Use branch prediction hints to make trace events cheaper

base/compiler_specific.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.
 
 #ifndef BASE_COMPILER_SPECIFIC_H_
 #define BASE_COMPILER_SPECIFIC_H_
 
 #include "build/build_config.h"
 
 #if defined(COMPILER_MSVC)
@@ skipping 200 matching lines @@
 
 // Macro useful for writing cross-platform function pointers.
 #if !defined(CDECL)
 #if defined(OS_WIN)
 #define CDECL __cdecl
 #else  // defined(OS_WIN)
 #define CDECL
 #endif  // defined(OS_WIN)
 #endif  // !defined(CDECL)
 
+// Macro for hinting that an expression is likely to be true.
+#if !defined(LIKELY)
+#if defined(COMPILER_GCC)
+#define LIKELY(x) __builtin_expect(!!(x), 1)
+#else
+#define LIKELY(x) (x)
+#endif  // defined(COMPILER_GCC)
+#endif  // !defined(LIKELY)
+
+// Macro for hinting that an expression is likely to be false.
+#if !defined(UNLIKELY)
+#if defined(COMPILER_GCC)
+#define UNLIKELY(x) __builtin_expect(!!(x), 0)
+#else
+#define UNLIKELY(x) (x)
+#endif  // defined(COMPILER_GCC)
+#endif  // !defined(UNLIKELY)
+
 #endif  // BASE_COMPILER_SPECIFIC_H_