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Micro-optimize timed wait loops. NFC #26849

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Merged
sbc100 merged 3 commits into from
May 5, 2026
Merged

Micro-optimize timed wait loops. NFC #26849

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a3f6f60
Micro-optimize timed wait loops. NFC
kleisauke May 4, 2026
ab6b2bf
Incorporate feedback
kleisauke May 4, 2026
79f46a5
Remove unnecessary `atomic_load()`
kleisauke May 4, 2026
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11 changes: 5 additions & 6 deletions system/lib/pthread/library_pthread.c
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Original file line number Diff line number Diff line change
Expand Up @@ -85,18 +85,17 @@ void emscripten_thread_sleep(double msecs) {
__pthread_testcancel();
if (msecs > 0) {
uint32_t dummyZeroAddress = 0;
double start = emscripten_get_now();
double elapsed = 0;
while (elapsed < msecs) {
double target = emscripten_get_now() + msecs;
do {
emscripten_conditional_set_current_thread_status(EM_THREAD_STATUS_RUNNING,
EM_THREAD_STATUS_SLEEPING);
emscripten_futex_wait(&dummyZeroAddress, 0, msecs - elapsed);
emscripten_futex_wait(&dummyZeroAddress, 0, msecs);
emscripten_conditional_set_current_thread_status(EM_THREAD_STATUS_SLEEPING,
EM_THREAD_STATUS_RUNNING);
emscripten_current_thread_process_queued_calls();
__pthread_testcancel();
elapsed = emscripten_get_now() - start;
}
msecs = target - emscripten_get_now();
} while (msecs > 0);
}
}

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31 changes: 15 additions & 16 deletions system/lib/pthread/proxying_legacy.c
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Original file line number Diff line number Diff line change
Expand Up @@ -530,24 +530,23 @@ em_queued_call* emscripten_async_waitable_run_in_main_runtime_thread_(
}

EMSCRIPTEN_RESULT emscripten_wait_for_call_v(em_queued_call* call, double timeoutMSecs) {
if (call->operationDone) {
return EMSCRIPTEN_RESULT_SUCCESS;
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}

emscripten_set_current_thread_status(EM_THREAD_STATUS_WAITPROXY);

int r;
double target = emscripten_get_now() + timeoutMSecs;
do {
r = -emscripten_futex_wait(&call->operationDone, 0, timeoutMSecs);

int done = atomic_load(&call->operationDone);

@sbc100 sbc100 May 4, 2026

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I don't think we want to remove this line.

IIUC that idea with the futex_wait API is that one should only call it once you know you need to block. i.e. its designed as the expensive syscall that you use once you know you need to block, and its common to place it after some amount of spinning (or at least an atomic check like this one).

@kleisauke kleisauke May 4, 2026

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Good point. Added this back as an early return with commit ab6b2bf.

if (!done) {
double now = emscripten_get_now();
double waitEndTime = now + timeoutMSecs;
emscripten_set_current_thread_status(EM_THREAD_STATUS_WAITPROXY);
while (!done && now < waitEndTime) {
r = emscripten_futex_wait(&call->operationDone, 0, waitEndTime - now);
done = atomic_load(&call->operationDone);
now = emscripten_get_now();
}
emscripten_set_current_thread_status(EM_THREAD_STATUS_RUNNING);
}
if (done)
return EMSCRIPTEN_RESULT_SUCCESS;
else
return EMSCRIPTEN_RESULT_TIMED_OUT;
timeoutMSecs = target - emscripten_get_now();
} while (r == EINTR && timeoutMSecs > 0);

emscripten_set_current_thread_status(EM_THREAD_STATUS_RUNNING);

return r == ETIMEDOUT ? EMSCRIPTEN_RESULT_TIMED_OUT : EMSCRIPTEN_RESULT_SUCCESS;
}

EMSCRIPTEN_RESULT emscripten_wait_for_call_i(
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