Define wasm-eh tags in native code. NFC (#25284)

These tags have also historically be weakly defined in each object file
by llvm (at least for non-PIC object files) but I hope to remove those
definitions going forward and instead rely on the toolchain/compiler-rt
to define them.

This will remove one of the distinctions between PIC and non-PIC
generated code.

This change can land here on the emscripten side before the
corresponding llvm changes since the strong definition I'm adding here
will always take precedence over any weak definitions in object files.

See llvm/llvm-project#159143

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?.value != 'undefined') {
         // a breaking change in the wasm spec, globals are now objects
         // https://github.com/WebAssembly/mutable-global/issues/1
         value = value.value;

system/lib/compiler-rt/__c_longjmp.S

@@ -0,0 +1,23 @@
+/*
+ * 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 `__c_longjmp` Wasm EH tag which is used to implment setjmp/longjmp
+ * in LLVM.
+ */
+
+#ifdef __wasm_exception_handling__
+
+#ifdef __wasm64__
+#define PTR i64
+#else
+#define PTR i32
+#endif
+
+.globl __c_longjmp
+.tagtype __c_longjmp PTR
+__c_longjmp:
+
+#endif // !__wasm_exception_handling__

system/lib/libcxxabi/src/__cpp_exception.S

@@ -0,0 +1,23 @@
+/*
+ * 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 `__cpp_exception` Wasm EH tag which is used to implmenent C++
+ * exception handling in LLVM.
+ */
+
+#ifdef __wasm_exception_handling__
+
+#ifdef __wasm64__
+#define PTR i64
+#else
+#define PTR i32
+#endif
+
+.globl __cpp_exception
+.tagtype __cpp_exception PTR
+__cpp_exception:
+
+#endif // !__wasm_exception_handling__

test/code_size/test_codesize_hello_dylink.json

@@ -1,10 +1,10 @@
 {
-  "a.out.js": 26969,
-  "a.out.js.gz": 11479,
+  "a.out.js": 26979,
+  "a.out.js.gz": 11483,
   "a.out.nodebug.wasm": 18567,
   "a.out.nodebug.wasm.gz": 9199,
-  "total": 45536,
-  "total_gz": 20678,
+  "total": 45546,
+  "total_gz": 20682,
   "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": 246031,
   "a.out.nodebug.wasm": 597767,
-  "total": 843788,
+  "total": 843798,
   "sent": [
     "IMG_Init",
     "IMG_Load",

test/test_core.py

@@ -4951,9 +4951,11 @@ 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

@@ -987,6 +987,7 @@ class libcompiler_rt(MTLibrary, SjLjLibrary):
         'emscripten_setjmp.c',
         'emscripten_exception_builtins.c',
         'emscripten_tempret.s',
+        '__c_longjmp.S',
         '__trap.c',
       ])
 
@@ -1661,6 +1662,7 @@ def get_files(self):
       'stdlib_typeinfo.cpp',
       'private_typeinfo.cpp',
       'cxa_exception_js_utils.cpp',
+      '__cpp_exception.S',
     ]
     if self.eh_mode == Exceptions.NONE:
       filenames += ['cxa_noexception.cpp']