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375 lines
12 KiB
C++
375 lines
12 KiB
C++
// Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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// Windows Timer Primer
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//
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// A good article: http://www.ddj.com/windows/184416651
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// A good mozilla bug: http://bugzilla.mozilla.org/show_bug.cgi?id=363258
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//
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// The default windows timer, GetSystemTimeAsFileTime is not very precise.
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// It is only good to ~15.5ms.
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//
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// QueryPerformanceCounter is the logical choice for a high-precision timer.
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// However, it is known to be buggy on some hardware. Specifically, it can
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// sometimes "jump". On laptops, QPC can also be very expensive to call.
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// It's 3-4x slower than timeGetTime() on desktops, but can be 10x slower
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// on laptops. A unittest exists which will show the relative cost of various
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// timers on any system.
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//
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// The next logical choice is timeGetTime(). timeGetTime has a precision of
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// 1ms, but only if you call APIs (timeBeginPeriod()) which affect all other
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// applications on the system. By default, precision is only 15.5ms.
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// Unfortunately, we don't want to call timeBeginPeriod because we don't
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// want to affect other applications. Further, on mobile platforms, use of
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// faster multimedia timers can hurt battery life. See the intel
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// article about this here:
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// http://softwarecommunity.intel.com/articles/eng/1086.htm
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//
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// To work around all this, we're going to generally use timeGetTime(). We
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// will only increase the system-wide timer if we're not running on battery
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// power. Using timeBeginPeriod(1) is a requirement in order to make our
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// message loop waits have the same resolution that our time measurements
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// do. Otherwise, WaitForSingleObject(..., 1) will no less than 15ms when
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// there is nothing else to waken the Wait.
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#include "base/time.h"
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#pragma comment(lib, "winmm.lib")
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#include <windows.h>
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#include <mmsystem.h>
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#include "base/basictypes.h"
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#include "base/lock.h"
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#include "base/logging.h"
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#include "base/cpu.h"
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#include "base/singleton.h"
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#include "mozilla/Casting.h"
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using base::Time;
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using base::TimeDelta;
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using base::TimeTicks;
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using mozilla::BitwiseCast;
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namespace {
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// From MSDN, FILETIME "Contains a 64-bit value representing the number of
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// 100-nanosecond intervals since January 1, 1601 (UTC)."
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int64_t FileTimeToMicroseconds(const FILETIME& ft) {
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// Need to BitwiseCast to fix alignment, then divide by 10 to convert
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// 100-nanoseconds to milliseconds. This only works on little-endian
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// machines.
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return BitwiseCast<int64_t>(ft) / 10;
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}
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void MicrosecondsToFileTime(int64_t us, FILETIME* ft) {
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DCHECK(us >= 0) << "Time is less than 0, negative values are not "
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"representable in FILETIME";
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// Multiply by 10 to convert milliseconds to 100-nanoseconds. BitwiseCast will
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// handle alignment problems. This only works on little-endian machines.
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*ft = BitwiseCast<FILETIME>(us * 10);
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}
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int64_t CurrentWallclockMicroseconds() {
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FILETIME ft;
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::GetSystemTimeAsFileTime(&ft);
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return FileTimeToMicroseconds(ft);
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}
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// Time between resampling the un-granular clock for this API. 60 seconds.
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const int kMaxMillisecondsToAvoidDrift = 60 * Time::kMillisecondsPerSecond;
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int64_t initial_time = 0;
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TimeTicks initial_ticks;
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void InitializeClock() {
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initial_ticks = TimeTicks::Now();
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initial_time = CurrentWallclockMicroseconds();
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}
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} // namespace
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// Time -----------------------------------------------------------------------
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// The internal representation of Time uses FILETIME, whose epoch is 1601-01-01
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// 00:00:00 UTC. ((1970-1601)*365+89)*24*60*60*1000*1000, where 89 is the
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// number of leap year days between 1601 and 1970: (1970-1601)/4 excluding
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// 1700, 1800, and 1900.
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// static
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const int64_t Time::kTimeTToMicrosecondsOffset = GG_INT64_C(11644473600000000);
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// static
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Time Time::Now() {
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if (initial_time == 0)
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InitializeClock();
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// We implement time using the high-resolution timers so that we can get
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// timeouts which are smaller than 10-15ms. If we just used
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// CurrentWallclockMicroseconds(), we'd have the less-granular timer.
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//
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// To make this work, we initialize the clock (initial_time) and the
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// counter (initial_ctr). To compute the initial time, we can check
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// the number of ticks that have elapsed, and compute the delta.
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//
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// To avoid any drift, we periodically resync the counters to the system
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// clock.
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while(true) {
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TimeTicks ticks = TimeTicks::Now();
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// Calculate the time elapsed since we started our timer
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TimeDelta elapsed = ticks - initial_ticks;
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// Check if enough time has elapsed that we need to resync the clock.
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if (elapsed.InMilliseconds() > kMaxMillisecondsToAvoidDrift) {
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InitializeClock();
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continue;
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}
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return Time(elapsed + Time(initial_time));
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}
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}
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// static
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Time Time::NowFromSystemTime() {
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// Force resync.
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InitializeClock();
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return Time(initial_time);
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}
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// static
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Time Time::FromFileTime(FILETIME ft) {
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return Time(FileTimeToMicroseconds(ft));
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}
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FILETIME Time::ToFileTime() const {
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FILETIME utc_ft;
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MicrosecondsToFileTime(us_, &utc_ft);
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return utc_ft;
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}
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// static
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Time Time::FromExploded(bool is_local, const Exploded& exploded) {
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// Create the system struct representing our exploded time. It will either be
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// in local time or UTC.
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SYSTEMTIME st;
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st.wYear = exploded.year;
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st.wMonth = exploded.month;
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st.wDayOfWeek = exploded.day_of_week;
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st.wDay = exploded.day_of_month;
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st.wHour = exploded.hour;
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st.wMinute = exploded.minute;
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st.wSecond = exploded.second;
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st.wMilliseconds = exploded.millisecond;
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// Convert to FILETIME.
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FILETIME ft;
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if (!SystemTimeToFileTime(&st, &ft)) {
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NOTREACHED() << "Unable to convert time";
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return Time(0);
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}
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// Ensure that it's in UTC.
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if (is_local) {
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FILETIME utc_ft;
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LocalFileTimeToFileTime(&ft, &utc_ft);
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return Time(FileTimeToMicroseconds(utc_ft));
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}
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return Time(FileTimeToMicroseconds(ft));
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}
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void Time::Explode(bool is_local, Exploded* exploded) const {
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// FILETIME in UTC.
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FILETIME utc_ft;
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MicrosecondsToFileTime(us_, &utc_ft);
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// FILETIME in local time if necessary.
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BOOL success = TRUE;
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FILETIME ft;
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if (is_local)
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success = FileTimeToLocalFileTime(&utc_ft, &ft);
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else
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ft = utc_ft;
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// FILETIME in SYSTEMTIME (exploded).
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SYSTEMTIME st;
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if (!success || !FileTimeToSystemTime(&ft, &st)) {
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NOTREACHED() << "Unable to convert time, don't know why";
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ZeroMemory(exploded, sizeof(*exploded));
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return;
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}
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exploded->year = st.wYear;
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exploded->month = st.wMonth;
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exploded->day_of_week = st.wDayOfWeek;
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exploded->day_of_month = st.wDay;
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exploded->hour = st.wHour;
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exploded->minute = st.wMinute;
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exploded->second = st.wSecond;
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exploded->millisecond = st.wMilliseconds;
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}
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// TimeTicks ------------------------------------------------------------------
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namespace {
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// We define a wrapper to adapt between the __stdcall and __cdecl call of the
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// mock function, and to avoid a static constructor. Assigning an import to a
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// function pointer directly would require setup code to fetch from the IAT.
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DWORD timeGetTimeWrapper() {
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return timeGetTime();
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}
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DWORD (*tick_function)(void) = &timeGetTimeWrapper;
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// We use timeGetTime() to implement TimeTicks::Now(). This can be problematic
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// because it returns the number of milliseconds since Windows has started,
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// which will roll over the 32-bit value every ~49 days. We try to track
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// rollover ourselves, which works if TimeTicks::Now() is called at least every
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// 49 days.
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class NowSingleton {
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public:
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NowSingleton()
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: rollover_(TimeDelta::FromMilliseconds(0)),
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last_seen_(0) {
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}
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TimeDelta Now() {
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AutoLock locked(lock_);
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// We should hold the lock while calling tick_function to make sure that
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// we keep our last_seen_ stay correctly in sync.
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DWORD now = tick_function();
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if (now < last_seen_)
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rollover_ += TimeDelta::FromMilliseconds(GG_LONGLONG(0x100000000)); // ~49.7 days.
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last_seen_ = now;
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return TimeDelta::FromMilliseconds(now) + rollover_;
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}
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private:
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Lock lock_; // To protected last_seen_ and rollover_.
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TimeDelta rollover_; // Accumulation of time lost due to rollover.
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DWORD last_seen_; // The last timeGetTime value we saw, to detect rollover.
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DISALLOW_COPY_AND_ASSIGN(NowSingleton);
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};
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// Overview of time counters:
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// (1) CPU cycle counter. (Retrieved via RDTSC)
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// The CPU counter provides the highest resolution time stamp and is the least
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// expensive to retrieve. However, the CPU counter is unreliable and should not
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// be used in production. Its biggest issue is that it is per processor and it
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// is not synchronized between processors. Also, on some computers, the counters
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// will change frequency due to thermal and power changes, and stop in some
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// states.
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//
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// (2) QueryPerformanceCounter (QPC). The QPC counter provides a high-
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// resolution (100 nanoseconds) time stamp but is comparatively more expensive
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// to retrieve. What QueryPerformanceCounter actually does is up to the HAL.
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// (with some help from ACPI).
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// According to http://blogs.msdn.com/oldnewthing/archive/2005/09/02/459952.aspx
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// in the worst case, it gets the counter from the rollover interrupt on the
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// programmable interrupt timer. In best cases, the HAL may conclude that the
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// RDTSC counter runs at a constant frequency, then it uses that instead. On
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// multiprocessor machines, it will try to verify the values returned from
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// RDTSC on each processor are consistent with each other, and apply a handful
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// of workarounds for known buggy hardware. In other words, QPC is supposed to
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// give consistent result on a multiprocessor computer, but it is unreliable in
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// reality due to bugs in BIOS or HAL on some, especially old computers.
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// With recent updates on HAL and newer BIOS, QPC is getting more reliable but
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// it should be used with caution.
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//
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// (3) System time. The system time provides a low-resolution (typically 10ms
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// to 55 milliseconds) time stamp but is comparatively less expensive to
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// retrieve and more reliable.
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class HighResNowSingleton {
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public:
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HighResNowSingleton()
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: ticks_per_microsecond_(0.0),
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skew_(0) {
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InitializeClock();
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// On Athlon X2 CPUs (e.g. model 15) QueryPerformanceCounter is
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// unreliable. Fallback to low-res clock.
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base::CPU cpu;
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if (cpu.vendor_name() == "AuthenticAMD" && cpu.family() == 15)
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DisableHighResClock();
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}
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bool IsUsingHighResClock() {
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return ticks_per_microsecond_ != 0.0;
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}
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void DisableHighResClock() {
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ticks_per_microsecond_ = 0.0;
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}
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TimeDelta Now() {
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// Our maximum tolerance for QPC drifting.
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const int kMaxTimeDrift = 50 * Time::kMicrosecondsPerMillisecond;
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if (IsUsingHighResClock()) {
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int64_t now = UnreliableNow();
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// Verify that QPC does not seem to drift.
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DCHECK(now - ReliableNow() - skew_ < kMaxTimeDrift);
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return TimeDelta::FromMicroseconds(now);
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}
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// Just fallback to the slower clock.
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return Singleton<NowSingleton>::get()->Now();
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}
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private:
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// Synchronize the QPC clock with GetSystemTimeAsFileTime.
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void InitializeClock() {
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LARGE_INTEGER ticks_per_sec = {{0}};
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if (!QueryPerformanceFrequency(&ticks_per_sec))
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return; // Broken, we don't guarantee this function works.
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ticks_per_microsecond_ = static_cast<float>(ticks_per_sec.QuadPart) /
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static_cast<float>(Time::kMicrosecondsPerSecond);
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skew_ = UnreliableNow() - ReliableNow();
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}
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// Get the number of microseconds since boot in a reliable fashion
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int64_t UnreliableNow() {
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LARGE_INTEGER now;
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QueryPerformanceCounter(&now);
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return static_cast<int64_t>(now.QuadPart / ticks_per_microsecond_);
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}
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// Get the number of microseconds since boot in a reliable fashion
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int64_t ReliableNow() {
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return Singleton<NowSingleton>::get()->Now().InMicroseconds();
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}
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// Cached clock frequency -> microseconds. This assumes that the clock
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// frequency is faster than one microsecond (which is 1MHz, should be OK).
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float ticks_per_microsecond_; // 0 indicates QPF failed and we're broken.
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int64_t skew_; // Skew between lo-res and hi-res clocks (for debugging).
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DISALLOW_COPY_AND_ASSIGN(HighResNowSingleton);
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};
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} // namespace
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// static
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TimeTicks::TickFunctionType TimeTicks::SetMockTickFunction(
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TickFunctionType ticker) {
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TickFunctionType old = tick_function;
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tick_function = ticker;
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return old;
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}
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// static
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TimeTicks TimeTicks::Now() {
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return TimeTicks() + Singleton<NowSingleton>::get()->Now();
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}
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// static
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TimeTicks TimeTicks::HighResNow() {
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return TimeTicks() + Singleton<HighResNowSingleton>::get()->Now();
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}
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