tor-browser

time_win.cc (31913B)

---

// Copyright 2012 The Chromium Authors
// 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.

#include "base/time/time.h"

#include <windows.foundation.h>
#include <windows.h>

#include <mmsystem.h>
#include <stdint.h>

#include <atomic>
#include <ostream>

#include "base/bit_cast.h"
#include "base/check_op.h"
#include "base/cpu.h"
#include "base/notreached.h"
#include "base/synchronization/lock.h"
#include "base/threading/platform_thread.h"
#include "base/time/time_override.h"
#include "build/build_config.h"

namespace base {

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 bit_cast to fix alignment, then divide by 10 to convert
 // 100-nanoseconds to microseconds. This only works on little-endian
 // machines.
 return bit_cast<int64_t, FILETIME>(ft) / 10;
}

bool CanConvertToFileTime(int64_t us) {
 return us >= 0 && us <= (std::numeric_limits<int64_t>::max() / 10);
}

FILETIME MicrosecondsToFileTime(int64_t us) {
 DCHECK(CanConvertToFileTime(us)) << "Out-of-range: Cannot convert " << us
                                  << " microseconds to FILETIME units.";

 // Multiply by 10 to convert microseconds to 100-nanoseconds. Bit_cast will
 // handle alignment problems. This only works on little-endian machines.
 return bit_cast<FILETIME, int64_t>(us * 10);
}

int64_t CurrentWallclockMicroseconds() {
 FILETIME ft;
 ::GetSystemTimeAsFileTime(&ft);
 return FileTimeToMicroseconds(ft);
}

// Time between resampling the un-granular clock for this API.
constexpr TimeDelta kMaxTimeToAvoidDrift = Seconds(60);

int64_t g_initial_time = 0;
TimeTicks g_initial_ticks;

void InitializeClock() {
 g_initial_ticks = subtle::TimeTicksNowIgnoringOverride();
 g_initial_time = CurrentWallclockMicroseconds();
}

// Track the last value passed to timeBeginPeriod so that we can cancel that
// call by calling timeEndPeriod with the same value. A value of zero means that
// the timer frequency is not currently raised.
UINT g_last_interval_requested_ms = 0;
// Track if kMinTimerIntervalHighResMs or kMinTimerIntervalLowResMs is active.
// For most purposes this could also be named g_is_on_ac_power.
bool g_high_res_timer_enabled = false;
// How many times the high resolution timer has been called.
uint32_t g_high_res_timer_count = 0;
// Start time of the high resolution timer usage monitoring. This is needed
// to calculate the usage as percentage of the total elapsed time.
TimeTicks g_high_res_timer_usage_start;
// The cumulative time the high resolution timer has been in use since
// |g_high_res_timer_usage_start| moment.
TimeDelta g_high_res_timer_usage;
// Timestamp of the last activation change of the high resolution timer. This
// is used to calculate the cumulative usage.
TimeTicks g_high_res_timer_last_activation;
// The lock to control access to the above set of variables.
Lock* GetHighResLock() {
 static auto* lock = new Lock();
 return lock;
}

// The two values that ActivateHighResolutionTimer uses to set the systemwide
// timer interrupt frequency on Windows. These control how precise timers are
// but also have a big impact on battery life.

// Used when a faster timer has been requested (g_high_res_timer_count > 0) and
// the computer is running on AC power (plugged in) so that it's okay to go to
// the highest frequency.
constexpr UINT kMinTimerIntervalHighResMs = 1;

// Used when a faster timer has been requested (g_high_res_timer_count > 0) and
// the computer is running on DC power (battery) so that we don't want to raise
// the timer frequency as much.
constexpr UINT kMinTimerIntervalLowResMs = 8;

// Calculate the desired timer interrupt interval. Note that zero means that the
// system default should be used.
UINT GetIntervalMs() {
 if (!g_high_res_timer_count)
   return 0;  // Use the default, typically 15.625
 if (g_high_res_timer_enabled)
   return kMinTimerIntervalHighResMs;
 return kMinTimerIntervalLowResMs;
}

// Compare the currently requested timer interrupt interval to the last interval
// requested and update if necessary (by cancelling the old request and making a
// new request). If there is no change then do nothing.
void UpdateTimerIntervalLocked() {
 UINT new_interval = GetIntervalMs();
 if (new_interval == g_last_interval_requested_ms)
   return;
 if (g_last_interval_requested_ms) {
   // Record how long the timer interrupt frequency was raised.
   g_high_res_timer_usage += subtle::TimeTicksNowIgnoringOverride() -
                             g_high_res_timer_last_activation;
   // Reset the timer interrupt back to the default.
   timeEndPeriod(g_last_interval_requested_ms);
 }
 g_last_interval_requested_ms = new_interval;
 if (g_last_interval_requested_ms) {
   // Record when the timer interrupt was raised.
   g_high_res_timer_last_activation = subtle::TimeTicksNowIgnoringOverride();
   timeBeginPeriod(g_last_interval_requested_ms);
 }
}

// Returns the current value of the performance counter.
int64_t QPCNowRaw() {
 LARGE_INTEGER perf_counter_now = {};
 // According to the MSDN documentation for QueryPerformanceCounter(), this
 // will never fail on systems that run XP or later.
 // https://msdn.microsoft.com/library/windows/desktop/ms644904.aspx
 ::QueryPerformanceCounter(&perf_counter_now);
 return perf_counter_now.QuadPart;
}

bool SafeConvertToWord(int in, WORD* out) {
 CheckedNumeric<WORD> result = in;
 *out = result.ValueOrDefault(std::numeric_limits<WORD>::max());
 return result.IsValid();
}

}  // namespace

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

namespace subtle {
Time TimeNowIgnoringOverride() {
 if (g_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 (g_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 = TimeTicksNowIgnoringOverride();

   // Calculate the time elapsed since we started our timer
   TimeDelta elapsed = ticks - g_initial_ticks;

   // Check if enough time has elapsed that we need to resync the clock.
   if (elapsed > kMaxTimeToAvoidDrift) {
     InitializeClock();
     continue;
   }

   return Time() + elapsed + Microseconds(g_initial_time);
 }
}

Time TimeNowFromSystemTimeIgnoringOverride() {
 // Force resync.
 InitializeClock();
 return Time() + Microseconds(g_initial_time);
}
}  // namespace subtle

// static
Time Time::FromFileTime(FILETIME ft) {
 if (bit_cast<int64_t, FILETIME>(ft) == 0)
   return Time();
 if (ft.dwHighDateTime == std::numeric_limits<DWORD>::max() &&
     ft.dwLowDateTime == std::numeric_limits<DWORD>::max())
   return Max();
 return Time(FileTimeToMicroseconds(ft));
}

FILETIME Time::ToFileTime() const {
 if (is_null())
   return bit_cast<FILETIME, int64_t>(0);
 if (is_max()) {
   FILETIME result;
   result.dwHighDateTime = std::numeric_limits<DWORD>::max();
   result.dwLowDateTime = std::numeric_limits<DWORD>::max();
   return result;
 }
 return MicrosecondsToFileTime(us_);
}

// static
// Enable raising of the system-global timer interrupt frequency to 1 kHz (when
// enable is true, which happens when on AC power) or some lower frequency when
// on battery power (when enable is false). If the g_high_res_timer_enabled
// setting hasn't actually changed or if if there are no outstanding requests
// (if g_high_res_timer_count is zero) then do nothing.
// TL;DR - call this when going from AC to DC power or vice-versa.
void Time::EnableHighResolutionTimer(bool enable) {
 AutoLock lock(*GetHighResLock());
 g_high_res_timer_enabled = enable;
 UpdateTimerIntervalLocked();
}

// static
// Request that the system-global Windows timer interrupt frequency be raised.
// How high the frequency is raised depends on the system's power state and
// possibly other options.
// TL;DR - call this at the beginning and end of a time period where you want
// higher frequency timer interrupts. Each call with activating=true must be
// paired with a subsequent activating=false call.
bool Time::ActivateHighResolutionTimer(bool activating) {
 // We only do work on the transition from zero to one or one to zero so we
 // can easily undo the effect (if necessary) when EnableHighResolutionTimer is
 // called.
 const uint32_t max = std::numeric_limits<uint32_t>::max();

 AutoLock lock(*GetHighResLock());
 if (activating) {
   DCHECK_NE(g_high_res_timer_count, max);
   ++g_high_res_timer_count;
 } else {
   DCHECK_NE(g_high_res_timer_count, 0u);
   --g_high_res_timer_count;
 }
 UpdateTimerIntervalLocked();
 return true;
}

// static
// See if the timer interrupt interval has been set to the lowest value.
bool Time::IsHighResolutionTimerInUse() {
 AutoLock lock(*GetHighResLock());
 return g_last_interval_requested_ms == kMinTimerIntervalHighResMs;
}

// static
void Time::ResetHighResolutionTimerUsage() {
 AutoLock lock(*GetHighResLock());
 g_high_res_timer_usage = TimeDelta();
 g_high_res_timer_usage_start = subtle::TimeTicksNowIgnoringOverride();
 if (g_high_res_timer_count > 0)
   g_high_res_timer_last_activation = g_high_res_timer_usage_start;
}

// static
double Time::GetHighResolutionTimerUsage() {
 AutoLock lock(*GetHighResLock());
 TimeTicks now = subtle::TimeTicksNowIgnoringOverride();
 TimeDelta elapsed_time = now - g_high_res_timer_usage_start;
 if (elapsed_time.is_zero()) {
   // This is unexpected but possible if TimeTicks resolution is low and
   // GetHighResolutionTimerUsage() is called promptly after
   // ResetHighResolutionTimerUsage().
   return 0.0;
 }
 TimeDelta used_time = g_high_res_timer_usage;
 if (g_high_res_timer_count > 0) {
   // If currently activated add the remainder of time since the last
   // activation.
   used_time += now - g_high_res_timer_last_activation;
 }
 return used_time / elapsed_time * 100;
}

// static
bool Time::FromExploded(bool is_local, const Exploded& exploded, Time* time) {
 // Create the system struct representing our exploded time. It will either be
 // in local time or UTC.If casting from int to WORD results in overflow,
 // fail and return Time(0).
 SYSTEMTIME st;
 if (!SafeConvertToWord(exploded.year, &st.wYear) ||
     !SafeConvertToWord(exploded.month, &st.wMonth) ||
     !SafeConvertToWord(exploded.day_of_week, &st.wDayOfWeek) ||
     !SafeConvertToWord(exploded.day_of_month, &st.wDay) ||
     !SafeConvertToWord(exploded.hour, &st.wHour) ||
     !SafeConvertToWord(exploded.minute, &st.wMinute) ||
     !SafeConvertToWord(exploded.second, &st.wSecond) ||
     !SafeConvertToWord(exploded.millisecond, &st.wMilliseconds)) {
   *time = Time(0);
   return false;
 }

 FILETIME ft;
 bool success = true;
 // Ensure that it's in UTC.
 if (is_local) {
   SYSTEMTIME utc_st;
   success = TzSpecificLocalTimeToSystemTime(nullptr, &st, &utc_st) &&
             SystemTimeToFileTime(&utc_st, &ft);
 } else {
   success = !!SystemTimeToFileTime(&st, &ft);
 }

 *time = Time(success ? FileTimeToMicroseconds(ft) : 0);
 return success;
}

void Time::Explode(bool is_local, Exploded* exploded) const {
 if (!CanConvertToFileTime(us_)) {
   // We are not able to convert it to FILETIME.
   ZeroMemory(exploded, sizeof(*exploded));
   return;
 }

 const FILETIME utc_ft = MicrosecondsToFileTime(us_);

 // FILETIME in local time if necessary.
 bool success = true;
 // FILETIME in SYSTEMTIME (exploded).
 SYSTEMTIME st = {0};
 if (is_local) {
   SYSTEMTIME utc_st;
   // We don't use FileTimeToLocalFileTime here, since it uses the current
   // settings for the time zone and daylight saving time. Therefore, if it is
   // daylight saving time, it will take daylight saving time into account,
   // even if the time you are converting is in standard time.
   success = FileTimeToSystemTime(&utc_ft, &utc_st) &&
             SystemTimeToTzSpecificLocalTime(nullptr, &utc_st, &st);
 } else {
   success = !!FileTimeToSystemTime(&utc_ft, &st);
 }

 if (!success) {
   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 (*g_tick_function)(void) = &timeGetTimeWrapper;

// A structure holding the most significant bits of "last seen" and a
// "rollover" counter.
union LastTimeAndRolloversState {
 // The state as a single 32-bit opaque value.
 std::atomic<int32_t> as_opaque_32{0};

 // The state as usable values.
 struct {
   // The top 8-bits of the "last" time. This is enough to check for rollovers
   // and the small bit-size means fewer CompareAndSwap operations to store
   // changes in state, which in turn makes for fewer retries.
   uint8_t last_8;
   // A count of the number of detected rollovers. Using this as bits 47-32
   // of the upper half of a 64-bit value results in a 48-bit tick counter.
   // This extends the total rollover period from about 49 days to about 8800
   // years while still allowing it to be stored with last_8 in a single
   // 32-bit value.
   uint16_t rollovers;
 } as_values;
};
std::atomic<int32_t> g_last_time_and_rollovers = 0;
static_assert(
   sizeof(LastTimeAndRolloversState) <= sizeof(g_last_time_and_rollovers),
   "LastTimeAndRolloversState does not fit in a single atomic word");

// 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
// 48.8 days (not 49 days because only changes in the top 8 bits get noticed).
TimeTicks RolloverProtectedNow() {
 LastTimeAndRolloversState state;
 DWORD now;  // DWORD is always unsigned 32 bits.

 while (true) {
   // Fetch the "now" and "last" tick values, updating "last" with "now" and
   // incrementing the "rollovers" counter if the tick-value has wrapped back
   // around. Atomic operations ensure that both "last" and "rollovers" are
   // always updated together.
   int32_t original =
       g_last_time_and_rollovers.load(std::memory_order_acquire);
   state.as_opaque_32 = original;
   now = g_tick_function();
   uint8_t now_8 = static_cast<uint8_t>(now >> 24);
   if (now_8 < state.as_values.last_8)
     ++state.as_values.rollovers;
   state.as_values.last_8 = now_8;

   // If the state hasn't changed, exit the loop.
   if (state.as_opaque_32 == original)
     break;

   // Save the changed state. If the existing value is unchanged from the
   // original so that the operation is successful. Exit the loop.
   bool success = g_last_time_and_rollovers.compare_exchange_strong(
       original, state.as_opaque_32, std::memory_order_release);
   if (success)
     break;

   // Another thread has done something in between so retry from the top.
 }

 return TimeTicks() +
        Milliseconds(now +
                     (static_cast<uint64_t>(state.as_values.rollovers) << 32));
}

// Discussion of tick counter options on Windows:
//
// (1) CPU cycle counter. (Retrieved via RDTSC)
// The CPU counter provides the highest resolution time stamp and is the least
// expensive to retrieve. However, on older CPUs, two issues can affect its
// reliability: First it is maintained per processor and not synchronized
// between processors. Also, 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 (<1 microsecond) time stamp. On most hardware running today, it
// auto-detects and uses the constant-rate RDTSC counter to provide extremely
// efficient and reliable time stamps.
//
// On older CPUs where RDTSC is unreliable, it falls back to using more
// expensive (20X to 40X more costly) alternate clocks, such as HPET or the ACPI
// PM timer, and can involve system calls; and all this 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 results on a multiprocessor computer, but for older CPUs it
// can be unreliable due bugs in BIOS or HAL.
//
// (3) System time. The system time provides a low-resolution (from ~1 to ~15.6
// milliseconds) time stamp but is comparatively less expensive to retrieve and
// more reliable. Time::EnableHighResolutionTimer() and
// Time::ActivateHighResolutionTimer() can be called to alter the resolution of
// this timer; and also other Windows applications can alter it, affecting this
// one.

TimeTicks InitialNowFunction();

// See "threading notes" in InitializeNowFunctionPointer() for details on how
// concurrent reads/writes to these globals has been made safe.
std::atomic<TimeTicksNowFunction> g_time_ticks_now_ignoring_override_function{
   &InitialNowFunction};
int64_t g_qpc_ticks_per_second = 0;

TimeDelta QPCValueToTimeDelta(LONGLONG qpc_value) {
 // Ensure that the assignment to |g_qpc_ticks_per_second|, made in
 // InitializeNowFunctionPointer(), has happened by this point.
 std::atomic_thread_fence(std::memory_order_acquire);

 DCHECK_GT(g_qpc_ticks_per_second, 0);

 // If the QPC Value is below the overflow threshold, we proceed with
 // simple multiply and divide.
 if (qpc_value < Time::kQPCOverflowThreshold) {
   return Microseconds(qpc_value * Time::kMicrosecondsPerSecond /
                       g_qpc_ticks_per_second);
 }
 // Otherwise, calculate microseconds in a round about manner to avoid
 // overflow and precision issues.
 int64_t whole_seconds = qpc_value / g_qpc_ticks_per_second;
 int64_t leftover_ticks = qpc_value - (whole_seconds * g_qpc_ticks_per_second);
 return Microseconds((whole_seconds * Time::kMicrosecondsPerSecond) +
                     ((leftover_ticks * Time::kMicrosecondsPerSecond) /
                      g_qpc_ticks_per_second));
}

TimeTicks QPCNow() {
 return TimeTicks() + QPCValueToTimeDelta(QPCNowRaw());
}

void InitializeNowFunctionPointer() {
 LARGE_INTEGER ticks_per_sec = {};
 if (!QueryPerformanceFrequency(&ticks_per_sec))
   ticks_per_sec.QuadPart = 0;

 // If Windows cannot provide a QPC implementation, TimeTicks::Now() must use
 // the low-resolution clock.
 //
 // If the QPC implementation is expensive and/or unreliable, TimeTicks::Now()
 // will still use the low-resolution clock. A CPU lacking a non-stop time
 // counter will cause Windows to provide an alternate QPC implementation that
 // works, but is expensive to use.
 //
 // Otherwise, Now uses the high-resolution QPC clock. As of 21 August 2015,
 // ~72% of users fall within this category.
 CPU cpu;
 const TimeTicksNowFunction now_function =
     (ticks_per_sec.QuadPart <= 0 || !cpu.has_non_stop_time_stamp_counter())
         ? &RolloverProtectedNow
         : &QPCNow;

 // Threading note 1: In an unlikely race condition, it's possible for two or
 // more threads to enter InitializeNowFunctionPointer() in parallel. This is
 // not a problem since all threads end up writing out the same values
 // to the global variables, and those variable being atomic are safe to read
 // from other threads.
 //
 // Threading note 2: A release fence is placed here to ensure, from the
 // perspective of other threads using the function pointers, that the
 // assignment to |g_qpc_ticks_per_second| happens before the function pointers
 // are changed.
 g_qpc_ticks_per_second = ticks_per_sec.QuadPart;
 std::atomic_thread_fence(std::memory_order_release);
 // Also set g_time_ticks_now_function to avoid the additional indirection via
 // TimeTicksNowIgnoringOverride() for future calls to TimeTicks::Now(), only
 // if it wasn't already overridden to a different value. memory_order_relaxed
 // is sufficient since an explicit fence was inserted above.
 base::TimeTicksNowFunction initial_time_ticks_now_function =
     &subtle::TimeTicksNowIgnoringOverride;
 internal::g_time_ticks_now_function.compare_exchange_strong(
     initial_time_ticks_now_function, now_function, std::memory_order_relaxed);
 g_time_ticks_now_ignoring_override_function.store(now_function,
                                                   std::memory_order_relaxed);
}

TimeTicks InitialNowFunction() {
 InitializeNowFunctionPointer();
 return g_time_ticks_now_ignoring_override_function.load(
     std::memory_order_relaxed)();
}

}  // namespace

// static
TimeTicks::TickFunctionType TimeTicks::SetMockTickFunction(
   TickFunctionType ticker) {
 TickFunctionType old = g_tick_function;
 g_tick_function = ticker;
 g_last_time_and_rollovers.store(0, std::memory_order_relaxed);
 return old;
}

namespace subtle {
TimeTicks TimeTicksNowIgnoringOverride() {
 return g_time_ticks_now_ignoring_override_function.load(
     std::memory_order_relaxed)();
}
}  // namespace subtle

// static
bool TimeTicks::IsHighResolution() {
 if (g_time_ticks_now_ignoring_override_function == &InitialNowFunction)
   InitializeNowFunctionPointer();
 return g_time_ticks_now_ignoring_override_function == &QPCNow;
}

// static
bool TimeTicks::IsConsistentAcrossProcesses() {
 // According to Windows documentation [1] QPC is consistent post-Windows
 // Vista. So if we are using QPC then we are consistent which is the same as
 // being high resolution.
 //
 // [1] https://msdn.microsoft.com/en-us/library/windows/desktop/dn553408(v=vs.85).aspx
 //
 // "In general, the performance counter results are consistent across all
 // processors in multi-core and multi-processor systems, even when measured on
 // different threads or processes. Here are some exceptions to this rule:
 // - Pre-Windows Vista operating systems that run on certain processors might
 // violate this consistency because of one of these reasons:
 //     1. The hardware processors have a non-invariant TSC and the BIOS
 //     doesn't indicate this condition correctly.
 //     2. The TSC synchronization algorithm that was used wasn't suitable for
 //     systems with large numbers of processors."
 return IsHighResolution();
}

// static
TimeTicks::Clock TimeTicks::GetClock() {
 return IsHighResolution() ? Clock::WIN_QPC
                           : Clock::WIN_ROLLOVER_PROTECTED_TIME_GET_TIME;
}

// LiveTicks ------------------------------------------------------------------

#if !defined(MOZ_SANDBOX)
namespace subtle {
LiveTicks LiveTicksNowIgnoringOverride() {
 ULONGLONG unbiased_interrupt_time;
 QueryUnbiasedInterruptTimePrecise(&unbiased_interrupt_time);
 // QueryUnbiasedInterruptTimePrecise gets the interrupt time in system time
 // units of 100 nanoseconds.
 return LiveTicks() + Nanoseconds(unbiased_interrupt_time * 100);
}
}  // namespace subtle
#endif

// ThreadTicks ----------------------------------------------------------------

namespace subtle {
ThreadTicks ThreadTicksNowIgnoringOverride() {
 return ThreadTicks::GetForThread(PlatformThread::CurrentHandle());
}
}  // namespace subtle

// static
ThreadTicks ThreadTicks::GetForThread(
   const PlatformThreadHandle& thread_handle) {
 DCHECK(IsSupported());

#if defined(ARCH_CPU_ARM64)
 // QueryThreadCycleTime versus TSCTicksPerSecond doesn't have much relation to
 // actual elapsed time on Windows on Arm, because QueryThreadCycleTime is
 // backed by the actual number of CPU cycles executed, rather than a
 // constant-rate timer like Intel. To work around this, use GetThreadTimes
 // (which isn't as accurate but is meaningful as a measure of elapsed
 // per-thread time).
 FILETIME creation_time, exit_time, kernel_time, user_time;
 ::GetThreadTimes(thread_handle.platform_handle(), &creation_time, &exit_time,
                  &kernel_time, &user_time);

 const int64_t us = FileTimeToMicroseconds(user_time);
#else
 // Get the number of TSC ticks used by the current thread.
 ULONG64 thread_cycle_time = 0;
 ::QueryThreadCycleTime(thread_handle.platform_handle(), &thread_cycle_time);

 // Get the frequency of the TSC.
 const double tsc_ticks_per_second = time_internal::TSCTicksPerSecond();
 if (tsc_ticks_per_second == 0)
   return ThreadTicks();

 // Return the CPU time of the current thread.
 const double thread_time_seconds = thread_cycle_time / tsc_ticks_per_second;
 const int64_t us =
     static_cast<int64_t>(thread_time_seconds * Time::kMicrosecondsPerSecond);
#endif

 return ThreadTicks(us);
}

// static
bool ThreadTicks::IsSupportedWin() {
#if defined(ARCH_CPU_ARM64)
 // The Arm implementation does not use QueryThreadCycleTime and therefore does
 // not care about the time stamp counter.
 return true;
#else
 return time_internal::HasConstantRateTSC();
#endif
}

// static
void ThreadTicks::WaitUntilInitializedWin() {
#if !defined(ARCH_CPU_ARM64)
 while (time_internal::TSCTicksPerSecond() == 0)
   ::Sleep(10);
#endif
}

// static
TimeTicks TimeTicks::FromQPCValue(LONGLONG qpc_value) {
 return TimeTicks() + QPCValueToTimeDelta(qpc_value);
}

// TimeDelta ------------------------------------------------------------------

// static
TimeDelta TimeDelta::FromQPCValue(LONGLONG qpc_value) {
 return QPCValueToTimeDelta(qpc_value);
}

// static
TimeDelta TimeDelta::FromFileTime(FILETIME ft) {
 return Microseconds(FileTimeToMicroseconds(ft));
}

// static
TimeDelta TimeDelta::FromWinrtDateTime(ABI::Windows::Foundation::DateTime dt) {
 // UniversalTime is 100 ns intervals since January 1, 1601 (UTC)
 return Microseconds(dt.UniversalTime / 10);
}

ABI::Windows::Foundation::DateTime TimeDelta::ToWinrtDateTime() const {
 ABI::Windows::Foundation::DateTime date_time;
 date_time.UniversalTime = InMicroseconds() * 10;
 return date_time;
}

// static
TimeDelta TimeDelta::FromWinrtTimeSpan(ABI::Windows::Foundation::TimeSpan ts) {
 // Duration is 100 ns intervals
 return Microseconds(ts.Duration / 10);
}

ABI::Windows::Foundation::TimeSpan TimeDelta::ToWinrtTimeSpan() const {
 ABI::Windows::Foundation::TimeSpan time_span;
 time_span.Duration = InMicroseconds() * 10;
 return time_span;
}

#if !defined(ARCH_CPU_ARM64)
namespace time_internal {

bool HasConstantRateTSC() {
 static bool is_supported = CPU().has_non_stop_time_stamp_counter();
 return is_supported;
}

double TSCTicksPerSecond() {
 DCHECK(HasConstantRateTSC());
 // The value returned by QueryPerformanceFrequency() cannot be used as the TSC
 // frequency, because there is no guarantee that the TSC frequency is equal to
 // the performance counter frequency.
 // The TSC frequency is cached in a static variable because it takes some time
 // to compute it.
 static double tsc_ticks_per_second = 0;
 if (tsc_ticks_per_second != 0)
   return tsc_ticks_per_second;

 // Increase the thread priority to reduces the chances of having a context
 // switch during a reading of the TSC and the performance counter.
 const int previous_priority = ::GetThreadPriority(::GetCurrentThread());
 ::SetThreadPriority(::GetCurrentThread(), THREAD_PRIORITY_HIGHEST);

 // The first time that this function is called, make an initial reading of the
 // TSC and the performance counter.

 static const uint64_t tsc_initial = __rdtsc();
 static const int64_t perf_counter_initial = QPCNowRaw();

 // Make a another reading of the TSC and the performance counter every time
 // that this function is called.
 const uint64_t tsc_now = __rdtsc();
 const int64_t perf_counter_now = QPCNowRaw();

 // Reset the thread priority.
 ::SetThreadPriority(::GetCurrentThread(), previous_priority);

 // Make sure that at least 50 ms elapsed between the 2 readings. The first
 // time that this function is called, we don't expect this to be the case.
 // Note: The longer the elapsed time between the 2 readings is, the more
 //   accurate the computed TSC frequency will be. The 50 ms value was
 //   chosen because local benchmarks show that it allows us to get a
 //   stddev of less than 1 tick/us between multiple runs.
 // Note: According to the MSDN documentation for QueryPerformanceFrequency(),
 //   this will never fail on systems that run XP or later.
 //   https://msdn.microsoft.com/library/windows/desktop/ms644905.aspx
 LARGE_INTEGER perf_counter_frequency = {};
 ::QueryPerformanceFrequency(&perf_counter_frequency);
 DCHECK_GE(perf_counter_now, perf_counter_initial);
 const int64_t perf_counter_ticks = perf_counter_now - perf_counter_initial;
 const double elapsed_time_seconds =
     perf_counter_ticks / static_cast<double>(perf_counter_frequency.QuadPart);

 constexpr double kMinimumEvaluationPeriodSeconds = 0.05;
 if (elapsed_time_seconds < kMinimumEvaluationPeriodSeconds)
   return 0;

 // Compute the frequency of the TSC.
 DCHECK_GE(tsc_now, tsc_initial);
 const uint64_t tsc_ticks = tsc_now - tsc_initial;
 tsc_ticks_per_second = tsc_ticks / elapsed_time_seconds;

 return tsc_ticks_per_second;
}

}  // namespace time_internal
#endif  // defined(ARCH_CPU_ARM64)

}  // namespace base