Well, it's a C macro in Apache Thrift code base [1]

#ifdef __GNUC__

#define TDB_LIKELY(val) (__builtin_expect((val), 1))

#define TDB_UNLIKELY(val) (__builtin_expect((val), 0))

#else

#define TDB_LIKELY(val) (val)

#define TDB_UNLIKELY(val) (val)

#endif

This code is beautiful when it deal with CPU cache-line effects to speed up your program.

[1] https://github.com/apache/thrift/blob/647501693bd14256df8839...