Add RBS_LIKELY and RBS_UNLIKELY

Author: soutaro

include/rbs/defines.h

@@ -32,6 +32,24 @@
 #define RBS_ATTRIBUTE_FORMAT(string_index, argument_index)
 #endif
 
+/**
+ * Support RBS_LIKELY and RBS_UNLIKELY to help the compiler optimize its
+ * branch predication.
+ */
+#if defined(__GNUC__) || defined(__clang__)
+/** The compiler should predicate that this branch will be taken. */
+#define RBS_LIKELY(x) __builtin_expect(!!(x), 1)
+
+/** The compiler should predicate that this branch will not be taken. */
+#define RBS_UNLIKELY(x) __builtin_expect(!!(x), 0)
+#else
+/** Void because this platform does not support branch prediction hints. */
+#define RBS_LIKELY(x) (x)
+
+/** Void because this platform does not support branch prediction hints. */
+#define RBS_UNLIKELY(x) (x)
+#endif
+
 /**
  * We use -Wimplicit-fallthrough to guard potentially unintended fall-through between cases of a switch.
  * Use RBS_FALLTHROUGH to explicitly annotate cases where the fallthrough is intentional.