[Emscripten-EH] Move JS unwinding implementation to src/lib/libunwind.js

Extract `_Unwind_*` functions and `uncaughtExceptionCount` /
`exceptionLast` JS state from libcore.js and libexceptions.js into a
dedicated `libunwind.js`.

This new libunwind.js is included as long as wasm EH is not being
used.

Also, invert the dependency between `__cxa_throw` and
`_Unwind_RaiseException` so that `__cxa_throw` now calls
`_Unwind_RaiseException` rather than the other way around.  This is
important as it allows C programs (or rather non-C++ programs like Rust) to
call `_Unwind_RaiseException` without linking as C++.

Inspired by #27496

Author: sbc100

src/lib/libcore.js

@@ -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
 

src/lib/libexceptions.js

@@ -6,10 +6,6 @@
 
 var LibraryExceptions = {
 #if !WASM_EXCEPTIONS
-  $uncaughtExceptionCount: '0',
-#if !DISABLE_EXCEPTION_CATCHING
-  $exceptionLast: null,
-#endif
   $exceptionCaught: ' []',
 
   // This class is the exception metadata which is prepended to each thrown object (in WASM memory).
@@ -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',
@@ -99,6 +95,7 @@ var LibraryExceptions = {
     // 'throw' is used here.
     '$decrementExceptionRefcount', '$incrementExceptionRefcount',
 #endif
+    '_Unwind_RaiseException',
   ],
   __cxa_throw: (ptr, type, destructor) => {
 #if EXCEPTION_DEBUG
@@ -111,8 +108,7 @@ var LibraryExceptions = {
     ___cxa_increment_exception_refcount(ptr);
     exceptionLast = new CppException(ptr);
 #endif
-    uncaughtExceptionCount++;
-    {{{ makeThrow() }}}
+    __Unwind_RaiseException(ptr);
   },
 
   // This exception will be caught twice, but while begin_catch runs twice,

src/lib/libunwind.js

@@ -0,0 +1,49 @@
+/**
+ * @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
+    if (!exceptionLast) {
+      exceptionLast = ex;
+    }
+#endif
+    uncaughtExceptionCount++;
+    {{{ makeThrow() }}}
+  },
+
+  _Unwind_DeleteException: (ex) => err('TODO: Unwind_DeleteException'),
+};
+
+addToLibrary(LibraryUnwind);

src/modules.mjs

@@ -71,6 +71,10 @@ function calculateLibraries() {
     }
   }
 
+  if (!WASM_EXCEPTIONS) {
+    libraries.push('libunwind.js');
+  }
+
   if (!MINIMAL_RUNTIME) {
     libraries.push('libbrowser.js');
     libraries.push('libwget.js');

test/test_other.py

@@ -8832,6 +8832,29 @@ def test_exceptions_c_linker(self):
     # when not linking as C++.
     self.assert_fail([EMCC, '-sSTRICT', test_file('other/test_exceptions_c_linker.c')], 'error: undefined symbol: __cxa_find_matching_catch_1')
 
+  @parameterized({
+    # TODO: Add wasm_eh modes once the libunwind Wasm EH followup PR lands
+    '': ([],),
+    'exceptions': (['-fexceptions'],),
+  })
+  def test_libunwind(self, cflags):
+    src = r'''
+      #include <unwind.h>
+      #include <stdio.h>
+      #include <assert.h>
+
+      static struct _Unwind_Exception exc;
+
+      int main() {
+        assert(&_Unwind_RaiseException != NULL);
+        printf("About to raise exception...\n");
+        _Unwind_RaiseException(&exc);
+        printf("ERROR: _Unwind_RaiseException returned!\n");
+        return 0;
+      }
+    '''
+    self.do_run(src, 'About to raise exception...\n', cflags=cflags, assert_returncode=NON_ZERO)
+
   @with_all_eh_sjlj
   @no_bun('https://github.com/emscripten-core/emscripten/issues/26197')
   def test_exceptions_stack_trace_and_message(self):