Use a monotonic clock in the implementation of Timer.

runtime/bin/utils_macos.cc

Index: runtime/bin/utils_macos.cc
diff --git a/runtime/bin/utils_macos.cc b/runtime/bin/utils_macos.cc
index 1d792c9c7fae7390224418be2310402411915e24..80cd0386ed25114eeccbf48b5bdc4eb039a43291 100644
--- a/runtime/bin/utils_macos.cc
+++ b/runtime/bin/utils_macos.cc
@@ -7,6 +7,9 @@
 
 #include <errno.h>  // NOLINT
 #include <netdb.h>  // NOLINT
+#include <mach/mach.h>  // NOLINT
+#include <mach/clock.h>  // NOLINT
+#include <mach/mach_time.h>  // NOLINT
 #include <sys/time.h>  // NOLINT
 #include <time.h>  // NOLINT
 
@@ -71,17 +74,44 @@ bool ShellUtils::GetUtf8Argv(int argc, char** argv) {
   return false;
 }
 
-int64_t TimerUtils::GetCurrentTimeMilliseconds() {
-  return GetCurrentTimeMicros() / 1000;
+int64_t TimerUtils::GetCurrentMonotonicMillis() {
+  return GetCurrentMonotonicMicros() / 1000;
 }
 
-int64_t TimerUtils::GetCurrentTimeMicros() {
-  struct timeval tv;
-  if (gettimeofday(&tv, NULL) < 0) {
-    UNREACHABLE();
-    return 0;
+int64_t TimerUtils::GetCurrentMonotonicMicros() {
+#if TARGET_OS_IOS
+  // On iOS mach_absolute_time stops while the device is sleeping. Instead use
+  // now - KERN_BOOTTIME to get a time difference that is not impacted by clock
+  // changes. KERN_BOOTTIME will be updated by the system whenever the system
+  // clock change.
+  struct timeval boottime;
+  int mib[2] = {CTL_KERN, KERN_BOOTTIME};
+  size_t size = sizeof(boottime);
+  int kr = sysctl(mib, sizeof(mib) / sizeof(mib[0]), &boottime, &size, NULL, 0);
+  ASSERT(KERN_SUCCESS == kr);
+  int64_t now = GetCurrentTimeMicros();
+  int64_t origin = boottime.tv_sec * kMicrosecondsPerSecond;
+  origin += boottime.tv_usec;
+  return now - origin;
+#else
+  static mach_timebase_info_data_t timebase_info;
+  if (timebase_info.denom == 0) {
+    // Zero-initialization of statics guarantees that denom will be 0 before
+    // calling mach_timebase_info.  mach_timebase_info will never set denom to
+    // 0 as that would be invalid, so the zero-check can be used to determine
+    // whether mach_timebase_info has already been called.  This is
+    // recommended by Apple's QA1398.
+    kern_return_t kr = mach_timebase_info(&timebase_info);
+    ASSERT(KERN_SUCCESS == kr);
   }
-  return (static_cast<int64_t>(tv.tv_sec) * 1000000) + tv.tv_usec;
+
+  // timebase_info converts absolute time tick units into nanoseconds.  Convert
+  // to microseconds.
+  int64_t result = mach_absolute_time() / kNanosecondsPerMicrosecond;
+  result *= timebase_info.numer;
+  result /= timebase_info.denom;
+  return result;
+#endif  // TARGET_OS_IOS
 }
 
 void TimerUtils::Sleep(int64_t millis) {