Refactor monotonic clock interface and use raw tick values in stopwatch

runtime/vm/os_win.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"
 #if defined(TARGET_OS_WINDOWS)
 
 #include "vm/os.h"
 
 #include <malloc.h>  // NOLINT
@@ skipping 105 matching lines @@
     int64_t t_;
   };
   TimeStamp time;
   GetSystemTimeAsFileTime(&time.ft_);
   return (time.t_ - kTimeEpoc) / kTimeScaler;
 }
 
 
 static int64_t qpc_ticks_per_second = 0;
 
-int64_t OS::GetCurrentMonotonicMicros() {
+
+int64_t OS::GetCurrentMonotonicTicks() {
   if (qpc_ticks_per_second == 0) {
     // QueryPerformanceCounter not supported, fallback.
     return GetCurrentTimeMicros();
   }
   // Grab performance counter value.
   LARGE_INTEGER now;
   QueryPerformanceCounter(&now);
   int64_t qpc_value = static_cast<int64_t>(now.QuadPart);
+}
+
+
+int64_t OS::GetCurrentMonotonicFrequency() {
+  if (qpc_ticks_per_second == 0) {
+    // QueryPerformanceCounter not supported, fallback.
+    return kMicrosecondsPerSecond;
+  }
+  return qpc_ticks_per_second;
+}
+
+
+int64_t OS::GetCurrentMonotonicMicros() {
+  int64_t ticks = GetCurrentMonotonicTicks();
+  int64_t frequency = GetCurrentMonotonicFrequency();
+
   // Convert to microseconds.
-  int64_t seconds = qpc_value / qpc_ticks_per_second;
-  int64_t leftover_ticks = qpc_value - (seconds * qpc_ticks_per_second);
+  int64_t seconds = ticks / frequency;
+  int64_t leftover_ticks = ticks - (seconds * frequency);
   int64_t result = seconds * kMicrosecondsPerSecond;
-  result += ((leftover_ticks * kMicrosecondsPerSecond) / qpc_ticks_per_second);
+  result += ((leftover_ticks * kMicrosecondsPerSecond) / frequency);
   return result;
 }
 
 
 void* OS::AlignedAllocate(intptr_t size, intptr_t alignment) {
   const int kMinimumAlignment = 16;
   ASSERT(Utils::IsPowerOfTwo(alignment));
   ASSERT(alignment >= kMinimumAlignment);
   void* p = _aligned_malloc(size, alignment);
   if (p == NULL) {
@@ skipping 241 matching lines @@
 }
 
 
 void OS::Exit(int code) {
   exit(code);
 }
 
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_WINDOWS)