEPTIONS: objc_exception_throw with nil value\n");
        return;
    }
    if (chainLink == nil) {
        if (PrintExceptions) _objc_inform("EXCEPTIONS: No handler in place!\n");
        return;
    }
    if (PrintExceptions) _objc_inform("EXCEPTIONS: exception thrown, going to handler block %p\n", chainLink->topHandler);
    led = chainLink->topHandler;
    chainLink->topHandler = (LocalData_t *)
        led->pointers[1];       // pop top handler
    led->pointers[0] = value;                   // store exception that is thrown
#if TARGET_OS_WIN32
    longjmp(led->buf, 1);
#else
    _longjmp(led->buf, 1);
#endif
}
    
static void default_try_exit(void *led) {
    ThreadChainLink_t *chainLink = getChainLink();
    if (!chainLink || led != chainLink->topHandler) {
        if (PrintExceptions) _objc_inform("EXCEPTIONS: *** mismatched try block exit handlers\n");
        return;
    }
    if (PrintExceptions) _objc_inform("EXCEPTIONS: removing try block handler %p\n", chainLink->topHandler);
    chainLink->topHandler = (LocalData_t *)
        chainLink->topHandler->pointers[1];     // pop top handler
}

static id default_extract(void *localExceptionData) {
    LocalData_t *led = (LocalData_t *)localExceptionData;
    return (id)led->pointers[0];
}

static int default_match(Class exceptionClass, id exception) {
    //return [exception isKindOfClass:exceptionClass];
    Class cls;
    for (cls = exception->getIsa(); nil != cls; cls = cls->superclass) 
        if (cls == exceptionClass) return 1;
    return 0;
}

static void set_default_handlers() {
    objc_exception_functions_t default_functions = {
        0, default_throw, default_try_enter, default_try_exit, default_extract, default_match };

    // should this always print?
    if (PrintExceptions) _objc_inform("EXCEPTIONS: *** Setting default (non-Foundation) exception mechanism\n");
    objc_exception_set_functions(&default_functions);
}


void exception_init(void)
{
    // nothing to do
}

void _destroyAltHandlerList(struct alt_handler_list *list)
{
    // nothing to do
}


// !__OBJC2__
#else
// __OBJC2__

/***********************************************************************
* 64-bit implementation.
**********************************************************************/

#include "objc-private.h"
#include <objc/objc-exception.h>
#include <objc/NSObject.h>
#include <execinfo.h>

// unwind library types and functions
// Mostly adapted from Itanium C++ ABI: Exception Handling
//   http://www.codesourcery.com/cxx-abi/abi-eh.html

struct _Unwind_Exception;
struct _Unwind_Context;

typedef int _Unwind_Action;
static const _Unwind_Action _UA_SEARCH_PHASE = 1;
static const _Unwind_Action _UA_CLEANUP_PHASE = 2;
static const _Unwind_Action _UA_HANDLER_FRAME = 4;
static const _Unwind_Action _UA_FORCE_UNWIND = 8;

typedef int _Unwind_Reason_Code;
static const _Unwind_Reason_Code _URC_NO_REASON = 0;
static const _Unwind_Reason_Code _URC_FOREIGN_EXCEPTION_CAUGHT = 1;
static const _Unwind_Reason_Code _URC_FATAL_PHASE2_ERROR = 2;
static const _Unwind_Reason_Code _URC_FATAL_PHASE1_ERROR = 3;
static const _Unwind_Reason_Code _URC_NORMAL_STOP = 4;
static const _Unwind_Reason_Code _URC_END_OF_STACK = 5;
static const _Unwind_Reason_Code _URC_HANDLER_FOUND = 6;
static const _Unwind_Reason_Code _URC_INSTALL_CONTEXT = 7;
static const _Unwind_Reason_Code _URC_CONTINUE_UNWIND = 8;

struct dwarf_eh_bases
{
    uintptr_t tbase;
    uintptr_t dbase;
    uintptr_t func;
};

OBJC_EXTERN uintptr_t _Unwind_GetIP (struct _Unwind_Context *);
OBJC_EXTERN uintptr_t _Unwind_GetCFA (struct _Unwind_Context *);
OBJC_EXTERN uintptr_t _Unwind_GetLanguageSpecificData(struct _Unwind_Context *);


// C++ runtime types and functions
// copied from cxxabi.h

OBJC_EXTERN void *__cxa_allocate_exception(size_t thrown_size);
OBJC_EXTERN void __cxa_throw(void *exc, void *typeinfo, void (*destructor)(void *)) __attribute__((noreturn));
OBJC_EXTERN void *__cxa_begin_catch(void *exc);
OBJC_EXTERN void __cxa_end_catch(void);
OBJC_EXTERN void __cxa_rethrow(void);
OBJC_EXTERN void *__cxa_current_exception_type(void);

#if SUPPORT_ZEROCOST_EXCEPTIONS
#   define CXX_PERSONALITY __gxx_personality_v0
#else
#   define CXX_PERSONALITY __gxx_personality_sj0
#endif

OBJC_EXTERN _Unwind_Reason_Code 
CXX_PERSONALITY(int version,
                _Unwind_Action actions,
                uint64_t exceptionClass,
                struct _Unwind_Exception *exceptionObject,
                struct _Unwind_Context *context);


// objc's internal exception types and data

struct objc_typeinfo {
    // Position of vtable and name fields must match C++ typeinfo object
    const void **vtable;  // always objc_ehtype_vtable+2
    const char *name;     // c++ typeinfo string

    Class cls_unremapped;
};

struct objc_exception {
    id obj;
    struct objc_typeinfo tinfo;
};


static void _objc_exception_noop(void) { } 
static bool _objc_exception_false(void) { return 0; } 
// static bool _objc_exception_true(void) { return 1; } 
static void _objc_exception_abort1(void) { 
    _objc_fatal("unexpected call into objc exception typeinfo vtable %d", 1); 
} 
static void _objc_exception_abort2(void) { 
    _objc_fatal("unexpected call into objc exception typeinfo vtable %d", 2); 
} 
static void _objc_exception_abort3(void) { 
    _objc_fatal("unexpected call into objc exception typeinfo vtable %d", 3); 
} 
static void _objc_exception_abort4(void) { 
    _objc_fatal("unexpected call into objc exception typeinfo vtable %d", 4); 
} 

static bool _objc_exception_do_catch(struct objc_typeinfo *catch_tinfo, 
                                     struct objc_typeinfo *throw_tinfo, 
                                     void **throw_obj_p, 
                                     unsigned outer);

// forward declaration
OBJC_EXPORT struct objc_typeinfo OBJC_EHTYPE_id;

OBJC_EXPORT
const void *objc_ehtype_vtable[] = {
    nil,  // typeinfo's vtable? - fixme 
    (void*)&OBJC_EHTYPE_id,  // typeinfo's typeinfo - hack
    (void*)_objc_exception_noop,      // in-place destructor?
    (void*)_objc_exception_noop,      // destructor?
    (void*)_objc_exception_false,     // OLD __is_pointer_p
    (void*)_objc_exception_false,     // OLD __is_function_p
    (void*)_objc_exception_do_catch,  // OLD __do_catch,  NEW can_catch
    (void*)_objc_exception_false,     // OLD __do_upcast, NEW search_above_dst
    (void*)_objc_exception_false,     //                  NEW search_below_dst
    (void*)_objc_exception_abort1,    // paranoia: blow up if libc++abi
    (void*)_objc_exception_abort2,    //           adds something new
    (void*)_objc_exception_abort3,
    (void*)_objc_exception_abort4,
};

OBJC_EXPORT
struct objc_typeinfo OBJC_EHTYPE_id = {
    objc_ehtype_vtable+2, 
    "id", 
    nil
};



/***********************************************************************
* Foundation customization
**********************************************************************/

/***********************************************************************
* _objc_default_exception_preprocessor
* Default exception preprocessor. Expected to be overridden by Foundation.
**********************************************************************/
static id _objc_default_exception_preprocessor(id exception)
{
    return exception;
}
static objc_exception_preprocessor exception_preprocessor = _objc_default_exception_preprocessor;


/***********************************************************************
* _objc_default_exception_matcher
* Default exception matcher. Expected to be overridden by Foundation.
**********************************************************************/
static int _objc_default_exception_matcher(Class catch_cls, id exception)
{
    Class cls;
    for (cls = exception->getIsa();
         cls != nil; 
         cls = cls->superclass)
    {
        if (cls == catch_cls) return 1;
    }

    return 0;
}
static objc_exception_matcher exception_matcher = _objc_default_exception_matcher;


/***********************************************************************
* _objc_default_uncaught_exception_handler
* Default uncaught exception handler. Expected to be overridden by Foundation.
**********************************************************************/
static void _objc_default_uncaught_exception_handler(id exception)
{
}
static objc_uncaught_exception_handler uncaught_handler = _objc_default_uncaught_exception_handler;


/***********************************************************************
* objc_setExceptionPreprocessor
* Set a handler for preprocessing Objective-C exceptions. 
* Returns the previous handler. 
**********************************************************************/
objc_exception_preprocessor
objc_setExceptionPreprocessor(objc_exception_preprocessor fn)
{
    objc_exception_preprocessor result = exception_preprocessor;
    exception_preprocessor = fn;
    return result;
}


/***********************************************************************
* objc_setExceptionMatcher
* Set a handler for matching Objective-C exceptions. 
* Returns the previous handler. 
**********************************************************************/
objc_exception_matcher
objc_setExceptionMatcher(objc_exception_matcher fn)
{
    objc_exception_matcher result = exception_matcher;
    exception_matcher = fn;
    return result;
}


/***********************************************************************
* objc_setUncaughtExceptionHandler
* Set a handler for uncaught Objective-C exceptions. 
* Returns the previous handler. 
**********************************************************************/
objc_uncaught_exception_handler 
objc_setUncaughtExceptionHandler(objc_uncaught_exception_handler fn)
{
    objc_uncaught_exception_handler result = uncaught_handler;
    uncaught_handler = fn;
    return result;
}


/***********************************************************************
* Exception personality
**********************************************************************/

static void call_alt_handlers(struct _Unwind_Context *ctx);

_Unwind_Reason_Code 
__objc_personality_v0(int version,
                      _Unwind_Action actions,
                      uint64_t exceptionClass,
                      struct _Unwind_Exception *exceptionObject,
                      struct _Unwind_Context *context)
{
    BOOL unwinding = ((actions & _UA_CLEANUP_PHASE)  ||  
                      (actions & _UA_FORCE_UNWIND));

    if (PrintExceptions) {
        _objc_inform("EXCEPTIONS: %s through frame [ip=%p sp=%p] "
                     "for exception %p", 
                     unwinding ? "unwinding" : "searching", 
                     (void*)(_Unwind_GetIP(context)-1),
                     (void*)_Unwind_GetCFA(context), exceptionObject);
    }

    // If we're executing the unwind, call this frame's alt handlers, if any.
    if (unwinding) {
        call_alt_handlers(context);
    }

    // Let C++ handle the unwind itself.
    return CXX_PERSONALITY(version, actions, exceptionClass, 
                           exceptionObject, context);
}


/***********************************************************************
* Compiler ABI
**********************************************************************/

static void _objc_exception_destructor(void *exc_gen) 
{
    // Release the retain from objc_exception_throw().

    struct objc_exception *exc = (struct objc_exception *)exc_gen;
    id obj = exc->obj;

    if (PrintExceptions) {
        _objc_inform("EXCEPTIONS: releasing completed exception %p (object %p, a %s)", 
                     exc, obj, object_getClassName(obj));
    }

#if SUPPORT_GC
    if (UseGC) {
        if (auto_zone_is_valid_pointer(gc_zone, obj)) {
            auto_zone_release(gc_zone, exc->obj);
        }
    }
    else 
#endif
    {
        [obj release];
    }
}


void objc_exception_throw(id obj)
{
    struct objc_exception *exc = (struct objc_exception *)
        __cxa_allocate_exception(sizeof(struct objc_exception));

    obj = (*exception_preprocessor)(obj);

    // Retain the exception object during unwinding.
    // GC: because `exc` is unscanned memory
    // Non-GC: because otherwise an autorelease pool pop can cause a crash
#if SUPPORT_GC
    if (UseGC) {
        if (auto_zone_is_valid_pointer(gc_zone, obj)) {
            auto_zone_retain(gc_zone, obj);
        }
    }
    else 
#endif
    {
        [obj retain];
    }

    exc->obj = obj;
    exc->tinfo.vtable = objc_ehtype_vtable+2;
    exc->tinfo.name = object_getClassName(obj);
    exc->tinfo.cls_unremapped = obj ? obj->getIsa() : Nil;

    if (PrintExceptions) {
        _objc_inform("EXCEPTIONS: throwing %p (object %p, a %s)", 
                     exc, (void*)obj, object_getClassName(obj));
    }

    if (PrintExceptionThrow) {
        if (!PrintExceptions)
            _objc_inform("EXCEPTIONS: throwing %p (object %p, a %s)", 
                         exc, (void*)obj, object_getClassName(obj));
        void* callstack[500];
        int frameCount = backtrace(callstack, 500);
        backtrace_symbols_fd(callstack, frameCount, fileno(stderr));
    }
    
    OBJC_RUNTIME_OBJC_EXCEPTION_THROW(obj);  // dtrace probe to log throw activity
    __cxa_throw(exc, &exc->tinfo, &_objc_exception_destructor);
    __builtin_trap();
}


void objc_exception_rethrow(void)
{
    // exception_preprocessor doesn't get another bite of the apple
    if (PrintExceptions) {
        _objc_inform("EXCEPTIONS: rethrowing current exception");
    }
    
    OBJC_RUNTIME_OBJC_EXCEPTION_RETHROW(); // dtrace probe to log throw activity.
    __cxa_rethrow();
    __builtin_trap();
}


id objc_begin_catch(void *exc_gen)
{
    if (PrintExceptions) {
        _objc_inform("EXCEPTIONS: handling exception %p at %p", 
                     exc_gen, __builtin_return_address(0));
    }
    // NOT actually an id in the catch(...) case!
    return (id)__cxa_begin_catch(exc_gen);
}


void objc_end_catch(void)
{
    if (PrintExceptions) {
        _objc_inform("EXCEPTIONS: finishing handler");
    }
    __cxa_end_catch();
}


// `outer` is not passed by the new libcxxabi
static bool _objc_exception_do_catch(struct objc_typeinfo *catch_tinfo, 
                                     struct objc_typeinfo *throw_tinfo, 
                                     void **throw_obj_p, 
                                     unsigned outer UNAVAILABLE_ATTRIBUTE)
{
    id exception;

    if (throw_tinfo->vtable != objc_ehtype_vtable+2) {
        // Only objc types can be caught here.
        if (PrintExceptions) _objc_inform("EXCEPTIONS: skipping catch(?)");
        return false;
    }

    // Adjust exception pointer.
    // Old libcppabi: we lied about __is_pointer_p() so we have to do it here
    // New libcxxabi: we have to do it here regardless
    *throw_obj_p = **(void***)throw_obj_p;

    // `catch (id)` always catches objc types.
    if (catch_tinfo == &OBJC_EHTYPE_id) {
        if (PrintExceptions) _objc_inform("EXCEPTIONS: catch(id)");
        return true;
    }

    exception = *(id *)throw_obj_p;

    Class handler_cls = _class_remap(catch_tinfo->cls_unremapped);
    if (!handler_cls) {
        // catch handler's class is weak-linked and missing. Not a match.
    }
    else if ((*exception_matcher)(handler_cls, exception)) {
        if (PrintExceptions) _objc_inform("EXCEPTIONS: catch(%s)", 
                                          handler_cls->nameForLogging());
        return true;
    }

    if (PrintExceptions) _objc_inform("EXCEPTIONS: skipping catch(%s)", 
                                      handler_cls->nameForLogging());

    return false;
}


/***********************************************************************
* _objc_terminate
* Custom std::terminate handler.
*
* The uncaught exception callback is implemented as a std::terminate handler. 
* 1. Check if there's an active exception
* 2. If so, check if it's an Objective-C exception
* 3. If so, call our registered callback with the object.
* 4. Finally, call the previous terminate handler.
**********************************************************************/
static void (*old_terminate)(void) = nil;
static void _objc_terminate(void)
{
    if (PrintExceptions) {
        _objc_inform("EXCEPTIONS: terminating");
    }

    if (! __cxa_current_exception_type()) {
        // No current exception.
        (*old_terminate)();
    }
    else {
        // There is a current exception. Check if it's an objc exception.
        @try {
            __cxa_rethrow();
        } @catch (id e) {
            // It's an objc object. Call Foundation's handler, if any.
            (*uncaught_handler)((id)e);
            (*old_terminate)();
        } @catch (...) {
            // It's not an objc object. Continue to C++ terminate.
            (*old_terminate)();
        }
    }
}


/***********************************************************************
* objc_terminate
* Calls std::terminate for clients who don't link to C++ themselves.
* Called by the compiler if an exception is thrown 
* from a context where exceptions may not be thrown. 
**********************************************************************/
void objc_terminate(void)
{
    std::terminate();
}


/***********************************************************************
* alt handler support - zerocost implementation only
**********************************************************************/

#if !SUPPORT_ALT_HANDLERS

void _destroyAltHandlerList(struct alt_handler_list *list)
{
}

static void call_alt_handlers(struct _Unwind_Context *ctx)
{
    // unsupported in sjlj environments
}

#else

#include <libunwind.h>
#include <execinfo.h>
#include <dispatch/dispatch.h>

// Dwarf eh data encodings
#define DW_EH_PE_omit      0xff  // no data follows

#define DW_EH_PE_absptr    0x00
#define DW_EH_PE_uleb128   0x01
#define DW_EH_PE_udata2    0x02
#define DW_EH_PE_udata4    0x03
#define DW_EH_PE_udata8    0x04
#define DW_EH_PE_sleb128   0x09
#define DW_EH_PE_sdata2    0x0A
#define DW_EH_PE_sdata4    0x0B
#define DW_EH_PE_sdata8    0x0C

#define DW_EH_PE_pcrel     0x10
#define DW_EH_PE_textrel   0x20
#define DW_EH_PE_datarel   0x30
#define DW_EH_PE_funcrel   0x40
#define DW_EH_PE_aligned   0x50  // fixme

#define DW_EH_PE_indirect  0x80  // gcc extension


/***********************************************************************
* read_uleb
* Read a LEB-encoded unsigned integer from the address stored in *pp.
* Increments *pp past the bytes read.
* Adapted from DWARF Debugging Information Format 1.1, appendix 4
**********************************************************************/
static uintptr_t read_uleb(uintptr_t *pp)
{
    uintptr_t result = 0;
    uintptr_t shift = 0;
    unsigned char byte;
    do {
        byte = *(const unsigned char *)(*pp)++;
        result |= (byte & 0x7f) << shift;
        shift += 7;
    } while (byte & 0x80);
    return result;
}


/***********************************************************************
* read_sleb
* Read a LEB-encoded signed integer from the address stored in *pp.
* Increments *pp past the bytes read.
* Adapted from DWARF Debugging Information Format 1.1, appendix 4
**********************************************************************/
static intptr_t read_sleb(uintptr_t *pp)
{
    uintptr_t result = 0;
    uintptr_t shift = 0;
    unsigned char byte;
    do {
        byte = *(const unsigned char *)(*pp)++;
        result |= (byte & 0x7f) << shift;
        shift += 7;
    } while (byte & 0x80);
    if ((shift < 8*sizeof(intptr_t))  &&  (byte & 0x40)) {
        result |= ((intptr_t)-1) << shift;
    }
    return result;
}


/***********************************************************************
* read_address
* Reads an encoded address from the address stored in *pp.
* Increments *pp past the bytes read.
* The data is interpreted according to the given dwarf encoding 
* and base addresses.
**********************************************************************/
static uintptr_t read_address(uintptr_t *pp, 
                              const struct dwarf_eh_bases *bases, 
                              unsigned char encoding)
{
    uintptr_t result = 0;
    uintptr_t oldp = *pp;

    // fixme need DW_EH_PE_aligned?

#define READ(type) \
    result = *(type *)(*pp); \
    *pp += sizeof(type);

    if (encoding == DW_EH_PE_omit) return 0;

    switch (encoding & 0x0f) {
    case DW_EH_PE_absptr:
        READ(uintptr_t);
        break;
    case DW_EH_PE_uleb128:
        result = read_uleb(pp);
        break;
    case DW_EH_PE_udata2:
        READ(uint16_t);
        break;
    case DW_EH_PE_udata4:
        READ(uint32_t);
        break;
#if __LP64__
    case DW_EH_PE_udata8:
        READ(uint64_t);
        break;
#endif
    case DW_EH_PE_sleb128:
        result = read_sleb(pp);
        break;
    case DW_EH_PE_sdata2:
        READ(int16_t);
        break;
    case DW_EH_PE_sdata4:
        READ(int32_t);
        break;
#if __LP64__
    case DW_EH_PE_sdata8:
        READ(int64_t);
        break;
#endif
    default:
        _objc_inform("unknown DWARF EH encoding 0x%x at %p", 
                     encoding, (void *)*pp);
        break;
    }

#undef READ

    if (result) {
        switch (encoding & 0x70) {
        case DW_EH_PE_pcrel:
            // fixme correct?
            result += (uintptr_t)oldp;
            break;
        case DW_EH_PE_textrel:
            result += bases->tbase;
            break;
        case DW_EH_PE_datarel:
            result += bases->dbase;
            break;
        case DW_EH_PE_funcrel:
            result += bases->func;
            break;
        case DW_EH_PE_aligned:
            _objc_inform("unknown DWARF EH encoding 0x%x at %p", 
                         encoding, (void *)*pp);
            break;
        default:
            // no adjustment
            break;
        }

        if (encoding & DW_EH_PE_indirect) {
            result = *(uintptr_t *)result;
        }
    }

    return (uintptr_t)result;
}


struct frame_ips {
    uintptr_t start;
    uintptr_t end;
};
struct frame_range {
    uintptr_t ip_start;
    uintptr_t ip_end;
    uintptr_t cfa;
    // precise ranges within ip_start..ip_end; nil or {0,0} terminated
    frame_ips *ips;
};


static bool isObjCExceptionCatcher(uintptr_t lsda, uintptr_t ip, 
                                   const struct dwarf_eh_bases* bases,
                                   struct frame_range *frame)
{
    unsigned char LPStart_enc = *(const unsigned char *)lsda++;    

    if (LPStart_enc != DW_EH_PE_omit) {
        read_address(&lsda, bases, LPStart_enc); // LPStart
    }

    unsigned char TType_enc = *(const unsigned char *)lsda++;
    if (TType_enc != DW_EH_PE_omit) {
        read_uleb(&lsda);  // TType
    }

    unsigned char call_site_enc = *(const unsigned char *)lsda++;
    uintptr_t length = read_uleb(&lsda);
    uintptr_t call_site_table = lsda;
    uintptr_t call_site_table_end = call_site_table + length;
    uintptr_t action_record_table = call_site_table_end;

    uintptr_t action_record = 0;
    uintptr_t p = call_site_table;

    uintptr_t try_start;
    uintptr_t try_end;
    uintptr_t try_landing_pad;

    while (p < call_site_table_end) {
        uintptr_t start   = read_address(&p, bases, call_site_enc)+bases->func;
        uintptr_t len     = read_address(&p, bases, call_site_enc);
        uintptr_t pad     = read_address(&p, bases, call_site_enc);
        uintptr_t action  = read_uleb(&p);

        if (ip < start) {
            // no more source ranges
            return false;
        } 
        else if (ip < start + len) {
            // found the range
            if (!pad) return false;  // ...but it has no landing pad
            // found the landing pad
            action_record = action ? action_record_table + action - 1 : 0;
            try_start = start;
            try_end = start + len;
            try_landing_pad = pad;
            break;
        }        
    }
    
    if (!action_record) return false;  // no catch handlers

    // has handlers, destructors, and/or throws specifications
    // Use this frame if it has any handlers
    bool has_handler = false;
    p = action_record;
    intptr_t offset;
    do {
        intptr_t filter = read_sleb(&p);
        uintptr_t temp = p;
        offset = read_sleb(&temp);
        p += offset;
        
        if (filter < 0) {
            // throws specification - ignore
        } else if (filter == 0) {
            // destructor - ignore
        } else /* filter >= 0 */ {
            // catch handler - use this frame
            has_handler = true;
            break;
        }
    } while (offset);

    if (!has_handler) return false;
    
    // Count the number of source ranges with the same landing pad as our match
    unsigned int range_count = 0;
    p = call_site_table;
    while (p < call_site_table_end) {
                /*start*/  read_address(&p, bases, call_site_enc)/*+bases->func*/;
                /*len*/    read_address(&p, bases, call_site_enc);
        uintptr_t pad    = read_address(&p, bases, call_site_enc);
                /*action*/ read_uleb(&p);
        
        if (pad == try_landing_pad) {
            range_count++;
        }
    }

    if (range_count == 1) {
        // No other source ranges with the same landing pad. We're done here.
        frame->ips = nil;
    }
    else {
        // Record all ranges with the same landing pad as our match.
        frame->ips = (frame_ips *)
            _malloc_internal((range_count + 1) * sizeof(frame->ips[0]));
        unsigned int r = 0;
        p = call_site_table;
        while (p < call_site_table_end) {
            uintptr_t start  = read_address(&p, bases, call_site_enc)+bases->func;
            uintptr_t len    = read_address(&p, bases, call_site_enc);
            uintptr_t pad    = read_address(&p, bases, call_site_enc);
                    /*action*/ read_uleb(&p);
            
            if (pad == try_landing_pad) {
                if (start < try_start) try_start = start;
                if (start+len > try_end) try_end = start+len;
                frame->ips[r].start = start;
                frame->ips[r].end = start+len;
                r++;
            }
        }

        frame->ips[r].start = 0;
        frame->ips[r].end = 0;
    }

    frame->ip_start = try_start;
    frame->ip_end = try_end;

    return true;
}


static struct frame_range findHandler(void)
{
    // walk stack looking for frame with objc catch handler
    unw_context_t    uc;
    unw_cursor_t    cursor; 
    unw_proc_info_t    info;
    unw_getcontext(&uc);
    unw_init_local(&cursor, &uc);
    while ( (unw_step(&cursor) > 0) && (unw_get_proc_info(&cursor, &info) == UNW_ESUCCESS) ) {
        // must use objc personality handler
        if ( info.handler != (uintptr_t)__objc_personality_v0 )
            continue;
        // must have landing pad
        if ( info.lsda == 0 )
            continue;
        // must have landing pad that catches objc exceptions
        struct dwarf_eh_bases bases;
        bases.tbase = 0;  // from unwind-dw2-fde-darwin.c:examine_objects()
        bases.dbase = 0;  // from unwind-dw2-fde-darwin.c:examine_objects()
        bases.func = info.start_ip;
        unw_word_t ip;
        unw_get_reg(&cursor, UNW_REG_IP, &ip);
        ip -= 1;
        struct frame_range try_range = {0, 0, 0, 0};
        if ( isObjCExceptionCatcher(info.lsda, ip, &bases, &try_range) ) {
            unw_word_t cfa;
            unw_get_reg(&cursor, UNW_REG_SP, &cfa);
            try_range.cfa = cfa;
            return try_range;
        }
    }

    return (struct frame_range){0, 0, 0, 0};
}


// This data structure assumes the number of 
// active alt handlers per frame is small.

// for OBJC_DEBUG_ALT_HANDLERS, record the call to objc_addExceptionHandler.
#define BACKTRACE_COUNT 46
#define THREADNAME_COUNT 64
struct alt_handler_debug {
    uintptr_t token;
    int backtraceSize;
    void *backtrace[BACKTRACE_COUNT];
    char thread[THREADNAME_COUNT];
    char queue[THREADNAME_COUNT];
};

struct alt_handler_data {
    struct frame_range frame;
    objc_exception_handler fn;
    void *context;
    struct alt_handler_debug *debug;
};

struct alt_handler_list {
    unsigned int allocated;
    unsigned int used;
    struct alt_handler_data *handlers;
    struct alt_handler_list *next_DEBUGONLY;
};

static pthread_mutex_t DebugLock = PTHREAD_MUTEX_INITIALIZER;
static struct alt_handler_list *DebugLists;
static uintptr_t DebugCounter;

void alt_handler_error(uintptr_t token) __attribute__((noinline));

static struct alt_handler_list *
fetch_handler_list(BOOL create)
{
    _objc_pthread_data *data = _objc_fetch_pthread_data(create);
    if (!data) return nil;

    struct alt_handler_list *list = data->handlerList;
    if (!list) {
        if (!create) return nil;
        list = (struct alt_handler_list *)_calloc_internal(1, sizeof(*list));
        data->handlerList = list;

        if (DebugAltHandlers) {
            // Save this list so the debug code can find it from other threads
            pthread_mutex_lock(&DebugLock);
            list->next_DEBUGONLY = DebugLists;
            DebugLists = list;
            pthread_mutex_unlock(&DebugLock);
        }
    }

    return list;
}


void _destroyAltHandlerList(struct alt_handler_list *list)
{
    if (list) {
        if (DebugAltHandlers) {
            // Detach from the list-of-lists.
            pthread_mutex_lock(&DebugLock);
            struct alt_handler_list **listp = &DebugLists;
            while (*listp && *listp != list) listp = &(*listp)->next_DEBUGONLY;
            if (*listp) *listp = (*listp)->next_DEBUGONLY;
            pthread_mutex_unlock(&DebugLock);
        }

        if (list->handlers) {
            for (unsigned int i = 0; i < list->allocated; i++) {
                if (list->handlers[i].frame.ips) {
                    _free_internal(list->handlers[i].frame.ips);
                }
            }
            _free_internal(list->handlers);
        }
        _free_internal(list);
    }
}


uintptr_t objc_addExceptionHandler(objc_exception_handler fn, void *context)
{ 
    // Find the closest enclosing frame with objc catch handlers
    struct frame_range target_frame = findHandler();
    if (!target_frame.ip_start) {
        // No suitable enclosing handler found.
        return 0;
    }

    // Record this alt handler for the discovered frame.
    struct alt_handler_list *list = fetch_handler_list(YES);
    unsigned int i = 0;

    if (list->used == list->allocated) {
        list->allocated = list->allocated*2 ?: 4;
        list->handlers = (struct alt_handler_data *)
            _realloc_internal(list->handlers, 
                              list->allocated * sizeof(list->handlers[0]));
        bzero(&list->handlers[list->used], (list->allocated - list->used) * sizeof(list->handlers[0]));
        i = list->used;
    }
    else {
        for (i = 0; i < list->allocated; i++) {
            if (list->handlers[i].frame.ip_start == 0  &&  
                list->handlers[i].frame.ip_end == 0  &&  
                list->handlers[i].frame.cfa == 0) 
            {
                break;
            }
        }
        if (i == list->allocated) {
            _objc_fatal("alt handlers in objc runtime are buggy!");
        }
    }

    struct alt_handler_data *data = &list->handlers[i];

    data->frame = target_frame;
    data->fn = fn;
    data->context = context;
    list->used++;

    uintptr_t token = i+1;

    if (DebugAltHandlers) {
        // Record backtrace in case this handler is misused later.
        pthread_mutex_lock(&DebugLock);

        token = DebugCounter++;
        if (token == 0) token = DebugCounter++;

        if (!data->debug) {
            data->debug = (struct alt_handler_debug *)
                _calloc_internal(sizeof(*data->debug), 1);
        } else {
            bzero(data->debug, sizeof(*data->debug));
        }

        pthread_getname_np(pthread_self(), data->debug->thread, THREADNAME_COUNT);
        strlcpy(data->debug->queue, 
                dispatch_queue_get_label(dispatch_get_current_queue()), 
                THREADNAME_COUNT);
        data->debug->backtraceSize = 
            backtrace(data->debug->backtrace, BACKTRACE_COUNT);
        data->debug->token = token;

        pthread_mutex_unlock(&DebugLock);
    }

    if (PrintAltHandlers) {
        _objc_inform("ALT HANDLERS: installing alt handler #%lu %p(%p) on "
                     "frame [ip=%p..%p sp=%p]", (unsigned long)token, 
                     data->fn, data->context, (void *)data->frame.ip_start, 
                     (void *)data->frame.ip_end, (void *)data->frame.cfa);
        if (data->frame.ips) {
            unsigned int r = 0;
            while (1) {
                uintptr_t start = data->frame.ips[r].start;
                uintptr_t end = data->frame.ips[r].end;
                r++;
                if (start == 0  &&  end == 0) break;
                _objc_inform("ALT HANDLERS:     ip=%p..%p", 
                             (void*)start, (void*)end);
            }
        }
    }

    if (list->used > 1000) {
        static int warned = 0;
        if (!warned) {
            _objc_inform("ALT HANDLERS: *** over 1000 alt handlers installed; "
                         "this is probably a bug");
            warned = 1;
        }
    }

    return token;
}


void objc_removeExceptionHandler(uintptr_t token)
{
    if (!token) {
        // objc_addExceptionHandler failed
        return;
    }
    
    struct alt_handler_list *list = fetch_handler_list(NO);
    if (!list  ||  !list->handlers) {
        // no alt handlers active
        alt_handler_error(token);
        __builtin_trap();
    }

    uintptr_t i = token-1;
    
    if (DebugAltHandlers) {
        // search for the token instead of using token-1
        for (i = 0; i < list->allocated; i++) {
            struct alt_handler_data *data = &list->handlers[i];
            if (data->debug  &&  data->debug->token == token) break;
        }
    }
    
    if (i >= list->allocated) {
        // token out of range
        alt_handler_error(token);
        __builtin_trap();
    }

    struct alt_handler_data *data = &list->handlers[i];

    if (data->frame.ip_start == 0  &&  data->frame.ip_end == 0  &&  data->frame.cfa == 0) {
        // token in range, but invalid
        alt_handler_error(token);
        __builtin_trap();
    }

    if (PrintAltHandlers) {
        _objc_inform("ALT HANDLERS: removing   alt handler #%lu %p(%p) on "
                     "frame [ip=%p..%p sp=%p]", (unsigned long)token, 
                     data->fn, data->context, (void *)data->frame.ip_start, 
                     (void *)data->frame.ip_end, (void *)data->frame.cfa);
    }

    if (data->debug) _free_internal(data->debug);
    if (data->frame.ips) _free_internal(data->frame.ips);
    bzero(data, sizeof(*data));
    list->used--;
}

void objc_alt_handler_error(void) __attribute__((noinline));

void alt_handler_error(uintptr_t token)
{
    if (!DebugAltHandlers) {
        _objc_inform_now_and_on_crash
            ("objc_removeExceptionHandler() called with unknown alt handler; "
             "this is probably a bug in multithreaded AppKit use. "
             "Set environment variable OBJC_DEBUG_ALT_HANDLERS=YES "
             "or break in objc_alt_handler_error() to debug.");
        objc_alt_handler_error();
    }

    pthread_mutex_lock(&DebugLock);

    // Search other threads' alt handler lists for this handler.
    struct alt_handler_list *list;
    for (list = DebugLists; list; list = list->next_DEBUGONLY) {
        unsigned h;
        for (h = 0; h < list->allocated; h++) {
            struct alt_handler_data *data = &list->handlers[h];
            if (data->debug  &&  data->debug->token == token) {
                // found it
                int i;

                // Build a string from the recorded backtrace
                char *symbolString;
                char **symbols = 
                    backtrace_symbols(data->debug->backtrace, 
                                      data->debug->backtraceSize);
                size_t len = 1;
                for (i = 0; i < data->debug->backtraceSize; i++){
                    len += 4 + strlen(symbols[i]) + 1;
                }
                symbolString = (char *)_calloc_internal(len, 1);
                for (i = 0; i < data->debug->backtraceSize; i++){
                    strcat(symbolString, "    ");
                    strcat(symbolString, symbols[i]);
                    strcat(symbolString, "\n");
                }

                free(symbols);

                _objc_inform_now_and_on_crash
                    ("objc_removeExceptionHandler() called with "
                     "unknown alt handler; this is probably a bug in "
                     "multithreaded AppKit use. \n"
                     "The matching objc_addExceptionHandler() was called by:\n"
                     "Thread '%s': Dispatch queue: '%s': \n%s", 
                     data->debug->thread, data->debug->queue, symbolString);

                pthread_mutex_unlock(&DebugLock);
                _free_internal(symbolString);
                
                objc_alt_handler_error();
            }
        }
    }

    pthread_mutex_lock(&DebugLock);

    // not found
    _objc_inform_now_and_on_crash
        ("objc_removeExceptionHandler() called with unknown alt handler; "
         "this is probably a bug in multithreaded AppKit use");
    objc_alt_handler_error();
}

void objc_alt_handler_error(void)
{
    __builtin_trap();
}

// called in order registered, to match 32-bit _NSAddAltHandler2
// fixme reverse registration order matches c++ destructors better
static void call_alt_handlers(struct _Unwind_Context *ctx)
{
    uintptr_t ip = _Unwind_GetIP(ctx) - 1;
    uintptr_t cfa = _Unwind_GetCFA(ctx);
    unsigned int i;
    
    struct alt_handler_list *list = fetch_handler_list(NO);
    if (!list  ||  list->used == 0) return;

    for (i = 0; i < list->allocated; i++) {
        struct alt_handler_data *data = &list->handlers[i];
        if (ip >= data->frame.ip_start  &&  ip < data->frame.ip_end  &&  data->frame.cfa == cfa) 
        {
            if (data->frame.ips) {
                unsigned int r = 0;
                bool found;
                while (1) {
                    uintptr_t start = data->frame.ips[r].start;
                    uintptr_t end = data->frame.ips[r].end;
                    r++;
                    if (start == 0  &&  end == 0) {
                        found = false;
                        break;
                    }
                    if (ip >= start  &&  ip < end) {
                        found = true; 
                        break;
                    }
                }
                if (!found) continue;
            }

            // Copy and clear before the callback, in case the 
            // callback manipulates the alt handler list.
            struct alt_handler_data copy = *data;
            bzero(data, sizeof(*data));
            list->used--;
            if (PrintExceptions || PrintAltHandlers) {
                _objc_inform("EXCEPTIONS: calling alt handler %p(%p) from "
                             "frame [ip=%p..%p sp=%p]", copy.fn, copy.context, 
                             (void *)copy.frame.ip_start, 
                             (void *)copy.frame.ip_end, 
                             (void *)copy.frame.cfa);
            }
            if (copy.fn) (*copy.fn)(nil, copy.context);
            if (copy.frame.ips) _free_internal(copy.frame.ips);
        }
    }
}

// SUPPORT_ALT_HANDLERS
#endif


/***********************************************************************
* exception_init
* Initialize libobjc's exception handling system.
* Called by map_images().
**********************************************************************/
void exception_init(void)
{
    old_terminate = std::set_terminate(&_objc_terminate);
}


// __OBJC2__
#endif