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Enable string builtins with SHARED_WASMGC #27473

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3 changes: 3 additions & 0 deletions site/source/docs/tools_reference/settings_reference.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -2536,6 +2536,9 @@ are not present. Since LLVM cannot emit Wasm GC instructions or shared anyref
globals, users are expected to use wasm-merge to add these globals and
additional Wasm GC code post-link.

Additionally, WebAssembly JS string builtins and imported string constants
(with module "'") are enabled when compiling the WebAssembly module.

.. note:: This is an experimental setting

Default value: false
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22 changes: 17 additions & 5 deletions src/lib/libdylink.js
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Expand Up @@ -935,22 +935,34 @@ var LibraryDylink = {
return moduleExports;
}

#if SHARED_WASMGC
var compileOptions = {
builtins: ['js-string'],
importedStringConstants: "'",
};
#endif

if (flags.loadAsync) {
return (async () => {
var instance;
if (binary instanceof WebAssembly.Module) {
instance = new WebAssembly.Instance(binary, info);
} else {
// Destructuring assignment without declaration has to be wrapped
// with parens or parser will treat the l-value as an object
// literal instead.
({ module: binary, instance } = await WebAssembly.instantiate(binary, info));
({ module: binary, instance } = await WebAssembly.instantiate(binary, info
#if SHARED_WASMGC
, compileOptions
#endif
));
}
return postInstantiation(binary, instance);
})();
}

var module = binary instanceof WebAssembly.Module ? binary : new WebAssembly.Module(binary);
var module = binary instanceof WebAssembly.Module ? binary : new WebAssembly.Module(binary
#if SHARED_WASMGC
, compileOptions
#endif
);
var instance = new WebAssembly.Instance(module, info);
return postInstantiation(module, instance);
}
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30 changes: 26 additions & 4 deletions src/postamble_minimal.js
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Expand Up @@ -202,6 +202,12 @@ const moduleUrl = `ENVIRONMENT_IS_AUDIO_WORKLET ? '${TARGET_BASENAME}.wasm' : ne
// precompiled WebAssembly Module.
assert(WebAssembly.instantiateStreaming || Module['wasm'], 'Must load WebAssembly Module in to variable Module.wasm before adding compiled output .js script to the DOM');
#endif
#if SHARED_WASMGC
var compileOptions = {
builtins: ['js-string'],
importedStringConstants: "'",
};
#endif
#if MODULARIZE || AUDIO_WORKLET
instantiatePromise =
#endif
Expand All @@ -210,13 +216,25 @@ instantiatePromise =
// Node's fetch API cannot be used for local files, so we cannot use instantiateStreaming
&& !ENVIRONMENT_IS_NODE
#endif
? WebAssembly.instantiateStreaming(fetch({{{ moduleUrl }}}), imports)
: WebAssembly.instantiate(Module['wasm'], imports)).then((output) => {
? WebAssembly.instantiateStreaming(fetch({{{ moduleUrl }}}), imports
#if SHARED_WASMGC
, compileOptions
#endif
)
: WebAssembly.instantiate(Module['wasm'], imports
#if SHARED_WASMGC
, compileOptions
#endif
)).then((output) => {
#else
#if MODULARIZE || AUDIO_WORKLET
instantiatePromise =
#endif
WebAssembly.instantiateStreaming(fetch({{{ moduleUrl }}}), imports).then((output) => {
WebAssembly.instantiateStreaming(fetch({{{ moduleUrl }}}), imports
#if SHARED_WASMGC
, compileOptions
#endif
).then((output) => {
#endif

#else // Non-streaming instantiation
Expand All @@ -235,7 +253,11 @@ assert(Module['wasm'], 'Must load WebAssembly Module in to variable Module.wasm
#if MODULARIZE || AUDIO_WORKLET
instantiatePromise =
#endif
WebAssembly.instantiate(Module['wasm'], imports).then(/** @suppress {missingProperties} */ (output) => {
WebAssembly.instantiate(Module['wasm'], imports
#if SHARED_WASMGC
, compileOptions
#endif
).then(/** @suppress {missingProperties} */ (output) => {
#endif

#if !LibraryManager.has('libexports.js') && ASYNCIFY != 1
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43 changes: 37 additions & 6 deletions src/preamble.js
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Expand Up @@ -543,6 +543,13 @@ var splitModuleProxyHandler = {
};
#endif

#if SHARED_WASMGC
var compileOptions = {
builtins: ['js-string'],
importedStringConstants: "'",
};
#endif

#if SPLIT_MODULE || !WASM_ASYNC_COMPILATION
function instantiateSync(file, info) {
var module;
Expand All @@ -569,15 +576,23 @@ function instantiateSync(file, info) {
}
}
}
module ||= new WebAssembly.Module(binary);
module ||= new WebAssembly.Module(binary
#if SHARED_WASMGC
, compileOptions
#endif
);
if (ENVIRONMENT_IS_NODE && !hasCached) {
#if RUNTIME_DEBUG
dbg('NODE_CODE_CACHING: saving module');
#endif
fs.writeFileSync(cachedCodeFile, v8.serialize(module));
}
#else // NODE_CODE_CACHING
module = new WebAssembly.Module(binary);
module = new WebAssembly.Module(binary
#if SHARED_WASMGC
, compileOptions
#endif
);
#endif // NODE_CODE_CACHING
var instance = new WebAssembly.Instance(module, info);
return [instance, module];
Expand All @@ -588,7 +603,11 @@ function instantiateSync(file, info) {
async function instantiateArrayBuffer(binaryFile, imports) {
try {
var binary = await getWasmBinary(binaryFile);
var instance = await WebAssembly.instantiate(binary, imports);
var instance = await WebAssembly.instantiate(binary, imports
#if SHARED_WASMGC
, compileOptions
#endif
);
return instance;
} catch (reason) {
err(`failed to asynchronously prepare wasm: ${reason}`);
Expand Down Expand Up @@ -637,7 +656,11 @@ async function instantiateAsync(binary, binaryFile, imports) {
#if expectToReceiveOnModule('onCOSCacheHit')
Module['onCOSCacheHit']?.(cosHash.value);
#endif
return WebAssembly.instantiate(cosBytes, imports);
return WebAssembly.instantiate(cosBytes, imports
#if SHARED_WASMGC
, compileOptions
#endif
);
} catch {
// Any error (not found, not allowed, …) — fetch from the network and
// attempt to store in COS for future page loads.
Expand Down Expand Up @@ -670,7 +693,11 @@ async function instantiateAsync(binary, binaryFile, imports) {
err(`COS store failed: ${storeErr}`);
}
})();
return WebAssembly.instantiate(wasmBytes, imports);
return WebAssembly.instantiate(wasmBytes, imports
#if SHARED_WASMGC
, compileOptions
#endif
);
} catch (fetchErr) {
// Network fetch failed; fall through to the standard path below.
err(`COS fallback fetch failed: ${fetchErr}`);
Expand Down Expand Up @@ -704,7 +731,11 @@ async function instantiateAsync(binary, binaryFile, imports) {
) {
try {
var response = fetch(binaryFile, {{{ makeModuleReceiveExpr('fetchSettings', "{ credentials: 'same-origin' }") }}});
var instantiationResult = await WebAssembly.instantiateStreaming(response, imports);
var instantiationResult = await WebAssembly.instantiateStreaming(response, imports
#if SHARED_WASMGC
, compileOptions
#endif
);
return instantiationResult;
} catch (reason) {
// We expect the most common failure cause to be a bad MIME type for the binary,
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3 changes: 3 additions & 0 deletions src/settings.js
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Expand Up @@ -1682,6 +1682,9 @@ var SHARED_MEMORY = false;
// are not present. Since LLVM cannot emit Wasm GC instructions or shared anyref
// globals, users are expected to use wasm-merge to add these globals and
// additional Wasm GC code post-link.
//
// Additionally, WebAssembly JS string builtins and imported string constants
// (with module "'") are enabled when compiling the WebAssembly module.
// [link]
// [experimental]
var SHARED_WASMGC = false;
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54 changes: 54 additions & 0 deletions test/test_other.py
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Expand Up @@ -13452,6 +13452,60 @@ def test_shared_wasmgc_thread_state(self):
output = self.run_js(out_js)
self.assertEqual(sorted(output.splitlines()), ['100', '300', '42'])

@requires_pthreads
@requires_node_25
def test_shared_wasmgc_strings(self):
create_file('test_shared_wasmgc_strings.c', r'''
#include <emscripten.h>
#include <emscripten/console.h>

__attribute__((import_module("wat"))) void run_test(void);

void print_int(int val) {
emscripten_console_logf("%d", val);
}

int main() {
run_test();
return 0;
}
''')

create_file('strings.wat', r'''
(module
(import "app" "print_int" (func $print_int (param i32)))
(import "wasm:js-string" "length" (func $length (param (ref null (shared extern))) (result i32)))
(import "wasm:js-string" "fromCharCode" (func $fromCharCode (param i32) (result (ref (shared extern)))))
(import "'" "hello world" (global $msg (ref (shared extern))))

(func (export "run_test")
(call $print_int (call $length (global.get $msg)))
(call $print_int (call $length (call $fromCharCode (i32.const 65))))
)
)

@sbc100 sbc100 Aug 1, 2026

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I wonder if this could be written in assembly? (Thus avoiding the need to wasm-merge?) If not, I wonder how hard it would be to enable llvm assembly to support this?

@tlively tlively Aug 2, 2026

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Almost, or even C given that clang nominally supports externrefs. The thing that prevents it from working in this case is that we're using shared externrefs in the import signatures to test the use of strings allocated on the shared heap. Clang definitely doesn't support that.

''')

out_js = self.in_dir('test_shared_wasmgc_strings.js')
out_wasm = self.in_dir('test_shared_wasmgc_strings.wasm')

self.run_process([
EMCC, '-pthread', '-sSHARED_WASMGC', '-sERROR_ON_UNDEFINED_SYMBOLS=0',
'-sEXIT_RUNTIME', '-sPROXY_TO_PTHREAD',
'-sEXPORTED_FUNCTIONS=_main,_print_int',
'test_shared_wasmgc_strings.c', '-o', out_js,
])

self.run_process([
WASM_MERGE, '--enable-threads', '--enable-reference-types',
'--enable-gc', '--enable-shared-everything', '--enable-strings',
out_wasm, 'app', 'strings.wat', 'wat', '-o', out_wasm,
])

self.node_args.append('--experimental-wasm-shared')

output = self.run_js(out_js)
self.assertEqual(output.splitlines(), ['11', '1'])

@crossplatform
def test_config_closure_compiler(self):
self.run_process([EMCC, test_file('hello_world.c'), '--closure=1'])
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