diff -r 000000000000 -r 6474c204b198 xpcom/ds/TimeStamp_darwin.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/ds/TimeStamp_darwin.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,195 @@
+/* -*- 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/. */
+
+//
+// Implement TimeStamp::Now() with mach_absolute_time
+//
+// The "tick" unit for mach_absolute_time is defined using mach_timebase_info() which
+// gives a conversion ratio to nanoseconds. For more information see Apple's QA1398.
+//
+// This code is inspired by Chromium's time_mac.cc. The biggest
+// differences are that we explicitly initialize using
+// TimeStamp::Initialize() instead of lazily in Now() and that
+// we store the time value in ticks and convert when needed instead
+// of storing the time value in nanoseconds.
+
+#include <mach/mach_time.h>
+#include <sys/time.h>
+#include <sys/sysctl.h>
+#include <time.h>
+#include <unistd.h>
+
+#include "mozilla/TimeStamp.h"
+#include "nsDebug.h"
+
+// Estimate of the smallest duration of time we can measure.
+static uint64_t sResolution;
+static uint64_t sResolutionSigDigs;
+
+static const uint64_t kNsPerMs   =    1000000;
+static const uint64_t kUsPerSec  =    1000000;
+static const double kNsPerMsd    =    1000000.0;
+static const double kNsPerSecd   = 1000000000.0;
+
+static bool gInitialized = false;
+static double sNsPerTick;
+
+static uint64_t
+ClockTime()
+{
+  // mach_absolute_time is it when it comes to ticks on the Mac.  Other calls
+  // with less precision (such as TickCount) just call through to
+  // mach_absolute_time.
+  //
+  // At the time of writing mach_absolute_time returns the number of nanoseconds
+  // since boot. This won't overflow 64bits for 500+ years so we aren't going
+  // to worry about that possiblity
+  return mach_absolute_time();
+}
+
+static uint64_t
+ClockResolutionNs()
+{
+  uint64_t start = ClockTime();
+  uint64_t end = ClockTime();
+  uint64_t minres = (end - start);
+
+  // 10 total trials is arbitrary: what we're trying to avoid by
+  // looping is getting unlucky and being interrupted by a context
+  // switch or signal, or being bitten by paging/cache effects
+  for (int i = 0; i < 9; ++i) {
+    start = ClockTime();
+    end = ClockTime();
+
+    uint64_t candidate = (start - end);
+    if (candidate < minres)
+      minres = candidate;
+  }
+
+  if (0 == minres) {
+    // measurable resolution is either incredibly low, ~1ns, or very
+    // high.  fall back on NSPR's resolution assumption
+    minres = 1 * kNsPerMs;
+  }
+
+  return minres;
+}
+
+namespace mozilla {
+
+double
+TimeDuration::ToSeconds() const
+{
+  NS_ABORT_IF_FALSE(gInitialized, "calling TimeDuration too early");
+  return (mValue * sNsPerTick) / kNsPerSecd;
+}
+
+double
+TimeDuration::ToSecondsSigDigits() const
+{
+  NS_ABORT_IF_FALSE(gInitialized, "calling TimeDuration too early");
+  // don't report a value < mResolution ...
+  int64_t valueSigDigs = sResolution * (mValue / sResolution);
+  // and chop off insignificant digits
+  valueSigDigs = sResolutionSigDigs * (valueSigDigs / sResolutionSigDigs);
+  return (valueSigDigs * sNsPerTick) / kNsPerSecd;
+}
+
+TimeDuration
+TimeDuration::FromMilliseconds(double aMilliseconds)
+{
+  NS_ABORT_IF_FALSE(gInitialized, "calling TimeDuration too early");
+  return TimeDuration::FromTicks(int64_t((aMilliseconds * kNsPerMsd) / sNsPerTick));
+}
+
+TimeDuration
+TimeDuration::Resolution()
+{
+  NS_ABORT_IF_FALSE(gInitialized, "calling TimeDuration too early");
+  return TimeDuration::FromTicks(int64_t(sResolution));
+}
+
+nsresult
+TimeStamp::Startup()
+{
+  if (gInitialized)
+    return NS_OK;
+
+  mach_timebase_info_data_t timebaseInfo;
+  // Apple's QA1398 suggests that the output from mach_timebase_info
+  // will not change while a program is running, so it should be safe
+  // to cache the result.
+  kern_return_t kr = mach_timebase_info(&timebaseInfo);
+  if (kr != KERN_SUCCESS)
+    NS_RUNTIMEABORT("mach_timebase_info failed");
+
+  sNsPerTick = double(timebaseInfo.numer) / timebaseInfo.denom;
+
+  sResolution = ClockResolutionNs();
+
+  // find the number of significant digits in sResolution, for the
+  // sake of ToSecondsSigDigits()
+  for (sResolutionSigDigs = 1;
+       !(sResolutionSigDigs == sResolution
+         || 10*sResolutionSigDigs > sResolution);
+       sResolutionSigDigs *= 10);
+
+  gInitialized = true;
+
+  return NS_OK;
+}
+
+void
+TimeStamp::Shutdown()
+{
+}
+
+TimeStamp
+TimeStamp::Now(bool aHighResolution)
+{
+  return TimeStamp(ClockTime());
+}
+
+// Computes and returns the process uptime in microseconds.
+// Returns 0 if an error was encountered.
+
+uint64_t
+TimeStamp::ComputeProcessUptime()
+{
+  struct timeval tv;
+  int rv = gettimeofday(&tv, nullptr);
+
+  if (rv == -1) {
+    return 0;
+  }
+
+  int mib[] = {
+    CTL_KERN,
+    KERN_PROC,
+    KERN_PROC_PID,
+    getpid(),
+  };
+  u_int mibLen = sizeof(mib) / sizeof(mib[0]);
+
+  struct kinfo_proc proc;
+  size_t bufferSize = sizeof(proc);
+  rv = sysctl(mib, mibLen, &proc, &bufferSize, nullptr, 0);
+
+  if (rv == -1)
+    return 0;
+
+  uint64_t startTime =
+    ((uint64_t)proc.kp_proc.p_un.__p_starttime.tv_sec * kUsPerSec) +
+    proc.kp_proc.p_un.__p_starttime.tv_usec;
+  uint64_t now = (tv.tv_sec * kUsPerSec) + tv.tv_usec;
+
+  if (startTime > now)
+    return 0;
+
+  return now - startTime;
+}
+
+} // namespace mozilla