The Tor Browser: xpcom/ds/TimeStamp.h@b8a032363ba2 (annotated)

xpcom/ds/TimeStamp.h@b8a032363ba2 (annotated)

xpcom/ds/TimeStamp.h

Thu, 22 Jan 2015 13:21:57 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Thu, 22 Jan 2015 13:21:57 +0100
branch: TOR_BUG_9701
changeset 15: b8a032363ba2
permissions: -rw-r--r--

Incorporate requested changes from Mozilla in review:
https://bugzilla.mozilla.org/show_bug.cgi?id=1123480#c6

 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim:set ts=2 sw=2 sts=2 et cindent: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 #ifndef mozilla_TimeStamp_h
 #define mozilla_TimeStamp_h
 #include <stdint.h>
 #include "mozilla/Assertions.h"
 #include "mozilla/Attributes.h"
 #include "nscore.h"
 namespace IPC {
 template <typename T> struct ParamTraits;
 }
 #ifdef XP_WIN
 // defines TimeStampValue as a complex value keeping both
 // GetTickCount and QueryPerformanceCounter values
 #include "TimeStamp_windows.h"
 #endif
 namespace mozilla {
 #ifndef XP_WIN
 typedef uint64_t TimeStampValue;
 #endif
 class TimeStamp;
 /**
  * Instances of this class represent the length of an interval of time.
  * Negative durations are allowed, meaning the end is before the start.
  *
  * Internally the duration is stored as a int64_t in units of
  * PR_TicksPerSecond() when building with NSPR interval timers, or a
  * system-dependent unit when building with system clocks.  The
  * system-dependent unit must be constant, otherwise the semantics of
  * this class would be broken.
  */
 class TimeDuration
 {
 public:
   // The default duration is 0.
   MOZ_CONSTEXPR TimeDuration() : mValue(0) {}
   // Allow construction using '0' as the initial value, for readability,
   // but no other numbers (so we don't have any implicit unit conversions).
   struct _SomethingVeryRandomHere;
   TimeDuration(_SomethingVeryRandomHere* aZero) : mValue(0) {
     MOZ_ASSERT(!aZero, "Who's playing funny games here?");
   }
   // Default copy-constructor and assignment are OK
   double ToSeconds() const;
   // Return a duration value that includes digits of time we think to
   // be significant.  This method should be used when displaying a
   // time to humans.
   double ToSecondsSigDigits() const;
   double ToMilliseconds() const {
     return ToSeconds() * 1000.0;
   }
   double ToMicroseconds() const {
     return ToMilliseconds() * 1000.0;
   }
   // Using a double here is safe enough; with 53 bits we can represent
   // durations up to over 280,000 years exactly.  If the units of
   // mValue do not allow us to represent durations of that length,
   // long durations are clamped to the max/min representable value
   // instead of overflowing.
   static inline TimeDuration FromSeconds(double aSeconds) {
     return FromMilliseconds(aSeconds * 1000.0);
   }
   static TimeDuration FromMilliseconds(double aMilliseconds);
   static inline TimeDuration FromMicroseconds(double aMicroseconds) {
     return FromMilliseconds(aMicroseconds / 1000.0);
   }
   static TimeDuration Forever() {
     return FromTicks(INT64_MAX);
   }
   TimeDuration operator+(const TimeDuration& aOther) const {
     return TimeDuration::FromTicks(mValue + aOther.mValue);
   }
   TimeDuration operator-(const TimeDuration& aOther) const {
     return TimeDuration::FromTicks(mValue - aOther.mValue);
   }
   TimeDuration& operator+=(const TimeDuration& aOther) {
     mValue += aOther.mValue;
     return *this;
   }
   TimeDuration& operator-=(const TimeDuration& aOther) {
     mValue -= aOther.mValue;
     return *this;
   }
 private:
   // Block double multiplier (slower, imprecise if long duration) - Bug 853398.
   // If required, use MultDouble explicitly and with care.
   TimeDuration operator*(const double aMultiplier) const MOZ_DELETE;
 public:
   TimeDuration MultDouble(double aMultiplier) const {
     return TimeDuration::FromTicks(static_cast<int64_t>(mValue * aMultiplier));
   }
   TimeDuration operator*(const int32_t aMultiplier) const {
     return TimeDuration::FromTicks(mValue * int64_t(aMultiplier));
   }
   TimeDuration operator*(const uint32_t aMultiplier) const {
     return TimeDuration::FromTicks(mValue * int64_t(aMultiplier));
   }
   TimeDuration operator*(const int64_t aMultiplier) const {
     return TimeDuration::FromTicks(mValue * int64_t(aMultiplier));
   }
   TimeDuration operator/(const int64_t aDivisor) const {
     return TimeDuration::FromTicks(mValue / aDivisor);
   }
   double operator/(const TimeDuration& aOther) const {
     return static_cast<double>(mValue) / aOther.mValue;
   }
   bool operator<(const TimeDuration& aOther) const {
     return mValue < aOther.mValue;
   }
   bool operator<=(const TimeDuration& aOther) const {
     return mValue <= aOther.mValue;
   }
   bool operator>=(const TimeDuration& aOther) const {
     return mValue >= aOther.mValue;
   }
   bool operator>(const TimeDuration& aOther) const {
     return mValue > aOther.mValue;
   }
   bool operator==(const TimeDuration& aOther) const {
     return mValue == aOther.mValue;
   }
   // Return a best guess at the system's current timing resolution,
   // which might be variable.  TimeDurations below this order of
   // magnitude are meaningless, and those at the same order of
   // magnitude or just above are suspect.
   static TimeDuration Resolution();
   // We could define additional operators here:
   // -- convert to/from other time units
   // -- scale duration by a float
   // but let's do that on demand.
   // Comparing durations for equality will only lead to bugs on
   // platforms with high-resolution timers.
 private:
   friend class TimeStamp;
   friend struct IPC::ParamTraits<mozilla::TimeDuration>;
   static TimeDuration FromTicks(int64_t aTicks) {
     TimeDuration t;
     t.mValue = aTicks;
     return t;
   }
   static TimeDuration FromTicks(double aTicks) {
     // NOTE: this MUST be a >= test, because int64_t(double(INT64_MAX))
     // overflows and gives INT64_MIN.
     if (aTicks >= double(INT64_MAX))
       return TimeDuration::FromTicks(INT64_MAX);
     // This MUST be a <= test.
     if (aTicks <= double(INT64_MIN))
       return TimeDuration::FromTicks(INT64_MIN);
     return TimeDuration::FromTicks(int64_t(aTicks));
   }
   // Duration, result is implementation-specific difference of two TimeStamps
   int64_t mValue;
 };
 /**
  * Instances of this class represent moments in time, or a special
  * "null" moment. We do not use the non-monotonic system clock or
  * local time, since they can be reset, causing apparent backward
  * travel in time, which can confuse algorithms. Instead we measure
  * elapsed time according to the system.  This time can never go
  * backwards (i.e. it never wraps around, at least not in less than
  * five million years of system elapsed time). It might not advance
  * while the system is sleeping. If TimeStamp::SetNow() is not called
  * at all for hours or days, we might not notice the passage of some
  * of that time.
  *
  * We deliberately do not expose a way to convert TimeStamps to some
  * particular unit. All you can do is compute a difference between two
  * TimeStamps to get a TimeDuration. You can also add a TimeDuration
  * to a TimeStamp to get a new TimeStamp. You can't do something
  * meaningless like add two TimeStamps.
  *
  * Internally this is implemented as either a wrapper around
  *   - high-resolution, monotonic, system clocks if they exist on this
  *     platform
  *   - PRIntervalTime otherwise.  We detect wraparounds of
  *     PRIntervalTime and work around them.
  *
  * This class is similar to C++11's time_point, however it is
  * explicitly nullable and provides an IsNull() method. time_point
  * is initialized to the clock's epoch and provides a
  * time_since_epoch() method that functions similiarly. i.e.
  * t.IsNull() is equivalent to t.time_since_epoch() == decltype(t)::duration::zero();
  */
 class TimeStamp
 {
 public:
   /**
    * Initialize to the "null" moment
    */
   MOZ_CONSTEXPR TimeStamp() : mValue(0) {}
   // Default copy-constructor and assignment are OK
   /**
    * Return true if this is the "null" moment
    */
   bool IsNull() const { return mValue == 0; }
   /**
    * Return a timestamp reflecting the current elapsed system time. This
    * is monotonically increasing (i.e., does not decrease) over the
    * lifetime of this process' XPCOM session.
    *
    * Now() is trying to ensure the best possible precision on each platform,
    * at least one millisecond.
    *
    * NowLoRes() has been introduced to workaround performance problems of
    * QueryPerformanceCounter on the Windows platform.  NowLoRes() is giving
    * lower precision, usually 15.6 ms, but with very good performance benefit.
    * Use it for measurements of longer times, like >200ms timeouts.
    */
   static TimeStamp Now() { return Now(true); }
   static TimeStamp NowLoRes() { return Now(false); }
   /**
    * Return a timestamp representing the time when the current process was
    * created which will be comparable with other timestamps taken with this
    * class. If the actual process creation time is detected to be inconsistent
    * the @a aIsInconsistent parameter will be set to true, the returned
    * timestamp however will still be valid though inaccurate.
    *
    * @param aIsInconsistent Set to true if an inconsistency was detected in the
    * process creation time
    * @returns A timestamp representing the time when the process was created,
    * this timestamp is always valid even when errors are reported
    */
   static TimeStamp ProcessCreation(bool& aIsInconsistent);
   /**
    * Records a process restart. After this call ProcessCreation() will return
    * the time when the browser was restarted instead of the actual time when
    * the process was created.
    */
   static void RecordProcessRestart();
   /**
    * Compute the difference between two timestamps. Both must be non-null.
    */
   TimeDuration operator-(const TimeStamp& aOther) const {
     MOZ_ASSERT(!IsNull(), "Cannot compute with a null value");
     MOZ_ASSERT(!aOther.IsNull(), "Cannot compute with aOther null value");
     static_assert(-INT64_MAX > INT64_MIN, "int64_t sanity check");
     int64_t ticks = int64_t(mValue - aOther.mValue);
     // Check for overflow.
     if (mValue > aOther.mValue) {
       if (ticks < 0) {
         ticks = INT64_MAX;
       }
     } else {
       if (ticks > 0) {
         ticks = INT64_MIN;
       }
     }
     return TimeDuration::FromTicks(ticks);
   }
   TimeStamp operator+(const TimeDuration& aOther) const {
     MOZ_ASSERT(!IsNull(), "Cannot compute with a null value");
     return TimeStamp(mValue + aOther.mValue);
   }
   TimeStamp operator-(const TimeDuration& aOther) const {
     MOZ_ASSERT(!IsNull(), "Cannot compute with a null value");
     return TimeStamp(mValue - aOther.mValue);
   }
   TimeStamp& operator+=(const TimeDuration& aOther) {
     MOZ_ASSERT(!IsNull(), "Cannot compute with a null value");
     mValue += aOther.mValue;
     return *this;
   }
   TimeStamp& operator-=(const TimeDuration& aOther) {
     MOZ_ASSERT(!IsNull(), "Cannot compute with a null value");
     mValue -= aOther.mValue;
     return *this;
   }
   bool operator<(const TimeStamp& aOther) const {
     MOZ_ASSERT(!IsNull(), "Cannot compute with a null value");
     MOZ_ASSERT(!aOther.IsNull(), "Cannot compute with aOther null value");
     return mValue < aOther.mValue;
   }
   bool operator<=(const TimeStamp& aOther) const {
     MOZ_ASSERT(!IsNull(), "Cannot compute with a null value");
     MOZ_ASSERT(!aOther.IsNull(), "Cannot compute with aOther null value");
     return mValue <= aOther.mValue;
   }
   bool operator>=(const TimeStamp& aOther) const {
     MOZ_ASSERT(!IsNull(), "Cannot compute with a null value");
     MOZ_ASSERT(!aOther.IsNull(), "Cannot compute with aOther null value");
     return mValue >= aOther.mValue;
   }
   bool operator>(const TimeStamp& aOther) const {
     MOZ_ASSERT(!IsNull(), "Cannot compute with a null value");
     MOZ_ASSERT(!aOther.IsNull(), "Cannot compute with aOther null value");
     return mValue > aOther.mValue;
   }
   bool operator==(const TimeStamp& aOther) const {
     // Maybe it's ok to check == with null timestamps?
     MOZ_ASSERT(!IsNull() && "Cannot compute with a null value");
     MOZ_ASSERT(!aOther.IsNull(), "Cannot compute with aOther null value");
     return mValue == aOther.mValue;
   }
   bool operator!=(const TimeStamp& aOther) const {
     // Maybe it's ok to check != with null timestamps?
     MOZ_ASSERT(!IsNull(), "Cannot compute with a null value");
     MOZ_ASSERT(!aOther.IsNull(), "Cannot compute with aOther null value");
     return mValue != aOther.mValue;
   }
   // Comparing TimeStamps for equality should be discouraged. Adding
   // two TimeStamps, or scaling TimeStamps, is nonsense and must never
   // be allowed.
   static NS_HIDDEN_(nsresult) Startup();
   static NS_HIDDEN_(void) Shutdown();
 private:
   friend struct IPC::ParamTraits<mozilla::TimeStamp>;
   friend void StartupTimelineRecordExternal(int, uint64_t);
   TimeStamp(TimeStampValue aValue) : mValue(aValue) {}
   static TimeStamp Now(bool aHighResolution);
   /**
    * Computes the uptime of the current process in microseconds. The result
    * is platform-dependent and needs to be checked against existing timestamps
    * for consistency.
    *
    * @returns The number of microseconds since the calling process was started
    *          or 0 if an error was encountered while computing the uptime
    */
   static uint64_t ComputeProcessUptime();
   /**
    * When built with PRIntervalTime, a value of 0 means this instance
    * is "null". Otherwise, the low 32 bits represent a PRIntervalTime,
    * and the high 32 bits represent a counter of the number of
    * rollovers of PRIntervalTime that we've seen. This counter starts
    * at 1 to avoid a real time colliding with the "null" value.
    *
    * PR_INTERVAL_MAX is set at 100,000 ticks per second. So the minimum
    * time to wrap around is about 2^64/100000 seconds, i.e. about
    * 5,849,424 years.
    *
    * When using a system clock, a value is system dependent.
    */
   TimeStampValue mValue;
 };
 }
 #endif /* mozilla_TimeStamp_h */

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xpcom/ds/TimeStamp.h@b8a032363ba2 (annotated)

xpcom/ds/TimeStamp.h