[PATCH] D86559: [Sema, CodeGen] Allow [[likely]] and [[unlikely]] on labels

Tue Sep 1 10:41:30 PDT 2020

staffantj added a comment.

In D86559#2250034 <https://reviews.llvm.org/D86559#2250034>, @aaron.ballman wrote:

> In D86559#2243575 <https://reviews.llvm.org/D86559#2243575>, @staffantj wrote:
>
>> As one of the SG14 industry members driving this, I'm firmly in the camp that this is what we're expecting. In the first case the 1/2 case are "neutral". This is a very explicit, and local, marker. Anything else makes it so vague as to be unusable for fine tuned code.
>
> Thank you for chiming in!
>
>> I should also make the point that we are not talking about a feature that is expected, or indeed should be, used by anyone other than someone with an exceedingly good understanding of what is going on.
>
> That doesn't mean we should design something that's really hard to use for average folks too.
>
>> This is not a "teach everyone about it, it's safe" feature. It's there to produce a very fine-grained control in those cases where it really matters, and where profiling-guided optimizations would produce exactly the wrong result. Using this feature should be an automatic "is this needed" question in a code review. It is needed sometimes, just rarely.
>
> +1 but I would point out that when PGO is enabled, this attribute is ignored, so it's an either/or feature. Either you get tuned optimizations, or you get to guess at them, but the current design doesn't let you mix and match. Do see that as being a major issue with the design?

We did have discussions in SG14 about the possible need for "inverse PGO" optimization passes, but we didn't have the compiler dev expertise to know if we were on a good track or not. From the 8 years I've spent working on this kind of code, I've never mixed the two, since the interactions are just too unpredictable. What I will frequently do however, is compare benchmarks between PGO generated code, and our hand-specified optimizations. Sometimes the PGO detects interesting program flow that a mere human brain could not have envisioned, which we can then take advantage of (or de-prioritize) as needed.

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