Improve & simplify time sync in pthreads

This supersedes solution from #6084 / #10137 with `performance.timeOrigin`.

This property provides the high resolution baseline for `performance.now()` results, which is exactly what we need. Unlike custom `__performance_now_clock_drift` sent as a message to a worker, it won't suffer from the small time difference incurred by `postMessage` itself, making synchronisation more precise, while at the same time allows to remove some custom code, making it a bit simpler too.

I checked caniuse and it's supported in all browsers where SAB are supported, so should be always safe to use when pthreads are enabled.

Note that this changes the absolute value returned from emscripten_get_now, but this is fine because that value is already different across engines (e.g. it might already return Date.now() in some browser), and it's explicitly documented as "is only meaningful in comparison to other calls to this function".

I verified that test_pthread_reltime still passes too.

Author: RReverser

src/library.js

@@ -2328,12 +2328,10 @@ mergeInto(LibraryManager.library, {
                                "} else " +
 #endif
 #if USE_PTHREADS
-// Pthreads need their clocks synchronized to the execution of the main thread, so give them a special form of the function.
-// N.b. Wasm workers do not provide this kind of clock synchronization.
-                               "if (ENVIRONMENT_IS_PTHREAD) {\n" +
-                               "  _emscripten_get_now = () => performance.now() - Module['__performance_now_clock_drift'];\n" +
-                               "} else " +
-#endif
+// Pthreads need their clocks synchronized to the execution of the main thread, so, when using them,
+// make sure to adjust all timings to the respective time origins.
+                               "_emscripten_get_now = () => performance.timeOrigin + performance.now();\n",
+#else
 #if ENVIRONMENT_MAY_BE_SHELL
                                "if (typeof dateNow != 'undefined') {\n" +
                                "  _emscripten_get_now = dateNow;\n" +
@@ -2349,6 +2347,7 @@ mergeInto(LibraryManager.library, {
                                // Modern environment where performance.now() is supported:
                                // N.B. a shorter form "_emscripten_get_now = return performance.now;" is unfortunately not allowed even in current browsers (e.g. FF Nightly 75).
                                "_emscripten_get_now = () => performance.now();\n",
+#endif
 #endif
 
   emscripten_get_now_res: function() { // return resolution of get_now, in nanoseconds

src/library_pthread.js

@@ -607,7 +607,6 @@ var LibraryPThread = {
         worker.ref();
       }
 #endif
-      msg.time = performance.now();
       worker.postMessage(msg, threadParams.transferList);
       delete worker.runPthread;
     };

src/worker.js

@@ -205,18 +205,6 @@ self.onmessage = (e) => {
 #endif
 #endif // MODULARIZE && EXPORT_ES6
     } else if (e.data.cmd === 'run') {
-      // This worker was idle, and now should start executing its pthread entry
-      // point.
-      // performance.now() is specced to return a wallclock time in msecs since
-      // that Web Worker/main thread launched. However for pthreads this can
-      // cause subtle problems in emscripten_get_now() as this essentially
-      // would measure time from pthread_create(), meaning that the clocks
-      // between each threads would be wildly out of sync. Therefore sync all
-      // pthreads to the clock on the main browser thread, so that different
-      // threads see a somewhat coherent clock across each of them
-      // (+/- 0.1msecs in testing).
-      Module['__performance_now_clock_drift'] = performance.now() - e.data.time;
-
       // Pass the thread address to wasm to store it for fast access.
       Module['__emscripten_thread_init'](e.data.pthread_ptr, /*isMainBrowserThread=*/0, /*isMainRuntimeThread=*/0, /*canBlock=*/1);