Make exported __cpp_exception into a global. NFC (#24282)

This still is not quite a generic as I would like so as a followup we
can probably make this work for all tags, and perhaps unify the updating
of the globals when the exports are available.

Author: sbc100

src/lib/libcore.js

@@ -2202,7 +2202,7 @@ addToLibrary({
   __asyncify_state: "new WebAssembly.Global({'value': 'i32', 'mutable': true}, 0)",
   __asyncify_data: "new WebAssembly.Global({'value': '{{{ POINTER_WASM_TYPE }}}', 'mutable': true}, {{{ to64(0) }}})",
 #endif
-#endif
+#endif // RELOCATABLE
 
   _emscripten_fs_load_embedded_files__deps: ['$FS', '$PATH'],
   _emscripten_fs_load_embedded_files: (ptr) => {

src/lib/libexceptions.js

@@ -283,16 +283,11 @@ var LibraryExceptions = {
   },
 #endif
 #if WASM_EXCEPTIONS
-  $getCppExceptionTag: () =>
-    // In static linking, tags are defined within the wasm module and are
-    // exported, whereas in dynamic linking, tags are defined in library.js in
-    // JS code and wasm modules import them.
-#if RELOCATABLE
-    ___cpp_exception // defined in library.js
-#else
-    wasmExports['__cpp_exception']
-#endif
-  ,
+  $getCppExceptionTag__deps: ['__cpp_exception'],
+  // In static linking, tags are defined within the wasm module and are
+  // exported, whereas in dynamic linking, tags are defined in libcore.js in
+  // JS code and wasm modules import them.
+  $getCppExceptionTag: () => ___cpp_exception,
 
 #if EXCEPTION_STACK_TRACES
   // Throw a WebAssembly.Exception object with the C++ tag with a stack trace

src/preamble.js

@@ -924,6 +924,11 @@ function getWasmImports() {
 #endif
 #endif
 
+#if hasExportedSymbol('__cpp_exception') && !RELOCATABLE
+    ___cpp_exception = wasmExports['__cpp_exception'];
+    {{{ receivedSymbol('___cpp_exception') }}};
+#endif
+
 #if hasExportedSymbol('__wasm_apply_data_relocs')
     __RELOC_FUNCS__.push(wasmExports['__wasm_apply_data_relocs']);
 #endif

tools/emscripten.py

@@ -280,6 +280,10 @@ def trim_asm_const_body(body):
   return body
 
 
+def create_other_export_declarations(tag_exports):
+  return '\n'.join(f'var {asmjs_mangle(name)};' for name in tag_exports)
+
+
 def create_global_exports(global_exports):
   lines = []
   for k, v in global_exports.items():
@@ -417,11 +421,13 @@ def emscript(in_wasm, out_wasm, outfile_js, js_syms, finalize=True, base_metadat
 
   if base_metadata:
     function_exports = base_metadata.function_exports
+    tag_exports = base_metadata.tag_exports
     # We want the real values from the final metadata but we only want to
     # include names from the base_metadata.  See phase_link() in link.py.
     global_exports = {k: v for k, v in metadata.global_exports.items() if k in base_metadata.global_exports}
   else:
     function_exports = metadata.function_exports
+    tag_exports = metadata.tag_exports
     global_exports = metadata.global_exports
 
   if settings.ASYNCIFY == 1:
@@ -431,7 +437,7 @@ def emscript(in_wasm, out_wasm, outfile_js, js_syms, finalize=True, base_metadat
     function_exports['asyncify_stop_rewind'] = webassembly.FuncType([], [])
 
   parts = [pre]
-  parts += create_module(metadata, function_exports, global_exports, forwarded_json['librarySymbols'])
+  parts += create_module(metadata, function_exports, global_exports, tag_exports, forwarded_json['librarySymbols'])
   parts.append(post)
 
   full_js_module = ''.join(parts)
@@ -1000,7 +1006,7 @@ def create_receiving(function_exports):
   return '\n'.join(receiving) + '\n'
 
 
-def create_module(metadata, function_exports, global_exports, library_symbols):
+def create_module(metadata, function_exports, global_exports, tag_exports,library_symbols):
   module = []
 
   receiving = create_receiving(function_exports)
@@ -1010,6 +1016,7 @@ def create_module(metadata, function_exports, global_exports, library_symbols):
     module.append(sending)
   else:
     receiving += create_global_exports(global_exports)
+    receiving += create_other_export_declarations(tag_exports)
 
     if settings.PTHREADS or settings.WASM_WORKERS:
       sending = textwrap.indent(sending, '  ').strip()

tools/extract_metadata.py

@@ -256,6 +256,14 @@ def get_function_exports(module):
   return rtn
 
 
+def get_tag_exports(module):
+  rtn = []
+  for e in module.get_exports():
+    if e.kind == webassembly.ExternType.TAG:
+      rtn.append(e.name)
+  return rtn
+
+
 def update_metadata(filename, metadata):
   imports = []
   invoke_funcs = []
@@ -270,6 +278,7 @@ def update_metadata(filename, metadata):
         imports.append(i.field)
 
     metadata.function_exports = get_function_exports(module)
+    metadata.tag_exports = get_tag_exports(module)
     metadata.all_exports = [utils.removeprefix(e.name, '__em_js__') for e in module.get_exports()]
 
   metadata.imports = imports
@@ -340,6 +349,7 @@ def extract_metadata(filename):
     metadata = Metadata()
     metadata.imports = import_names
     metadata.function_exports = get_function_exports(module)
+    metadata.tag_exports = get_tag_exports(module)
     metadata.all_exports = [utils.removeprefix(e.name, '__em_js__') for e in exports]
     metadata.em_asm_consts = get_section_strings(module, export_map, 'em_asm')
     metadata.js_deps = [d for d in get_section_strings(module, export_map, 'em_lib_deps').values() if d]