From a14048f0a23b0e50576ab3b5e87aebd616b35d43 Mon Sep 17 00:00:00 2001
From: wolfbeast <mcwerewolf@gmail.com>
Date: Mon, 19 Feb 2018 12:50:59 +0100
Subject: Use GetTickCount64 without a GetProcAddress check in
 Timestamp_windows.cpp.

Tag #22.
---
 mozglue/misc/TimeStamp_windows.cpp | 56 ++++----------------------------------
 1 file changed, 6 insertions(+), 50 deletions(-)

(limited to 'mozglue')

diff --git a/mozglue/misc/TimeStamp_windows.cpp b/mozglue/misc/TimeStamp_windows.cpp
index cd519affd..683c2209a 100644
--- a/mozglue/misc/TimeStamp_windows.cpp
+++ b/mozglue/misc/TimeStamp_windows.cpp
@@ -5,7 +5,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 // Implement TimeStamp::Now() with QueryPerformanceCounter() controlled with
-// values of GetTickCount().
+// values of GetTickCount64().
 
 #include "mozilla/MathAlgorithms.h"
 #include "mozilla/TimeStamp.h"
@@ -69,7 +69,7 @@ static const DWORD kDefaultTimeIncrement = 156001;
  * further just referred as [mt], meaning milli-ticks.
  *
  * This is needed to preserve maximum precision of the performance frequency
- * representation.  GetTickCount values in milliseconds are multiplied with
+ * representation.  GetTickCount64 values in milliseconds are multiplied with
  * frequency per second.  Therefor we need to multiply QPC value by 1000 to
  * have the same units to allow simple arithmentic with both QPC and GTC.
  */
@@ -156,41 +156,8 @@ static CRITICAL_SECTION sTimeStampLock;
 // Kept in [mt]
 static ULONGLONG sFaultIntoleranceCheckpoint = 0;
 
-// Used only when GetTickCount64 is not available on the platform.
-// Last result of GetTickCount call.
-//
-// Kept in [ms]
-static DWORD sLastGTCResult = 0;
-
-// Higher part of the 64-bit value of MozGetTickCount64,
-// incremented atomically.
-static DWORD sLastGTCRollover = 0;
-
 namespace mozilla {
 
-typedef ULONGLONG (WINAPI* GetTickCount64_t)();
-static GetTickCount64_t sGetTickCount64 = nullptr;
-
-// Function protecting GetTickCount result from rolling over,
-// result is in [ms]
-static ULONGLONG WINAPI
-MozGetTickCount64()
-{
-  DWORD GTC = ::GetTickCount();
-
-  // Cheaper then CMPXCHG8B
-  AutoCriticalSection lock(&sTimeStampLock);
-
-  // Pull the rollover counter forward only if new value of GTC goes way
-  // down under the last saved result
-  if ((sLastGTCResult > GTC) && ((sLastGTCResult - GTC) > (1UL << 30))) {
-    ++sLastGTCRollover;
-  }
-
-  sLastGTCResult = GTC;
-  return ULONGLONG(sLastGTCRollover) << 32 | sLastGTCResult;
-}
-
 // Result is in [mt]
 static inline ULONGLONG
 PerformanceCounter()
@@ -355,7 +322,7 @@ TimeStampValue::CheckQPC(const TimeStampValue& aOther) const
   if (duration < sHardFailureLimit) {
     // Interval between the two time stamps is very short, consider
     // QPC as unstable and record a failure.
-    uint64_t now = ms2mt(sGetTickCount64());
+    uint64_t now = ms2mt(GetTickCount64());
 
     AutoCriticalSection lock(&sTimeStampLock);
 
@@ -485,17 +452,6 @@ TimeStamp::Startup()
 
   gInitialized = true;
 
-  // Decide which implementation to use for the high-performance timer.
-
-  HMODULE kernelDLL = GetModuleHandleW(L"kernel32.dll");
-  sGetTickCount64 = reinterpret_cast<GetTickCount64_t>(
-    GetProcAddress(kernelDLL, "GetTickCount64"));
-  if (!sGetTickCount64) {
-    // If the platform does not support the GetTickCount64 (Windows XP doesn't),
-    // then use our fallback implementation based on GetTickCount.
-    sGetTickCount64 = MozGetTickCount64;
-  }
-
   InitializeCriticalSectionAndSpinCount(&sTimeStampLock, kLockSpinCount);
 
   sHasStableTSC = HasStableTSC();
@@ -504,10 +460,10 @@ TimeStamp::Startup()
   LARGE_INTEGER freq;
   sUseQPC = ::QueryPerformanceFrequency(&freq);
   if (!sUseQPC) {
-    // No Performance Counter.  Fall back to use GetTickCount.
+    // No Performance Counter.  Fall back to use GetTickCount64.
     InitResolution();
 
-    LOG(("TimeStamp: using GetTickCount"));
+    LOG(("TimeStamp: using GetTickCount64"));
     return;
   }
 
@@ -534,7 +490,7 @@ TimeStamp::Now(bool aHighResolution)
 
   // Both values are in [mt] units.
   ULONGLONG QPC = useQPC ? PerformanceCounter() : uint64_t(0);
-  ULONGLONG GTC = ms2mt(sGetTickCount64());
+  ULONGLONG GTC = ms2mt(GetTickCount64());
   return TimeStamp(TimeStampValue(GTC, QPC, useQPC));
 }
 
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