Improve & simplify time sync in pthreads (#18267)

Author: RReverser

ChangeLog.md

@@ -26,6 +26,12 @@ See docs/process.md for more on how version tagging works.
   applications that wish to know where emscripten is installed looking for
   `emcc` in the `PATH` has long been the recommended method (i.e. `which emcc`).
   (#18279)
+- More accurate synchronisation of `emscripten_get_now` clocks between main
+  thread and pthreads.
+  This also changes the absolute value returned by the function, but it shouldn't
+  affect correct usages as the function has always returned different values on
+  different platforms, and is clearly documented as "only meaningful in
+  comparison to other calls to this function". (#18267)
 
 3.1.27 - 11/29/22
 -----------------

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);