Allow emscripten_atomic_wait_suspending to return synchronously when possible

src/lib/libatomic.js

@@ -75,11 +75,21 @@ addToLibrary({
   },
 
 #if ASYNCIFY
-  emscripten_atomic_wait_suspending__async: 'auto',
-  emscripten_atomic_wait_suspending__deps: ['$polyfillWaitAsync', '$atomicWaitStates'],
-  emscripten_atomic_wait_suspending: async (addr, val, maxWaitMilliseconds) => {
+  _emscripten_atomic_wait_promise__deps: ['$polyfillWaitAsync', '$atomicWaitStates', '$addPromise'],
+  _emscripten_atomic_wait_promise: (addr, val, maxWaitMilliseconds) => {
     var wait = Atomics.waitAsync(HEAP32, {{{ getHeapOffset('addr', 'i32') }}}, val, maxWaitMilliseconds);
-    return atomicWaitStates.indexOf(await wait.value)
+    if (wait.async) {
+      // In the async case return the promise ID.
+      var chainedPromise = wait.value.then((value) => atomicWaitStates.indexOf(value));
+      var id = addPromise(chainedPromise);
+      return id;
+    }
+    // In the synchronous case return the negative result code
+    return -atomicWaitStates.indexOf(wait.value);
+  },
+#else
+  _emscripten_atomic_wait_promise: (addr, val, maxWaitMilliseconds) => {
+    abort('Please compile your program with async support in order to use asynchronous operations like emscripten_atomic_wait_suspending');
   },
 #endif
 

src/lib/libsigs.js

@@ -314,6 +314,7 @@ sigs = {
   _embind_register_value_object__sig: 'vpppppp',
   _embind_register_value_object_field__sig: 'vpppppppppp',
   _embind_register_void__sig: 'vpp',
+  _emscripten_atomic_wait_promise__sig: 'ppid',
   _emscripten_create_audio_worklet__sig: 'viipippp',
   _emscripten_create_wasm_worker__sig: 'iipip',
   _emscripten_dlopen_js__sig: 'vpppp',
@@ -592,7 +593,6 @@ sigs = {
   emscripten_atomic_cancel_all_wait_asyncs_at_address__sig: 'ip',
   emscripten_atomic_cancel_wait_async__sig: 'ii',
   emscripten_atomic_wait_async__sig: 'ipippd',
-  emscripten_atomic_wait_suspending__sig: 'ipid',
   emscripten_atomics_is_lock_free__sig: 'ii',
   emscripten_audio_context_quantum_size__sig: 'ii',
   emscripten_audio_context_sample_rate__sig: 'ii',

system/lib/pthread/emscripten_atomic_wait_suspending.c

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2026 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.
+ */
+
+#include <emscripten/promise.h>
+#include <emscripten/atomic.h>
+#include "threading_internal.h"
+
+ATOMICS_WAIT_TOKEN_T emscripten_atomic_wait_suspending(volatile void * _Nonnull addr,
+                                                       uint32_t value,
+                                                       double maxWaitMilliseconds) {
+  intptr_t res = _emscripten_atomic_wait_promise(addr, value, maxWaitMilliseconds);
+  // A negative value is a synchronous result code.
+  if (res < 0) {
+     return (ATOMICS_WAIT_TOKEN_T)-res;
+  }
+  // Otherwise a positive value is a promise ID, and we can then `await` using
+  // ASYNCIFY/JSPI.
+  em_promise_t promise = (em_promise_t)res;
+  void* await_result = emscripten_promise_await_unchecked(promise);
+  return (ATOMICS_WAIT_TOKEN_T)(intptr_t)await_result;
+}

system/lib/pthread/threading_internal.h

@@ -7,6 +7,7 @@
 
 #pragma once
 
+#include <inttypes.h>
 #include <pthread.h>
 #include <stdbool.h>
 
@@ -131,3 +132,9 @@ void* _emscripten_init_pthread(void *base, size_t* size, pid_t tid);
 // __builtin_wasm_memory_atomic_waitXX then they will not be woken by
 // this method.
 void _emscripten_thread_notify(pthread_t thread);
+
+// Internal, promise-returning API used to implement
+// emscripten_atomic_wait_suspending.
+intptr_t _emscripten_atomic_wait_promise(volatile void *addr,
+                                         uint32_t value,
+                                         double maxWaitMilliseconds);

tools/maint/gen_sig_info.py

@@ -384,13 +384,15 @@ def main(args):
                               'MAX_WEBGL_VERSION': 0,
                               'BUILD_AS_WORKER': 1,
                               'LINK_AS_CXX': 1,
+                              'SHARED_MEMORY': 0,
                               'AUTO_JS_LIBRARIES': 0}, cxx=True)
   extract_sig_info(sig_info, {'AUDIO_WORKLET': 1, 'WASM_WORKERS': 1, 'JS_LIBRARIES': ['libwasm_worker.js', 'libwebaudio.js']})
   extract_sig_info(sig_info, {'USE_GLFW': 3}, ['-DGLFW3'])
   extract_sig_info(sig_info, {'JS_LIBRARIES': ['libembind.js', 'libemval.js'],
                               'USE_SDL': 0,
                               'MAX_WEBGL_VERSION': 0,
                               'AUTO_JS_LIBRARIES': 0,
+                              'SHARED_MEMORY': 0,
                               'ASYNCIFY': 1}, cxx=True, extra_cflags=['-std=c++20'])
   extract_sig_info(sig_info, {'LEGACY_GL_EMULATION': 1}, ['-DGLES'])
   extract_sig_info(sig_info, {'USE_GLFW': 2, 'FULL_ES3': 1, 'MAX_WEBGL_VERSION': 2})

tools/system_libs.py

@@ -1283,6 +1283,7 @@ def get_files(self):
           'emscripten_thread_primitives.c',
           'emscripten_futex_wait.c',
           'emscripten_futex_wake.c',
+          'emscripten_atomic_wait_suspending.c',
         ])
 
     # These files are in libc directories, but only built in libc_optz.