Link the Wasm EH runtime under -fwasm-exceptions without requiring C++

ChangeLog.md

@@ -23,6 +23,10 @@ See docs/process.md for more on how version tagging works.
 
 - JavaScript library symbols can now use the `__force` and `__export`
   decorators to control inclusion and export visibility. (#27436)
+- `-fwasm-exceptions` now links the Wasm EH runtime (libunwind and the
+  `__cpp_exception` tag) independently of C++ linking, so Wasm EH objects from
+  non-C++ frontends (e.g. rustc, which links via `emcc`) link without
+  requiring `em++` or `-sDEFAULT_TO_CXX`.
 
 6.0.6 - 08/05/26
 ----------------

system/lib/libunwind/src/Unwind-wasm.c

@@ -31,6 +31,21 @@ _Unwind_RaiseException(_Unwind_Exception *exception_object) {
   __builtin_wasm_throw(0, exception_object);
 }
 
+// Define the `__cpp_exception` symbol which `__builtin_wasm_throw` above will
+// reference. This is defined here in `libunwind` as the single canonical
+// definition for this API and it's required for users to ensure that there's
+// only one copy of `libunwind` within a wasm module to ensure this is only
+// defined once and exactly once.
+__asm__(".globl __cpp_exception\n"
+#if defined(__wasm32__)
+        ".tagtype __cpp_exception i32\n"
+#elif defined(__wasm64__)
+        ".tagtype __cpp_exception i64\n"
+#else
+#error "Unsupported Wasm architecture"
+#endif
+        "__cpp_exception:\n");
+
 /// Called by __cxa_end_catch.
 _LIBUNWIND_EXPORT void
 _Unwind_DeleteException(_Unwind_Exception *exception_object) {

test/test_other.py

@@ -8833,9 +8833,10 @@ def test_exceptions_c_linker(self):
     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'],),
+    'wasm_eh': (['-fwasm-exceptions'],),
+    'wasm_legacy_eh': (['-fwasm-exceptions', '-sWASM_LEGACY_EXCEPTIONS'],),
   })
   def test_libunwind(self, cflags):
     src = r'''
@@ -14621,10 +14622,36 @@ def test_wasi_with_sjlj(self):
 
     # When using Wasm exception, SUPPORT_LONGJMP defaults to 'wasm', which does
     # not use the JS-based support. This should succeed.
-    # -fwasm-exceptions exports __cpp_exception, so this is necessary
-    self.set_setting('DEFAULT_TO_CXX')
     self.do_runf('core/test_longjmp.c', cflags=['-fwasm-exceptions'])
 
+  def test_cpp_exception_tag(self):
+    # Wasm EH throw/catch sites reference the `__cpp_exception` tag directly,
+    # regardless of source language (e.g. rustc objects), so an object whose
+    # only undefined symbol is the tag must still pull in its libunwind definition.
+    create_file('throw.S', '''
+.tagtype __cpp_exception i32
+.text
+.globl throw_tag
+throw_tag:
+  .functype throw_tag (i32) -> ()
+  local.get 0
+  throw __cpp_exception
+  end_function
+''')
+    create_file('main.c', r'''
+      #include <stdio.h>
+      void throw_tag(int);
+      int main(int argc, char* argv[]) {
+        if (argc > 100) {
+          throw_tag(argc);
+        }
+        printf("done\n");
+        return 0;
+      }
+    ''')
+    # -mexception-handling is needed for the assembler to accept `throw`.
+    self.do_runf('main.c', 'done\n', cflags=['throw.S', '-fwasm-exceptions', '-mexception-handling'])
+
   def test_memory_init_file_unsupported(self):
     self.assert_fail([EMCC, test_file('hello_world.c'), '-Werror', '--memory-init-file=1'], 'error: --memory-init-file is no longer supported')
 

tools/system_libs.py

@@ -1686,7 +1686,6 @@ def get_files(self):
       'stdlib_typeinfo.cpp',
       'private_typeinfo.cpp',
       'cxa_exception_js_utils.cpp',
-      '__cpp_exception.S',
     ]
     match self.eh_mode:
       case Exceptions.NONE:
@@ -2484,8 +2483,13 @@ def add_sanitizer_libs():
     add_library('libc++')
   if settings.LINK_AS_CXX or sanitize:
     add_library('libc++abi')
-    if settings.WASM_EXCEPTIONS:
-      add_library('libunwind')
+  if settings.WASM_EXCEPTIONS:
+    # Wasm EH objects can come from any language frontend (e.g. rust), so the
+    # unwinding runtime and the `__cpp_exception` tag it defines are linked
+    # independently of C++. When WASM_EXCEPTIONS is not enabled, `_Unwind_*`
+    # symbols are instead provided by JS stubs in libcore.js (under
+    # LINK_AS_CXX).
+    add_library('libunwind')
 
   if settings.PROXY_POSIX_SOCKETS:
     add_library('libsockets_proxy')