C++20 standard will include [[likely]] and [[unlikely]] branch prediction attributes.
The latest revision of attribute proposal can be found from http://wg21.link/p0479
The original attribute proposal can be found from http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0479r0.html
Programmers should prefer PGO. Attributes can easily reduce performance if applied incorrectly or they later become incorrect when program changes.