Avoid twice clock reading

Add TRACE_EVENT_FLAG_EXPLICIT_TIMESTAMP which is set when an explicit
timestamp is specified for the trace event. Only re-read clock if the
flag is set.

BUG=457441

Review URL: https://codereview.chromium.org/928563003

Cr-Commit-Position: refs/heads/master@{#317618}

2 files changed, 7 insertions, 6 deletions

diff --git a/base/trace_event/trace_event.h b/base/trace_event/trace_event.h
index 07a2afa..e12d8f4 100644
--- a/base/trace_event/trace_event.h
+++ b/base/trace_event/trace_event.h
@@ -1008,7 +1008,8 @@ TRACE_EVENT_API_CLASS_EXPORT extern \
             phase, INTERNAL_TRACE_EVENT_UID(category_group_enabled), \
             name, trace_event_trace_id.data(), \
             thread_id, base::TimeTicks::FromInternalValue(timestamp), \
-            trace_event_flags, ##__VA_ARGS__); \
+            trace_event_flags | TRACE_EVENT_FLAG_EXPLICIT_TIMESTAMP, \
+            ##__VA_ARGS__); \
       } \
     } while (0)
 
@@ -1045,9 +1046,11 @@ TRACE_EVENT_API_CLASS_EXPORT extern \
 #define TRACE_EVENT_FLAG_HAS_ID       (static_cast<unsigned char>(1 << 1))
 #define TRACE_EVENT_FLAG_MANGLE_ID    (static_cast<unsigned char>(1 << 2))
 #define TRACE_EVENT_FLAG_SCOPE_OFFSET (static_cast<unsigned char>(1 << 3))
+#define TRACE_EVENT_FLAG_SCOPE_EXTRA  (static_cast<unsigned char>(1 << 4))
+#define TRACE_EVENT_FLAG_EXPLICIT_TIMESTAMP (static_cast<unsigned char>(1 << 5))
 
 #define TRACE_EVENT_FLAG_SCOPE_MASK   (static_cast<unsigned char>( \
-    TRACE_EVENT_FLAG_SCOPE_OFFSET | (TRACE_EVENT_FLAG_SCOPE_OFFSET << 1)))
+    TRACE_EVENT_FLAG_SCOPE_OFFSET | TRACE_EVENT_FLAG_SCOPE_EXTRA))
 
 // Type values for identifying types in the TraceValue union.
 #define TRACE_VALUE_TYPE_BOOL         (static_cast<unsigned char>(1))
diff --git a/base/trace_event/trace_event_impl.cc b/base/trace_event/trace_event_impl.cc
index 9ca0ddc..445cb6d 100644
--- a/base/trace_event/trace_event_impl.cc
+++ b/base/trace_event/trace_event_impl.cc
@@ -1914,7 +1914,8 @@ TraceEventHandle TraceLog::AddTraceEventWithThreadIdAndTimestamp(
     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;
@@ -2035,9 +2036,6 @@ TraceEventHandle TraceLog::AddTraceEventWithThreadIdAndTimestamp(
     }
   }
 
-  // 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);