Unconditionally define wasm-eh tags in native code. NFC

This tags are sometimes also weakly defined in each object file by llvm
but I hope to remove those definitions going forward and instead rely
on the toolchain/compiler-rt to define them.

Author: sbc100

src/lib/libcore.js

@@ -2239,13 +2239,6 @@ addToLibrary({
   __stack_high: '{{{ STACK_HIGH }}}',
   __stack_low: '{{{ STACK_LOW }}}',
   __global_base: '{{{ GLOBAL_BASE }}}',
-#if WASM_EXCEPTIONS
-  // In dynamic linking we define tags here and feed them to each module
-  __cpp_exception: "new WebAssembly.Tag({'parameters': ['{{{ POINTER_WASM_TYPE }}}']})",
-#endif
-#if SUPPORT_LONGJMP == 'wasm'
-  __c_longjmp: "new WebAssembly.Tag({'parameters': ['{{{ POINTER_WASM_TYPE }}}']})",
-#endif
 #if ASYNCIFY == 1
   __asyncify_state: "new WebAssembly.Global({'value': 'i32', 'mutable': true}, 0)",
   __asyncify_data: "new WebAssembly.Global({'value': '{{{ POINTER_WASM_TYPE }}}', 'mutable': true}, {{{ to64(0) }}})",

src/lib/libdylink.js

@@ -276,7 +276,7 @@ var LibraryDylink = {
         continue;
       }
 #endif
-      if (typeof value == 'object') {
+      if (typeof value == 'object' && typeof value.value != 'undefined') {
         // a breaking change in the wasm spec, globals are now objects
         // https://github.com/WebAssembly/mutable-global/issues/1
         value = value.value;

src/preamble.js

@@ -740,7 +740,7 @@ function getWasmImports() {
 #endif
 #endif
 
-#if hasExportedSymbol('__cpp_exception') && !RELOCATABLE
+#if hasExportedSymbol('__cpp_exception')
     ___cpp_exception = wasmExports['__cpp_exception'];
     {{{ receivedSymbol('___cpp_exception') }}};
 #endif

system/lib/compiler-rt/emscripten_setjmp.c

@@ -70,6 +70,7 @@ void __wasm_longjmp(void* env, int val) {
   __builtin_wasm_throw(C_LONGJMP, arg);
 }
 #else
+
 void emscripten_longjmp(uintptr_t env, int val) {
   // C standard:
   //   The longjmp function cannot cause the setjmp macro to return

system/lib/compiler-rt/wasm_eh_tags.S

@@ -0,0 +1,26 @@
+/*
+ * Copyright 2025 The Emscripten Authors.  All rights reserved.
+ * Emscripten is available under two separate licenses, the MIT license and the
+ * University of Illinois/NCSA Open Source License.  Both these licenses can be
+ * found in the LICENSE file.
+ *
+ * Define the Wasm EH tags needed by LLVM
+ */
+
+#ifdef __WASM_SJLJ__
+
+#ifdef __wasm64__
+#define PTR i64
+#else
+#define PTR i32
+#endif
+
+.globl __cpp_exception
+.tagtype __cpp_exception PTR
+__cpp_exception:
+
+.globl __c_longjmp
+.tagtype __c_longjmp PTR
+__c_longjmp:
+
+#endif // !__WASM_SJLJ__

test/code_size/test_codesize_hello_dylink.json

@@ -1,10 +1,10 @@
 {
-  "a.out.js": 26969,
-  "a.out.js.gz": 11479,
+  "a.out.js": 26998,
+  "a.out.js.gz": 11486,
   "a.out.nodebug.wasm": 18567,
   "a.out.nodebug.wasm.gz": 9199,
-  "total": 45536,
-  "total_gz": 20678,
+  "total": 45565,
+  "total_gz": 20685,
   "sent": [
     "__heap_base",
     "__indirect_function_table",

test/code_size/test_codesize_hello_dylink_all.json

@@ -1,7 +1,7 @@
 {
-  "a.out.js": 246021,
+  "a.out.js": 246049,
   "a.out.nodebug.wasm": 597767,
-  "total": 843788,
+  "total": 843816,
   "sent": [
     "IMG_Init",
     "IMG_Load",

test/test_core.py

@@ -4958,9 +4958,11 @@ def test_dylink_exceptions_try_catch_2(self):
   @no_js_math('JS_MATH is not compatible with MAIN_MODULE')
   def test_dylink_exceptions_try_catch_6(self):
     create_file('main.cpp', r'''
+      #include <assert.h>
       #include <dlfcn.h>
       int main() {
         void* handle = dlopen("liblib.so", RTLD_LAZY);
+        assert(handle);
         void (*side)(void) = (void (*)(void))dlsym(handle, "side");
         (side)();
         return 0;

tools/system_libs.py

@@ -984,6 +984,7 @@ class libcompiler_rt(MTLibrary, SjLjLibrary):
       filenames=[
         'stack_ops.S',
         'stack_limits.S',
+        'wasm_eh_tags.S',
         'emscripten_setjmp.c',
         'emscripten_exception_builtins.c',
         'emscripten_tempret.s',