ck_cc: Add branch execution hint support for GCC.

include/ck_cc.h

@@ -77,4 +77,12 @@
 #define CK_CC_CACHELINE
 #endif
 
+#ifndef CK_CC_LIKELY
+#define CK_CC_LIKELY(x)
+#endif
+
+#ifndef CK_CC_UNLIKELY
+#define CK_CC_UNLIKELY(x)
+#endif
+
 #endif /* _CK_CC_H */

include/gcc/ck_cc.h

@@ -75,6 +75,12 @@
 #pragma GCC poison malloc free
 #endif
 
+/*
+ * Branch execution hints.
+ */
+#define CK_CC_LIKELY(x) (__builtin_expect(!!(x), 1))
+#define CK_CC_UNLIKELY(x) (__builtin_expect(!!(x), 0))
+
 /*
  * Some compilers are overly strict regarding aliasing semantics.
  * Unfortunately, in many cases it makes more sense to pay aliasing