Avoid twice clock reading

base/trace_event/trace_event_impl.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/trace_event/trace_event_impl.h"
 
 #include <algorithm>
 
 #include "base/base_switches.h"
 #include "base/bind.h"
@@ skipping 1896 matching lines @@
 
   AutoThreadLocalBoolean thread_is_in_trace_event(&thread_is_in_trace_event_);
 
   DCHECK(name);
   DCHECK(!timestamp.is_null());
 
   if (flags & TRACE_EVENT_FLAG_MANGLE_ID)
     id ^= process_id_hash_;
 
   TimeTicks offset_event_timestamp = OffsetTimestamp(timestamp);
-  TimeTicks now = OffsetNow();
+  TimeTicks now = flags & TRACE_EVENT_FLAG_EXPLICIT_TIMESTAMP ?
+      OffsetNow() : offset_event_timestamp;
   TimeTicks thread_now = ThreadNow();
 
   ThreadLocalEventBuffer* thread_local_event_buffer = NULL;
   // A ThreadLocalEventBuffer needs the message loop
   // - to know when the thread exits;
   // - to handle the final flush.
   // For a thread without a message loop or the message loop may be blocked, the
   // trace events will be added into the main buffer directly.
   if (!thread_blocks_message_loop_.Get() && MessageLoop::current()) {
     thread_local_event_buffer = thread_local_event_buffer_.Get();
@@ skipping 100 matching lines @@
     if (event_callback) {
       event_callback(offset_event_timestamp,
                      phase == TRACE_EVENT_PHASE_COMPLETE ?
                          TRACE_EVENT_PHASE_BEGIN : phase,
                      category_group_enabled, name, id,
                      num_args, arg_names, arg_types, arg_values,
                      flags);
     }
   }
 
-  // Use |now| instead of |offset_event_timestamp| to compute overhead, because
-  // event timestamp may be not the real time that we started to add the event
-  // (e.g. event with zero timestamp or that was generated some time ago).
   if (thread_local_event_buffer)
     thread_local_event_buffer->ReportOverhead(now, thread_now);
 
   return handle;
 }
 
 // May be called when a COMPELETE event ends and the unfinished event has been
 // recycled (phase == TRACE_EVENT_PHASE_END and trace_event == NULL).
 std::string TraceLog::EventToConsoleMessage(unsigned char phase,
                                             const TimeTicks& timestamp,
@@ skipping 539 matching lines @@
 }
 
 ScopedTraceBinaryEfficient::~ScopedTraceBinaryEfficient() {
   if (*category_group_enabled_) {
     TRACE_EVENT_API_UPDATE_TRACE_EVENT_DURATION(category_group_enabled_,
                                                 name_, event_handle_);
   }
 }
 
 }  // namespace trace_event_internal