utilize @branchHint more

[Rexicon226]

No description provided.

[mlugg]

Note that the LLVM backend doesn't currently do anything with function-level branch hints which aren't .cold. @Rexicon226, if you know of a sane way we could lower likely/unlikely without doing the work of propagating the hint to callsites ourselves, please do open an issue. (Otherwise, this will hopefully be improved once we implement our own optimization passes.)

[rohlem]

if you know of a sane way we could lower likely/unlikely without doing the work of propagating the hint to callsites ourselves

Uninformed drive-by idea, does if (true) count as a branch? Could we wrap the entire function body with if (true) { ... } and place the branch hint on that?
(Optimizations might still only happen when LLVM inlines at the callsite, but I think that's kind of expected - if functions are analyzed in isolation, and we don't manually propagate the branch hint information, then there's no information to act on.)

[mlugg]

All that would do is tell LLVM that the body of the if (true) was likely to be reached; which is obvious, because it's guaranteed! In other words, such a hint would be a nop.

[Rexicon226]

If you know of a sane way we could lower likely/unlikely without doing the work of propagating the hint to callsites ourselves, please do open an issue.

Hmm, I did not consider that. I'll need to think about it. Afaik, no LLVM function attribute other than cold affects this sort of thing, and cold does something different than what unlikely does. I'll leave the code as is since this is LLVM's fault and not the code's :)

Optimizations might still only happen when LLVM inlines at the callsite, but I think that's kind of expected

To add to what mlugg said, this isn't the case. One of the key points of the cold attribute is that it prevents LLVM from inlining the function as well as influencing branch weight. The same thing should apply to an unlikely function, doing

if (x) {
    unlikely();
} else {
    likely();
}

Should clearly fallthrough to the likely function call.