Avoid 64-bit divide when sampling timestamp

base/time/time_posix.cc

 // Copyright (c) 2012 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.
 
 #include "base/time/time.h"
 
 #include <stdint.h>
 #include <sys/time.h>
 #include <time.h>
 #if defined(OS_ANDROID) && !defined(__LP64__)
@@ skipping 80 matching lines @@
   uint64_t absolute_micro;
 
   struct timespec ts;
   if (clock_gettime(clk_id, &ts) != 0) {
     NOTREACHED() << "clock_gettime(" << clk_id << ") failed.";
     return base::TimeTicks();
   }
 
   absolute_micro =
       (static_cast<int64>(ts.tv_sec) * base::Time::kMicrosecondsPerSecond) +
-      (static_cast<int64>(ts.tv_nsec) / base::Time::kNanosecondsPerMicrosecond);
+      (static_cast<int64>(ts.tv_nsec / base::Time::kNanosecondsPerMicrosecond));

[jamesr, 2014/09/29 23:37:31]

i'm pretty sure this doesn't actually help, since kNanosecondsPerMicrosecond is:

static const int64 kNanosecondsPerMicrosecond = 1000;

so thanks to type promotion this is still a 64 bit divide.

[Peter Kasting, 2014/09/30 00:22:15]

This.  I'm in the process of cleaning up our 64-bit math around time constants
in various places in the codebase and frankly the right change here is probably
to simply remove both casts entirely.  Neither one will have any effect on the
generated code, no matter how you parenthesize things.

 
   return base::TimeTicks::FromInternalValue(absolute_micro);
 }
 #else  // _POSIX_MONOTONIC_CLOCK
 #error No usable tick clock function on this platform.
 #endif  // _POSIX_MONOTONIC_CLOCK
 #endif  // !defined(OS_MACOSX)
 
 }  // namespace
 
@@ skipping 279 matching lines @@
     result.tv_usec = static_cast<suseconds_t>(Time::kMicrosecondsPerSecond) - 1;
     return result;
   }
   int64 us = us_ - kTimeTToMicrosecondsOffset;
   result.tv_sec = us / Time::kMicrosecondsPerSecond;
   result.tv_usec = us % Time::kMicrosecondsPerSecond;
   return result;
 }
 
 }  // namespace base