The Tor Browser / file revision

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.

 // Use of this source code is governed by a BSD-style license that can be

 // found in the LICENSE file.

 // Windows Timer Primer

 //

 // A good article:  http://www.ddj.com/windows/184416651

 // A good mozilla bug:  http://bugzilla.mozilla.org/show_bug.cgi?id=363258

 //

 // The default windows timer, GetSystemTimeAsFileTime is not very precise.

 // It is only good to ~15.5ms.

 //

 // QueryPerformanceCounter is the logical choice for a high-precision timer.

 // However, it is known to be buggy on some hardware.  Specifically, it can

 // sometimes "jump".  On laptops, QPC can also be very expensive to call.

 // It's 3-4x slower than timeGetTime() on desktops, but can be 10x slower

 // on laptops.  A unittest exists which will show the relative cost of various

 // timers on any system.

 //

 // The next logical choice is timeGetTime().  timeGetTime has a precision of

 // 1ms, but only if you call APIs (timeBeginPeriod()) which affect all other

 // applications on the system.  By default, precision is only 15.5ms.

 // Unfortunately, we don't want to call timeBeginPeriod because we don't

 // want to affect other applications.  Further, on mobile platforms, use of

 // faster multimedia timers can hurt battery life.  See the intel

 // article about this here:

 // http://softwarecommunity.intel.com/articles/eng/1086.htm

 //

 // To work around all this, we're going to generally use timeGetTime().  We

 // will only increase the system-wide timer if we're not running on battery

 // power.  Using timeBeginPeriod(1) is a requirement in order to make our

 // message loop waits have the same resolution that our time measurements

 // do.  Otherwise, WaitForSingleObject(..., 1) will no less than 15ms when

 // there is nothing else to waken the Wait.

 #include "base/time.h"

 #pragma comment(lib, "winmm.lib")

 #include <windows.h>

 #include <mmsystem.h>

 #include "base/basictypes.h"

 #include "base/lock.h"

 #include "base/logging.h"

 #include "base/cpu.h"

 #include "base/singleton.h"

 #include "base/system_monitor.h"

 #include "mozilla/Casting.h"

 using base::Time;

 using base::TimeDelta;

 using base::TimeTicks;

 using mozilla::BitwiseCast;

 namespace {

 // From MSDN, FILETIME "Contains a 64-bit value representing the number of

 // 100-nanosecond intervals since January 1, 1601 (UTC)."

 int64_t FileTimeToMicroseconds(const FILETIME& ft) {

   // Need to BitwiseCast to fix alignment, then divide by 10 to convert

   // 100-nanoseconds to milliseconds. This only works on little-endian

   // machines.

   return BitwiseCast<int64_t>(ft) / 10;

 }

 void MicrosecondsToFileTime(int64_t us, FILETIME* ft) {

   DCHECK(us >= 0) << "Time is less than 0, negative values are not "

       "representable in FILETIME";

   // Multiply by 10 to convert milliseconds to 100-nanoseconds. BitwiseCast will

   // handle alignment problems. This only works on little-endian machines.

   *ft = BitwiseCast<FILETIME>(us * 10);

 }

 int64_t CurrentWallclockMicroseconds() {

   FILETIME ft;

   ::GetSystemTimeAsFileTime(&ft);

   return FileTimeToMicroseconds(ft);

 }

 // Time between resampling the un-granular clock for this API.  60 seconds.

 const int kMaxMillisecondsToAvoidDrift = 60 * Time::kMillisecondsPerSecond;

 int64_t initial_time = 0;

 TimeTicks initial_ticks;

 void InitializeClock() {

   initial_ticks = TimeTicks::Now();

   initial_time = CurrentWallclockMicroseconds();

 }

 }  // namespace

 // Time -----------------------------------------------------------------------

 // The internal representation of Time uses FILETIME, whose epoch is 1601-01-01

 // 00:00:00 UTC.  ((1970-1601)*365+89)*24*60*60*1000*1000, where 89 is the

 // number of leap year days between 1601 and 1970: (1970-1601)/4 excluding

 // 1700, 1800, and 1900.

 // static

 const int64_t Time::kTimeTToMicrosecondsOffset = GG_INT64_C(11644473600000000);

 // static

 Time Time::Now() {

   if (initial_time == 0)

     InitializeClock();

   // We implement time using the high-resolution timers so that we can get

   // timeouts which are smaller than 10-15ms.  If we just used

   // CurrentWallclockMicroseconds(), we'd have the less-granular timer.

   //

   // To make this work, we initialize the clock (initial_time) and the

   // counter (initial_ctr).  To compute the initial time, we can check

   // the number of ticks that have elapsed, and compute the delta.

   //

   // To avoid any drift, we periodically resync the counters to the system

   // clock.

   while(true) {

     TimeTicks ticks = TimeTicks::Now();

     // Calculate the time elapsed since we started our timer

     TimeDelta elapsed = ticks - initial_ticks;

     // Check if enough time has elapsed that we need to resync the clock.

     if (elapsed.InMilliseconds() > kMaxMillisecondsToAvoidDrift) {

       InitializeClock();

       continue;

     }

     return Time(elapsed + initial_time);

   }

 }

 // static

 Time Time::NowFromSystemTime() {

   // Force resync.

   InitializeClock();

   return Time(initial_time);

 }

 // static

 Time Time::FromFileTime(FILETIME ft) {

   return Time(FileTimeToMicroseconds(ft));

 }

 FILETIME Time::ToFileTime() const {

   FILETIME utc_ft;

   MicrosecondsToFileTime(us_, &utc_ft);

   return utc_ft;

 }

 // static

 Time Time::FromExploded(bool is_local, const Exploded& exploded) {

   // Create the system struct representing our exploded time. It will either be

   // in local time or UTC.

   SYSTEMTIME st;

   st.wYear = exploded.year;

   st.wMonth = exploded.month;

   st.wDayOfWeek = exploded.day_of_week;

   st.wDay = exploded.day_of_month;

   st.wHour = exploded.hour;

   st.wMinute = exploded.minute;

   st.wSecond = exploded.second;

   st.wMilliseconds = exploded.millisecond;

   // Convert to FILETIME.

   FILETIME ft;

   if (!SystemTimeToFileTime(&st, &ft)) {

     NOTREACHED() << "Unable to convert time";

     return Time(0);

   }

   // Ensure that it's in UTC.

   if (is_local) {

     FILETIME utc_ft;

     LocalFileTimeToFileTime(&ft, &utc_ft);

     return Time(FileTimeToMicroseconds(utc_ft));

   }

   return Time(FileTimeToMicroseconds(ft));

 }

 void Time::Explode(bool is_local, Exploded* exploded) const {

   // FILETIME in UTC.

   FILETIME utc_ft;

   MicrosecondsToFileTime(us_, &utc_ft);

   // FILETIME in local time if necessary.

   BOOL success = TRUE;

   FILETIME ft;

   if (is_local)

     success = FileTimeToLocalFileTime(&utc_ft, &ft);

   else

     ft = utc_ft;

   // FILETIME in SYSTEMTIME (exploded).

   SYSTEMTIME st;

   if (!success || !FileTimeToSystemTime(&ft, &st)) {

     NOTREACHED() << "Unable to convert time, don't know why";

     ZeroMemory(exploded, sizeof(*exploded));

     return;

   }

   exploded->year = st.wYear;

   exploded->month = st.wMonth;

   exploded->day_of_week = st.wDayOfWeek;

   exploded->day_of_month = st.wDay;

   exploded->hour = st.wHour;

   exploded->minute = st.wMinute;

   exploded->second = st.wSecond;

   exploded->millisecond = st.wMilliseconds;

 }

 // TimeTicks ------------------------------------------------------------------

 namespace {

 // We define a wrapper to adapt between the __stdcall and __cdecl call of the

 // mock function, and to avoid a static constructor.  Assigning an import to a

 // function pointer directly would require setup code to fetch from the IAT.

 DWORD timeGetTimeWrapper() {

   return timeGetTime();

 }

 DWORD (*tick_function)(void) = &timeGetTimeWrapper;

 // We use timeGetTime() to implement TimeTicks::Now().  This can be problematic

 // because it returns the number of milliseconds since Windows has started,

 // which will roll over the 32-bit value every ~49 days.  We try to track

 // rollover ourselves, which works if TimeTicks::Now() is called at least every

 // 49 days.

 class NowSingleton {

  public:

   NowSingleton()

     : rollover_(TimeDelta::FromMilliseconds(0)),

       last_seen_(0) {

   }

   TimeDelta Now() {

     AutoLock locked(lock_);

     // We should hold the lock while calling tick_function to make sure that

     // we keep our last_seen_ stay correctly in sync.

     DWORD now = tick_function();

     if (now < last_seen_)

       rollover_ += TimeDelta::FromMilliseconds(GG_LONGLONG(0x100000000));  // ~49.7 days.

     last_seen_ = now;

     return TimeDelta::FromMilliseconds(now) + rollover_;

   }

  private:

   Lock lock_;  // To protected last_seen_ and rollover_.

   TimeDelta rollover_;  // Accumulation of time lost due to rollover.

   DWORD last_seen_;  // The last timeGetTime value we saw, to detect rollover.

   DISALLOW_COPY_AND_ASSIGN(NowSingleton);

 };

 // Overview of time counters:

 // (1) CPU cycle counter. (Retrieved via RDTSC)

 // The CPU counter provides the highest resolution time stamp and is the least

 // expensive to retrieve. However, the CPU counter is unreliable and should not

 // be used in production. Its biggest issue is that it is per processor and it

 // is not synchronized between processors. Also, on some computers, the counters

 // will change frequency due to thermal and power changes, and stop in some

 // states.

 //

 // (2) QueryPerformanceCounter (QPC). The QPC counter provides a high-

 // resolution (100 nanoseconds) time stamp but is comparatively more expensive

 // to retrieve. What QueryPerformanceCounter actually does is up to the HAL.

 // (with some help from ACPI).

 // According to http://blogs.msdn.com/oldnewthing/archive/2005/09/02/459952.aspx

 // in the worst case, it gets the counter from the rollover interrupt on the

 // programmable interrupt timer. In best cases, the HAL may conclude that the

 // RDTSC counter runs at a constant frequency, then it uses that instead. On

 // multiprocessor machines, it will try to verify the values returned from

 // RDTSC on each processor are consistent with each other, and apply a handful

 // of workarounds for known buggy hardware. In other words, QPC is supposed to

 // give consistent result on a multiprocessor computer, but it is unreliable in

 // reality due to bugs in BIOS or HAL on some, especially old computers.

 // With recent updates on HAL and newer BIOS, QPC is getting more reliable but

 // it should be used with caution.

 //

 // (3) System time. The system time provides a low-resolution (typically 10ms

 // to 55 milliseconds) time stamp but is comparatively less expensive to

 // retrieve and more reliable.

 class HighResNowSingleton {

  public:

   HighResNowSingleton()

     : ticks_per_microsecond_(0.0),

       skew_(0) {

     InitializeClock();

     // On Athlon X2 CPUs (e.g. model 15) QueryPerformanceCounter is

     // unreliable.  Fallback to low-res clock.

     base::CPU cpu;

     if (cpu.vendor_name() == "AuthenticAMD" && cpu.family() == 15)

       DisableHighResClock();

   }

   bool IsUsingHighResClock() {

     return ticks_per_microsecond_ != 0.0;

   }

   void DisableHighResClock() {

     ticks_per_microsecond_ = 0.0;

   }

   TimeDelta Now() {

     // Our maximum tolerance for QPC drifting.

     const int kMaxTimeDrift = 50 * Time::kMicrosecondsPerMillisecond;

     if (IsUsingHighResClock()) {

       int64_t now = UnreliableNow();

       // Verify that QPC does not seem to drift.

       DCHECK(now - ReliableNow() - skew_ < kMaxTimeDrift);

       return TimeDelta::FromMicroseconds(now);

     }

     // Just fallback to the slower clock.

     return Singleton<NowSingleton>::get()->Now();

   }

  private:

   // Synchronize the QPC clock with GetSystemTimeAsFileTime.

   void InitializeClock() {

     LARGE_INTEGER ticks_per_sec = {0};

     if (!QueryPerformanceFrequency(&ticks_per_sec))

       return;  // Broken, we don't guarantee this function works.

     ticks_per_microsecond_ = static_cast<float>(ticks_per_sec.QuadPart) /

       static_cast<float>(Time::kMicrosecondsPerSecond);

     skew_ = UnreliableNow() - ReliableNow();

   }

   // Get the number of microseconds since boot in a reliable fashion

   int64_t UnreliableNow() {

     LARGE_INTEGER now;

     QueryPerformanceCounter(&now);

     return static_cast<int64_t>(now.QuadPart / ticks_per_microsecond_);

   }

   // Get the number of microseconds since boot in a reliable fashion

   int64_t ReliableNow() {

     return Singleton<NowSingleton>::get()->Now().InMicroseconds();

   }

   // Cached clock frequency -> microseconds. This assumes that the clock

   // frequency is faster than one microsecond (which is 1MHz, should be OK).

   float ticks_per_microsecond_;  // 0 indicates QPF failed and we're broken.

   int64_t skew_;  // Skew between lo-res and hi-res clocks (for debugging).

   DISALLOW_COPY_AND_ASSIGN(HighResNowSingleton);

 };

 }  // namespace

 // static

 TimeTicks::TickFunctionType TimeTicks::SetMockTickFunction(

     TickFunctionType ticker) {

   TickFunctionType old = tick_function;

   tick_function = ticker;

   return old;

 }

 // static

 TimeTicks TimeTicks::Now() {

   return TimeTicks() + Singleton<NowSingleton>::get()->Now();

 }

 // static

 TimeTicks TimeTicks::HighResNow() {

   return TimeTicks() + Singleton<HighResNowSingleton>::get()->Now();

 }