To my knowledge this does not have any direct impact to the CPU branch prediction mechanism. If that had been the case then we would at least have some x86-(64) instruction to manipulate with the BP. If I write a quick example such as https://godbolt.org/z/j7Y81j5fe, I also see no such instruction in the generated output.

But what likely/unlikely mechanism can do is that it can serve as a hint to the compiler which the compiler can then use to generate more optimal code layout. For example, if we provide the compiler with likely/unlikely hints in our code, compiler will try to use that hint to stitch together the more probable code paths first. In theory this approach should result with better utilization of CPU instruction cache and thus it may lead to better performance.

There are no ways to hint the branch predictor on some common CPUs. I think Intel says it's fully dynamic and has no static predictions.

If you want to cause a pipeline stall, there's usually serializing instructions like cpuid/eieio.

I believe intel used to always predict forward branches not taken and backward branches always taken if it didn’t have any history.

Regardless, you can at least lay out code in a way that heavily favors one branch over the other, and potentially optimize for one branch (I.e. give one branch very expensive prelude/exit to make another branch very cheap). The last thing I’ve seen compilers greatly struggle with though…

20 years of "progress" and hey, C++20 is gaining the likely attribute.