aaron.ballman added a comment. In D134456#3827410 <https://reviews.llvm.org/D134456#3827410>, @dexonsmith wrote:
> In D134456#3827377 <https://reviews.llvm.org/D134456#3827377>, @aaron.ballman > wrote: > >> Ah, I see, thank you for clarifying! Does that seem like a generally useful >> attribute to you? (It seems like it could be, but this isn't my area of >> expertise.) > > Well, "generally useful" might be a stretch. Probably most users wouldn't > want/need this. But if a user wants fine-grained control of the optimizer > (probably not achievable, really), then turning off PGO seems like a knob > they might want. Heh, you interpreted me properly. :-D I mostly meant "does this attribute seem like it'd be usable in other circumstances or is it likely to be really specific to just this situation? > On the other hand, maybe `[[nopgo]]` is all that the safety-critical code > should be using. IIRC, `[[likely]]` is really saying "hint: the other path is > cold", and the optimizer pessimists the other path. Perhaps the > safety-critical crowd just wants to turn off all pessimization; if so, > they'll find that adding `[[likely]]` to the must-fail-quickly error path > will do bad things to the non-error path. If "bad things" is "make the non-error path slow", my understanding is that's exactly their expectations. >> Another thought I had is: when PGO is enabled, can we diagnose when PGO >> countermands an explicit annotation? Something to at least alert the user >> "hey, look over here, this suggestion was wrong" so they have more of a >> chance of knowing something is up? > > Seems pretty noisy, much like when the inliner countermands an `inline` hint. > Maybe an optimization remark would be appropriate, allowing advanced users to > selectively enable them in a critical section when debugging a performance > issue? Hmmm, on the one hand, any optimization hint is really an advanced user feature and so the opt remark might make sense. On the other hand, when I posted about `[[likely]]` and PGO on Twitter (not exactly what I'd call a scientific poll, take with an ocean of salt!), multiple people were surprised and expected PGO to honor the attribute, which suggests users may want this information. I don't have a good feel for what the user expectations are here, so I think an opt remark would be the most conservative way to start, and if we find that it's too hidden and non-expert users really need warnings, we can add a warning at that point. WDYT? Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D134456/new/ https://reviews.llvm.org/D134456 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
