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[Emscripten-EH] Implement __cxa_throw in terms of _Unwind_RaiseException #27498

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Merged
sbc100 merged 1 commit into from
Aug 8, 2026
Merged

[Emscripten-EH] Implement __cxa_throw in terms of _Unwind_RaiseException #27498

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24 changes: 0 additions & 24 deletions src/lib/libcore.js
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Original file line number Diff line number Diff line change
Expand Up @@ -1733,31 +1733,7 @@ addToLibrary({
$jstoi_q__docs: '/** @suppress {checkTypes} */',
$jstoi_q: (str) => parseInt(str),

#if LINK_AS_CXX
// libunwind

_Unwind_Backtrace__deps: ['$getCallstack'],
_Unwind_Backtrace: (func, arg) => {
var trace = getCallstack();
var parts = trace.split('\n');
for (var i = 0; i < parts.length; i++) {
var ret = {{{ makeDynCall('iii', 'func') }}}(0, arg);
if (ret) return;
}
},

_Unwind_GetIPInfo: (context, ipBefore) => abort('Unwind_GetIPInfo'),

_Unwind_FindEnclosingFunction: (ip) => 0, // we cannot succeed

_Unwind_RaiseException__deps: ['__cxa_throw'],
_Unwind_RaiseException: (ex) => {
err('Warning: _Unwind_RaiseException is not correctly implemented');
return ___cxa_throw(ex, 0, 0);
},

_Unwind_DeleteException: (ex) => err('TODO: Unwind_DeleteException'),
#endif

// special runtime support

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39 changes: 18 additions & 21 deletions src/lib/libexceptions.js
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Expand Up @@ -6,10 +6,6 @@

var LibraryExceptions = {
#if !WASM_EXCEPTIONS
$uncaughtExceptionCount: '0',
#if !DISABLE_EXCEPTION_CATCHING
$exceptionLast: null,
#endif
Comment on lines -9 to -12

@aheejin aheejin Aug 8, 2026

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In libcxxabi native code (and thus in Wasm EH), managing information like uncaught exception count and last exception is done by libcxxabi, not libunwind. It is true in libcxxabi/libunwind that __cxa_throw calls _Unwind_RaiseException, but all exception status managing is done within libcxxabi. Wouldn't it be consistent to manage it within libexceptions.js and leave libunwind simple?

@sbc100 sbc100 Aug 8, 2026

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Sure, we can make it consistent in that way. I'll make a followup PR.

$exceptionCaught: ' []',

// This class is the exception metadata which is prepended to each thrown object (in WASM memory).
Expand Down Expand Up @@ -84,7 +80,7 @@ var LibraryExceptions = {

// Here, we throw an exception after recording a couple of values that we need to remember
// We also remember that it was the last exception thrown as we need to know that later.
__cxa_throw__deps: ['$ExceptionInfo', '$uncaughtExceptionCount',
__cxa_throw__deps: ['$ExceptionInfo',
#if !DISABLE_EXCEPTION_CATCHING
'$exceptionLast',
'__cxa_increment_exception_refcount',
Expand All @@ -99,6 +95,7 @@ var LibraryExceptions = {
// 'throw' is used here.
'$decrementExceptionRefcount', '$incrementExceptionRefcount',
#endif
'_Unwind_RaiseException',
],
__cxa_throw: (ptr, type, destructor) => {
#if EXCEPTION_DEBUG
Expand All @@ -109,20 +106,20 @@ var LibraryExceptions = {
info.init(type, destructor);
#if !DISABLE_EXCEPTION_CATCHING
___cxa_increment_exception_refcount(ptr);
exceptionLast = new CppException(ptr);
ptr = new CppException(ptr);
#endif
uncaughtExceptionCount++;
{{{ makeThrow() }}}
__Unwind_RaiseException(ptr);
},

// This exception will be caught twice, but while begin_catch runs twice,
// we early-exit from end_catch when the exception has been rethrown, so
// pop that here from the caught exceptions.
__cxa_rethrow__deps: ['$exceptionCaught', '$uncaughtExceptionCount',
__cxa_rethrow__deps: ['$exceptionCaught',
#if !DISABLE_EXCEPTION_CATCHING
'$exceptionLast',
'__cxa_increment_exception_refcount',
#endif
'_Unwind_RaiseException',
],
__cxa_rethrow: () => {
if (!exceptionCaught.length) {
Expand All @@ -132,16 +129,15 @@ var LibraryExceptions = {
var ptr = info.excPtr;
info.set_rethrown(true);
info.set_caught(false);
uncaughtExceptionCount++;
#if !DISABLE_EXCEPTION_CATCHING
___cxa_increment_exception_refcount(ptr);
#if EXCEPTION_DEBUG
dbg('__cxa_rethrow: ' +
[ptrToString(ptr), exceptionLast, 'stack', exceptionCaught]);
#endif
exceptionLast = new CppException(ptr);
ptr = new CppException(ptr);
#endif
{{{ makeThrow() }}}
__Unwind_RaiseException(ptr);
},

llvm_eh_typeid_for: (type) => type,
Expand Down Expand Up @@ -212,11 +208,12 @@ var LibraryExceptions = {
return info.get_type();
},

__cxa_rethrow_primary_exception__deps: ['$ExceptionInfo', '$uncaughtExceptionCount',
__cxa_rethrow_primary_exception__deps: ['$ExceptionInfo',
#if !DISABLE_EXCEPTION_CATCHING
'$exceptionLast',
'__cxa_increment_exception_refcount',
#endif
'_Unwind_RaiseException',
],
__cxa_rethrow_primary_exception: (ptr) => {
if (!ptr) return;
Expand All @@ -226,12 +223,11 @@ var LibraryExceptions = {
var info = new ExceptionInfo(ptr);
info.set_rethrown(true);
info.set_caught(false);
uncaughtExceptionCount++;
#if !DISABLE_EXCEPTION_CATCHING
___cxa_increment_exception_refcount(ptr);
exceptionLast = new CppException(ptr);
ptr = new CppException(ptr);
#endif
{{{ makeThrow('exceptionLast') }}}
__Unwind_RaiseException(ptr);
},

// Finds a suitable catch clause for when an exception is thrown.
Expand Down Expand Up @@ -293,19 +289,20 @@ var LibraryExceptions = {
#endif
},

__resumeException__deps: [
#if !DISABLE_EXCEPTION_CATCHING
__resumeException__deps: ['$exceptionLast'],
'$exceptionLast',
#endif
'_Unwind_Resume',
],
__resumeException: (ptr) => {
#if !DISABLE_EXCEPTION_CATCHING
#if EXCEPTION_DEBUG
dbg("__resumeException " + [ptrToString(ptr), exceptionLast]);
#endif
if (!exceptionLast) {
exceptionLast = new CppException(ptr);
}
ptr = exceptionLast ?? new CppException(ptr);
#endif
{{{ makeThrow() }}}
__Unwind_Resume(ptr);
},

#endif
Expand Down
1 change: 0 additions & 1 deletion src/lib/libsigs.js
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Expand Up @@ -213,7 +213,6 @@ sigs = {
_Unwind_DeleteException__sig: 'vp',
_Unwind_FindEnclosingFunction__sig: 'pp',
_Unwind_GetIPInfo__sig: 'ppp',
_Unwind_RaiseException__sig: 'ip',
__asctime_r__sig: 'ppp',
__assert_fail__sig: 'vppip',
__call_sighandler__sig: 'vpi',
Expand Down
57 changes: 57 additions & 0 deletions src/lib/libunwind.js
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Original file line number Diff line number Diff line change
@@ -0,0 +1,57 @@
/**
* @license
* Copyright 2026 The Emscripten Authors
* SPDX-License-Identifier: MIT
*/

#if WASM_EXCEPTIONS
#error "Internal error! WASM_EXCEPTIONS should not be enabled when including libunwind.js."
#endif

var LibraryUnwind = {
$uncaughtExceptionCount: '0',
#if !DISABLE_EXCEPTION_CATCHING
$exceptionLast: null,
#endif

_Unwind_Backtrace__deps: ['$getCallstack'],
_Unwind_Backtrace: (func, arg) => {
var trace = getCallstack();
var parts = trace.split('\n');
for (var i = 0; i < parts.length; i++) {
var ret = {{{ makeDynCall('iii', 'func') }}}(0, arg);
if (ret) return;
}
},

_Unwind_GetIPInfo: (context, ipBefore) => abort('Unwind_GetIPInfo'),

_Unwind_FindEnclosingFunction: (ip) => 0, // we cannot succeed

_Unwind_RaiseException__deps: ['$uncaughtExceptionCount',
#if !DISABLE_EXCEPTION_CATCHING
'$exceptionLast',
#endif
],
_Unwind_RaiseException: (ex) => {
#if !DISABLE_EXCEPTION_CATCHING
exceptionLast = ex;
uncaughtExceptionCount++;
#endif
{{{ makeThrow('ex') }}}
},

#if !DISABLE_EXCEPTION_CATCHING
_Unwind_Resume__deps: ['$exceptionLast'],
#endif
_Unwind_Resume: (ex) => {
#if !DISABLE_EXCEPTION_CATCHING
exceptionLast = ex;
#endif
{{{ makeThrow('ex') }}}
},

_Unwind_DeleteException: (ex) => err('TODO: Unwind_DeleteException'),
};

addToLibrary(LibraryUnwind);
4 changes: 4 additions & 0 deletions src/modules.mjs
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Original file line number Diff line number Diff line change
Expand Up @@ -71,6 +71,10 @@ function calculateLibraries() {
}
}

if (!WASM_EXCEPTIONS) {
libraries.push('libunwind.js');
}

if (!MINIMAL_RUNTIME) {
libraries.push('libbrowser.js');
libraries.push('libwget.js');
Expand Down
4 changes: 2 additions & 2 deletions src/parseTools.mjs
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Expand Up @@ -664,7 +664,7 @@ export function makeReturn64(value) {
return `(setTempRet0(${pair[1]}), ${pair[0]})`;
}

function makeThrow() {
function makeThrow(exc) {
if (DISABLE_EXCEPTION_CATCHING) {
if (ASSERTIONS) {
var assertInfo =
Expand All @@ -678,7 +678,7 @@ function makeThrow() {
return 'abort()';
}
}
return 'throw exceptionLast;';
return `throw ${exc};`;
}

function charCode(char) {
Expand Down
8 changes: 4 additions & 4 deletions test/codesize/test_codesize_cxx_ctors1.json
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@@ -1,10 +1,10 @@
{
"a.out.js": 19208,
"a.out.js.gz": 8121,
"a.out.js": 19198,
"a.out.js.gz": 8107,
"a.out.nodebug.wasm": 134729,
"a.out.nodebug.wasm.gz": 51522,
"total": 153937,
"total_gz": 59643,
"total": 153927,
"total_gz": 59629,
"sent": [
"__cxa_throw",
"_abort_js",
Expand Down
8 changes: 4 additions & 4 deletions test/codesize/test_codesize_cxx_ctors2.json
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@@ -1,10 +1,10 @@
{
"a.out.js": 19185,
"a.out.js.gz": 8104,
"a.out.js": 19175,
"a.out.js.gz": 8091,
"a.out.nodebug.wasm": 134158,
"a.out.nodebug.wasm.gz": 51187,
"total": 153343,
"total_gz": 59291,
"total": 153333,
"total_gz": 59278,
"sent": [
"__cxa_throw",
"_abort_js",
Expand Down
8 changes: 4 additions & 4 deletions test/codesize/test_codesize_cxx_except.json
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Original file line number Diff line number Diff line change
@@ -1,10 +1,10 @@
{
"a.out.js": 22895,
"a.out.js.gz": 9075,
"a.out.js": 22904,
"a.out.js.gz": 9078,
"a.out.nodebug.wasm": 177195,
"a.out.nodebug.wasm.gz": 59099,
"total": 200090,
"total_gz": 68174,
"total": 200099,
"total_gz": 68177,
"sent": [
"__cxa_begin_catch",
"__cxa_end_catch",
Expand Down
8 changes: 4 additions & 4 deletions test/codesize/test_codesize_cxx_mangle.json
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@@ -1,10 +1,10 @@
{
"a.out.js": 22945,
"a.out.js.gz": 9095,
"a.out.js": 22954,
"a.out.js.gz": 9099,
"a.out.nodebug.wasm": 243475,
"a.out.nodebug.wasm.gz": 81296,
"total": 266420,
"total_gz": 90391,
"total": 266429,
"total_gz": 90395,
"sent": [
"__cxa_begin_catch",
"__cxa_end_catch",
Expand Down
8 changes: 4 additions & 4 deletions test/codesize/test_codesize_cxx_noexcept.json
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@@ -1,10 +1,10 @@
{
"a.out.js": 19208,
"a.out.js.gz": 8121,
"a.out.js": 19198,
"a.out.js.gz": 8107,
"a.out.nodebug.wasm": 136639,
"a.out.nodebug.wasm.gz": 52140,
"total": 155847,
"total_gz": 60261,
"total": 155837,
"total_gz": 60247,
"sent": [
"__cxa_throw",
"_abort_js",
Expand Down
8 changes: 4 additions & 4 deletions test/codesize/test_codesize_cxx_wasmfs.json
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@@ -1,10 +1,10 @@
{
"a.out.js": 6604,
"a.out.js.gz": 3154,
"a.out.js": 6594,
"a.out.js.gz": 3145,
"a.out.nodebug.wasm": 174361,
"a.out.nodebug.wasm.gz": 64898,
"total": 180965,
"total_gz": 68052,
"total": 180955,
"total_gz": 68043,
"sent": [
"__cxa_throw",
"_abort_js",
Expand Down
8 changes: 4 additions & 4 deletions test/codesize/test_codesize_hello_O0.json
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@@ -1,10 +1,10 @@
{
"a.out.js": 23471,
"a.out.js.gz": 8555,
"a.out.js": 23494,
"a.out.js.gz": 8565,
"a.out.nodebug.wasm": 15115,
"a.out.nodebug.wasm.gz": 7464,
"total": 38586,
"total_gz": 16019,
"total": 38609,
"total_gz": 16029,
"sent": [
"fd_write"
],
Expand Down
10 changes: 8 additions & 2 deletions test/codesize/test_codesize_hello_dylink_all.json
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@@ -1,7 +1,7 @@
{
"a.out.js": 267525,
"a.out.js": 267896,
"a.out.nodebug.wasm": 588311,
"total": 855836,
"total": 856207,
"sent": [
"IMG_Init",
"IMG_Load",
Expand Down Expand Up @@ -212,6 +212,12 @@
"XSendEvent",
"XSetWMHints",
"XStoreName",
"_Unwind_Backtrace",
"_Unwind_DeleteException",
"_Unwind_FindEnclosingFunction",
"_Unwind_GetIPInfo",
"_Unwind_RaiseException",
"_Unwind_Resume",
"__asctime_r",
"__assert_fail",
"__call_sighandler",
Expand Down
1 change: 1 addition & 0 deletions test/codesize/test_codesize_minimal_O0.expected.js
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Expand Up @@ -1030,6 +1030,7 @@ missingLibrarySymbols.forEach(missingLibrarySymbol)
'emClearImmediate_deps',
'emClearImmediate',
'promiseMap',
'uncaughtExceptionCount',
'Browser',
'requestFullscreen',
'setCanvasSize',
Expand Down
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