Use branch prediction hints to make trace events cheaper

base/debug/trace_event.h

 // 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.
 
 // This header file defines the set of trace_event macros without specifying
 // how the events actually get collected and stored. If you need to expose trace
 // events to some other universe, you can copy-and-paste this file as well as
 // trace_event.h, modifying the macros contained there as necessary for the
 // target platform. The end result is that multiple libraries can funnel events
 // through to a shared trace event collector.
@@ skipping 708 matching lines @@
 #define TRACE_EVENT_OBJECT_SNAPSHOT_WITH_ID(category_group, name, id, snapshot) \
     INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_SNAPSHOT_OBJECT, \
         category_group, name, TRACE_ID_DONT_MANGLE(id), TRACE_EVENT_FLAG_NONE,\
         "snapshot", snapshot)
 
 #define TRACE_EVENT_OBJECT_DELETED_WITH_ID(category_group, name, id) \
     INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_DELETE_OBJECT, \
         category_group, name, TRACE_ID_DONT_MANGLE(id), TRACE_EVENT_FLAG_NONE)
 
 #define INTERNAL_TRACE_EVENT_CATEGORY_GROUP_ENABLED_FOR_RECORDING_MODE() \
-    *INTERNAL_TRACE_EVENT_UID(category_group_enabled) & \
+    UNLIKELY(*INTERNAL_TRACE_EVENT_UID(category_group_enabled) & \
         (base::debug::TraceLog::ENABLED_FOR_RECORDING | \
-         base::debug::TraceLog::ENABLED_FOR_EVENT_CALLBACK)
+         base::debug::TraceLog::ENABLED_FOR_EVENT_CALLBACK))
 
 // Macro to efficiently determine if a given category group is enabled.
 #define TRACE_EVENT_CATEGORY_GROUP_ENABLED(category_group, ret) \
     do { \
       INTERNAL_TRACE_EVENT_GET_CATEGORY_INFO(category_group); \
       if (INTERNAL_TRACE_EVENT_CATEGORY_GROUP_ENABLED_FOR_RECORDING_MODE()) { \
         *ret = true; \
       } else { \
         *ret = false; \
       } \
@@ skipping 105 matching lines @@
 
 // Implementation detail: internal macro to create static category.
 // No barriers are needed, because this code is designed to operate safely
 // even when the unsigned char* points to garbage data (which may be the case
 // on processors without cache coherency).
 #define INTERNAL_TRACE_EVENT_GET_CATEGORY_INFO_CUSTOM_VARIABLES( \
     category_group, atomic, category_group_enabled) \
     category_group_enabled = \
         reinterpret_cast<const unsigned char*>(TRACE_EVENT_API_ATOMIC_LOAD( \
             atomic)); \
-    if (!category_group_enabled) { \
+    if (UNLIKELY(!category_group_enabled)) { \
       category_group_enabled = \
           TRACE_EVENT_API_GET_CATEGORY_GROUP_ENABLED(category_group); \
       TRACE_EVENT_API_ATOMIC_STORE(atomic, \
           reinterpret_cast<TRACE_EVENT_API_ATOMIC_WORD>( \
               category_group_enabled)); \
     }
 
 #define INTERNAL_TRACE_EVENT_GET_CATEGORY_INFO(category_group) \
     static TRACE_EVENT_API_ATOMIC_WORD INTERNAL_TRACE_EVENT_UID(atomic) = 0; \
     const unsigned char* INTERNAL_TRACE_EVENT_UID(category_group_enabled); \
@@ skipping 624 matching lines @@
   const char* name_;
   IDType id_;
 
   DISALLOW_COPY_AND_ASSIGN(TraceScopedTrackableObject);
 };
 
 } // namespace debug
 } // namespace base
 
 #endif /* BASE_DEBUG_TRACE_EVENT_H_ */