[PATCH] D152328: InstrProf - don't emit 64 bit atomic operations on 32 bit platforms

[Ellis Hoag via Phabricator via llvm-commits]

ellis added a comment.

In D152328#4436415 <https://reviews.llvm.org/D152328#4436415>, @SeanMollet wrote:

> In D152328#4436337 <https://reviews.llvm.org/D152328#4436337>, @ellis wrote:
>
>> Even with a static flag, that doesn't stop the warning from showing up from each clang invocation if you aren't using LTO.
>
> True. At least it would only be once per file. Without a flag, it would throw at least twice per file. I don't know the call pattern into PGO, so maybe more than that. At least twice though, which seems worth a flag. Open to suggestions, I only want to do the "right" thing..
>
>> By the way, why not simply use `-fprofile-update=single` (which I think will set `Options.Atomic = false`) for your use case? Now that I look closely, all the atomic flags seem to be false by default. Where are you seeing atomic instructions emitted for you?
>
> Forgive me if I get the exact wording of this wrong, this is the my first foray into LLVM.
>
> Because the intrinsics aren't being emitted by the backend. They're being converted to an atomic directly by InstrProfiling.cpp which doesn't look at that option.
>
> PassBuilderPipelines.cpp line 787 effectively sets the default for InstrProfiling.cpp to true. DoCounterPromotion=true uses atomics.
>
>   // Add the profile lowering pass.
>   InstrProfOptions Options;
>   if (!ProfileFile.empty())
>     Options.InstrProfileOutput = ProfileFile;
>   // Do counter promotion at Level greater than O0.
>   Options.DoCounterPromotion = true;
>   Options.UseBFIInPromotion = IsCS;
>   MPM.addPass(InstrProfiling(Options, IsCS));

I still don't see how `DoCounterPromotion=true` emits atomics. As far as I can tell, all atomic instructions emitted in `InstrProfiling.cpp` are guarded behind an atomic flag that defaults to false.


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