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[libc++abi][WebAssembly] Support Wasm EH
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This adds Wasm-specific libc++abi changes to support Wasm exception
handling (https://github.com/WebAssembly/exception-handling).

Wasm EH requires `__USING_WASM_EXCEPTIONS__` to be defined. Wasm EH's
LSDA handling mostly shares that of SjLj EH.
Changes are:
- In Wasm, a destructor returns its argument.
- Wasm EH currently only has one phase (search) that does both search
  and cleanup. So added an additional `set_registers` to support that.

The bulk of these changes was added back in Mar 2020 in
emscripten-core/emscripten#10577 to emscripten
repo and has been used ever since. Now we'd like to upstream this so
that other toolchains that don't use emscripten libraries, e.g., WASI,
can use this too.

Companion patch: D158919

Reviewed By: dschuff, #libc_abi, phosek

Differential Revision: https://reviews.llvm.org/D158918
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@aheejin
aheejin committed Sep 22, 2023
1 parent 8ffe73b commit e6cbba7
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Showing 4 changed files with 40 additions and 18 deletions.
5 changes: 5 additions & 0 deletions libcxxabi/include/cxxabi.h
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Original file line number Diff line number Diff line change
Expand Up @@ -47,7 +47,12 @@ __cxa_free_exception(void *thrown_exception) throw();
// 2.4.3 Throwing the Exception Object
extern _LIBCXXABI_FUNC_VIS _LIBCXXABI_NORETURN void
__cxa_throw(void *thrown_exception, std::type_info *tinfo,
#ifdef __USING_WASM_EXCEPTIONS__
// In Wasm, a destructor returns its argument
void *(_LIBCXXABI_DTOR_FUNC *dest)(void *));
#else
void (_LIBCXXABI_DTOR_FUNC *dest)(void *));
#endif

// 2.5.3 Exception Handlers
extern _LIBCXXABI_FUNC_VIS void *
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5 changes: 5 additions & 0 deletions libcxxabi/src/cxa_exception.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -254,7 +254,12 @@ will call terminate, assuming that there was no handler for the
exception.
*/
void
#ifdef __USING_WASM_EXCEPTIONS__
// In Wasm, a destructor returns its argument
__cxa_throw(void *thrown_object, std::type_info *tinfo, void *(_LIBCXXABI_DTOR_FUNC *dest)(void *)) {
#else
__cxa_throw(void *thrown_object, std::type_info *tinfo, void (_LIBCXXABI_DTOR_FUNC *dest)(void *)) {
#endif
__cxa_eh_globals *globals = __cxa_get_globals();
__cxa_exception* exception_header = cxa_exception_from_thrown_object(thrown_object);

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5 changes: 5 additions & 0 deletions libcxxabi/src/cxa_exception.h
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Original file line number Diff line number Diff line change
Expand Up @@ -43,7 +43,12 @@ struct _LIBCXXABI_HIDDEN __cxa_exception {

// Manage the exception object itself.
std::type_info *exceptionType;
#ifdef __USING_WASM_EXCEPTIONS__
// In Wasm, a destructor returns its argument
void *(_LIBCXXABI_DTOR_FUNC *exceptionDestructor)(void *);
#else
void (_LIBCXXABI_DTOR_FUNC *exceptionDestructor)(void *);
#endif
std::unexpected_handler unexpectedHandler;
std::terminate_handler terminateHandler;

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43 changes: 25 additions & 18 deletions libcxxabi/src/cxa_personality.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -70,7 +70,7 @@ extern "C" EXCEPTION_DISPOSITION _GCC_specific_handler(PEXCEPTION_RECORD,
+------------------+--+-----+-----+------------------------+--------------------------+
| callSiteTableLength | (ULEB128) | Call Site Table length, used to find Action table |
+---------------------+-----------+---------------------------------------------------+
#ifndef __USING_SJLJ_EXCEPTIONS__
#if !defined(__USING_SJLJ_EXCEPTIONS__) && !defined(__USING_WASM_EXCEPTIONS__)
+---------------------+-----------+------------------------------------------------+
| Beginning of Call Site Table The current ip lies within the |
| ... (start, length) range of one of these |
Expand All @@ -84,7 +84,7 @@ extern "C" EXCEPTION_DISPOSITION _GCC_specific_handler(PEXCEPTION_RECORD,
| +-------------+---------------------------------+------------------------------+ |
| ... |
+----------------------------------------------------------------------------------+
#else // __USING_SJLJ_EXCEPTIONS__
#else // __USING_SJLJ_EXCEPTIONS__ || __USING_WASM_EXCEPTIONS__
+---------------------+-----------+------------------------------------------------+
| Beginning of Call Site Table The current ip is a 1-based index into |
| ... this table. Or it is -1 meaning no |
Expand All @@ -97,7 +97,7 @@ extern "C" EXCEPTION_DISPOSITION _GCC_specific_handler(PEXCEPTION_RECORD,
| +-------------+---------------------------------+------------------------------+ |
| ... |
+----------------------------------------------------------------------------------+
#endif // __USING_SJLJ_EXCEPTIONS__
#endif // __USING_SJLJ_EXCEPTIONS__ || __USING_WASM_EXCEPTIONS__
+---------------------------------------------------------------------+
| Beginning of Action Table ttypeIndex == 0 : cleanup |
| ... ttypeIndex > 0 : catch |
Expand Down Expand Up @@ -547,7 +547,7 @@ void
set_registers(_Unwind_Exception* unwind_exception, _Unwind_Context* context,
const scan_results& results)
{
#if defined(__USING_SJLJ_EXCEPTIONS__)
#if defined(__USING_SJLJ_EXCEPTIONS__) || defined(__USING_WASM_EXCEPTIONS__)
#define __builtin_eh_return_data_regno(regno) regno
#elif defined(__ibmxl__)
// IBM xlclang++ compiler does not support __builtin_eh_return_data_regno.
Expand Down Expand Up @@ -642,7 +642,7 @@ static void scan_eh_tab(scan_results &results, _Unwind_Action actions,
// Get beginning current frame's code (as defined by the
// emitted dwarf code)
uintptr_t funcStart = _Unwind_GetRegionStart(context);
#ifdef __USING_SJLJ_EXCEPTIONS__
#if defined(__USING_SJLJ_EXCEPTIONS__) || defined(__USING_WASM_EXCEPTIONS__)
if (ip == uintptr_t(-1))
{
// no action
Expand All @@ -652,9 +652,9 @@ static void scan_eh_tab(scan_results &results, _Unwind_Action actions,
else if (ip == 0)
call_terminate(native_exception, unwind_exception);
// ip is 1-based index into call site table
#else // !__USING_SJLJ_EXCEPTIONS__
#else // !__USING_SJLJ_EXCEPTIONS__ && !__USING_WASM_EXCEPTIONS__
uintptr_t ipOffset = ip - funcStart;
#endif // !defined(_USING_SLJL_EXCEPTIONS__)
#endif // !__USING_SJLJ_EXCEPTIONS__ && !__USING_WASM_EXCEPTIONS__
const uint8_t* classInfo = NULL;
// Note: See JITDwarfEmitter::EmitExceptionTable(...) for corresponding
// dwarf emission
Expand All @@ -676,8 +676,8 @@ static void scan_eh_tab(scan_results &results, _Unwind_Action actions,
// Walk call-site table looking for range that
// includes current PC.
uint8_t callSiteEncoding = *lsda++;
#ifdef __USING_SJLJ_EXCEPTIONS__
(void)callSiteEncoding; // When using SjLj exceptions, callSiteEncoding is never used
#if defined(__USING_SJLJ_EXCEPTIONS__) || defined(__USING_WASM_EXCEPTIONS__)
(void)callSiteEncoding; // When using SjLj/Wasm exceptions, callSiteEncoding is never used
#endif
uint32_t callSiteTableLength = static_cast<uint32_t>(readULEB128(&lsda));
const uint8_t* callSiteTableStart = lsda;
Expand All @@ -687,33 +687,33 @@ static void scan_eh_tab(scan_results &results, _Unwind_Action actions,
while (callSitePtr < callSiteTableEnd)
{
// There is one entry per call site.
#ifndef __USING_SJLJ_EXCEPTIONS__
#if !defined(__USING_SJLJ_EXCEPTIONS__) && !defined(__USING_WASM_EXCEPTIONS__)
// The call sites are non-overlapping in [start, start+length)
// The call sites are ordered in increasing value of start
uintptr_t start = readEncodedPointer(&callSitePtr, callSiteEncoding);
uintptr_t length = readEncodedPointer(&callSitePtr, callSiteEncoding);
uintptr_t landingPad = readEncodedPointer(&callSitePtr, callSiteEncoding);
uintptr_t actionEntry = readULEB128(&callSitePtr);
if ((start <= ipOffset) && (ipOffset < (start + length)))
#else // __USING_SJLJ_EXCEPTIONS__
#else // __USING_SJLJ_EXCEPTIONS__ || __USING_WASM_EXCEPTIONS__
// ip is 1-based index into this table
uintptr_t landingPad = readULEB128(&callSitePtr);
uintptr_t actionEntry = readULEB128(&callSitePtr);
if (--ip == 0)
#endif // __USING_SJLJ_EXCEPTIONS__
#endif // __USING_SJLJ_EXCEPTIONS__ || __USING_WASM_EXCEPTIONS__
{
// Found the call site containing ip.
#ifndef __USING_SJLJ_EXCEPTIONS__
#if !defined(__USING_SJLJ_EXCEPTIONS__) && !defined(__USING_WASM_EXCEPTIONS__)
if (landingPad == 0)
{
// No handler here
results.reason = _URC_CONTINUE_UNWIND;
return;
}
landingPad = (uintptr_t)lpStart + landingPad;
#else // __USING_SJLJ_EXCEPTIONS__
#else // __USING_SJLJ_EXCEPTIONS__ || __USING_WASM_EXCEPTIONS__
++landingPad;
#endif // __USING_SJLJ_EXCEPTIONS__
#endif // __USING_SJLJ_EXCEPTIONS__ || __USING_WASM_EXCEPTIONS__
results.landingPad = landingPad;
if (actionEntry == 0)
{
Expand Down Expand Up @@ -841,15 +841,15 @@ static void scan_eh_tab(scan_results &results, _Unwind_Action actions,
action += actionOffset;
} // there is no break out of this loop, only return
}
#ifndef __USING_SJLJ_EXCEPTIONS__
#if !defined(__USING_SJLJ_EXCEPTIONS__) && !defined(__USING_WASM_EXCEPTIONS__)
else if (ipOffset < start)
{
// There is no call site for this ip
// Something bad has happened. We should never get here.
// Possible stack corruption.
call_terminate(native_exception, unwind_exception);
}
#endif // !__USING_SJLJ_EXCEPTIONS__
#endif // !__USING_SJLJ_EXCEPTIONS__ && !__USING_WASM_EXCEPTIONS__
} // there might be some tricky cases which break out of this loop

// It is possible that no eh table entry specify how to handle
Expand Down Expand Up @@ -906,7 +906,9 @@ _UA_CLEANUP_PHASE
*/

#if !defined(_LIBCXXABI_ARM_EHABI)
#if defined(__SEH__) && !defined(__USING_SJLJ_EXCEPTIONS__)
#ifdef __USING_WASM_EXCEPTIONS__
_Unwind_Reason_Code __gxx_personality_wasm0
#elif defined(__SEH__) && !defined(__USING_SJLJ_EXCEPTIONS__)
static _Unwind_Reason_Code __gxx_personality_imp
#else
_LIBCXXABI_FUNC_VIS _Unwind_Reason_Code
Expand Down Expand Up @@ -973,6 +975,11 @@ __gxx_personality_v0
exc->languageSpecificData = results.languageSpecificData;
exc->catchTemp = reinterpret_cast<void*>(results.landingPad);
exc->adjustedPtr = results.adjustedPtr;
#ifdef __USING_WASM_EXCEPTIONS__
// Wasm only uses a single phase (_UA_SEARCH_PHASE), so save the
// results here.
set_registers(unwind_exception, context, results);
#endif
}
return _URC_HANDLER_FOUND;
}
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