Based on example from Jacksonville’18 ISO C++ Report the syntax is correct, but it seems that it is not implemented yet: if (a>b) [[likely]] { 10.6.6 Likelihood attributes [dcl.attr.likelihood] draft
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.
Imagine the assembly code that would be generated from: if (__builtin_expect(x, 0)) { foo(); … } else { bar(); … } I guess it should be something like: cmp $x, 0 jne _foo _bar: call bar … jmp after_if _foo: call foo … after_if: You can see that the instructions are arranged in such an … Read more
They are hint to the compiler to emit instructions that will cause branch prediction to favour the “likely” side of a jump instruction. This can be a big win, if the prediction is correct it means that the jump instruction is basically free and will take zero cycles. On the other hand if the prediction … Read more