Workaround for Rosetta 2 on Darwin

rdar://FB9546856

https://openradar.appspot.com/radar?id=5055988478509056

darwin/PlatformHelpers.c

@@ -58,14 +58,68 @@ bool Platform_KernelVersionIsBetween(KernelVersion lowerBound, KernelVersion upp
       && Platform_CompareKernelVersion(upperBound) < 0;
 }
 
+void Platform_getCPUBrandString(char *cpuBrandString, size_t cpuBrandStringSize) {
+   if (sysctlbyname("machdep.cpu.brand_string", cpuBrandString, &cpuBrandStringSize, NULL, 0) == -1) {
+      fprintf(stderr,
+         "WARN: Unable to determine the CPU brand string.\n"
+         "errno: %i, %s\n", errno, strerror(errno));
+
+      String_safeStrncpy(cpuBrandString, "UNKNOWN!", cpuBrandStringSize);
+   }
+}
+
+// Adapted from https://developer.apple.com/documentation/apple-silicon/about-the-rosetta-translation-environment
+bool Platform_isRunningTranslated() {
+   int ret = 0;
+   size_t size = sizeof(ret);
+   errno = 0;
+   if (sysctlbyname("sysctl.proc_translated", &ret, &size, NULL, 0) == -1) {
+      if (errno == ENOENT) return false;
+
+      fprintf(stderr,
+         "WARN: Could not determine if this process was running in a translation environment like Rosetta 2.\n"
+         "Assuming that we're not.\n"
+         "errno: %i, %s\n", errno, strerror(errno));
+
+      return false;
+   }
+   return ret;
+}
+
 double Platform_calculateNanosecondsPerMachTick() {
    // Check if we can determine the timebase used on this system.
    // If the API is unavailable assume we get our timebase in nanoseconds.
-#ifdef HAVE_MACH_TIMEBASE_INFO
+#ifndef HAVE_MACH_TIMEBASE_INFO
-   mach_timebase_info_data_t info;
-   mach_timebase_info(&info);
-   return (double)info.numer / (double)info.denom;
-#else
    return 1.0;
+#else
+   mach_timebase_info_data_t info;
+
+   /* WORKAROUND for `mach_timebase_info` giving incorrect values on M1 under Rosetta 2.
+    *    rdar://FB9546856 https://openradar.appspot.com/radar?id=5055988478509056
+    *
+    *    We don't know exactly what feature/attribute of the M1 chip causes this mistake under Rosetta 2.
+    *    Until we have more Apple ARM chips to compare against, the best we can do is special-case
+    *    the "Apple M1" chip specifically when running under Rosetta 2.
+    */
+
+   size_t cpuBrandStringSize = 1024;
+   char cpuBrandString[cpuBrandStringSize];
+   Platform_getCPUBrandString(cpuBrandString, cpuBrandStringSize);
+
+   bool isRunningUnderRosetta2 = Platform_isRunningTranslated();
+
+   // Kernel version 20.0.0 is macOS 11.0 (Big Sur)
+   bool isBuggedVersion = Platform_KernelVersionIsBetween((KernelVersion) {20, 0, 0}, (KernelVersion) {999, 999, 999});
+
+   if (isRunningUnderRosetta2 && String_eq(cpuBrandString, "Apple M1") && isBuggedVersion) {
+      // In this case `mach_timebase_info` provides the wrong value, so we hard-code the correct factor,
+      // as determined from `mach_timebase_info` when the process running natively.
+      info = (mach_timebase_info_data_t) { .numer = 125, .denom = 3 };
+   } else {
+      // No workarounds needed, use the OS-provided value.
+      mach_timebase_info(&info);
+   }
+
+   return (double)info.numer / (double)info.denom;
 #endif
 }

darwin/PlatformHelpers.h

@@ -31,4 +31,10 @@ bool Platform_KernelVersionIsBetween(KernelVersion lowerBound, KernelVersion upp
 
 double Platform_calculateNanosecondsPerMachTick(void);
 
+void Platform_getCPUBrandString(char *cpuBrandString, size_t cpuBrandStringSize);
+
+bool Platform_isRunningTranslated(void);
+
+double Platform_calculateNanosecondsPerMachTick(void);
+
 #endif