[llvm-dev] [profile] Heads Up: Please Double Check the Instrument Level of Your PGO Profile Files

[David Major via llvm-dev]

This sounds a lot like https://bugs.llvm.org/show_bug.cgi?id=45050. I'm
glad to hear you figured out the root cause, thank you!


On Thu, Jul 16, 2020 at 1:55 PM Min-Yih Hsu via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi All,
>
> Recently we discovered a (hidden) performance bug that happened when
> you’re using Full LTO setup + BFD linker to compile first-stage PGO
> executables that were expected to generate IR level profile files. That is,
> compiling an executable using following commands:
> ```
> clang -flto -fprofile-generate -c sample.c -o sample.c.bc.o
> clang -flto sample.c.bc.o -o sample_exe -fprofile-generate
> ```
> The  resulting sample_exe executable will *always* emit Frontend level
> profile files despite the presence of the -fprofile-generate flag, which
> should make the executable generate IR level profile files. Other
> prerequisites to reproduce this problem including:
> • Using (latest version of) BFD linker
> • Using LLVM’s own LTO linker plugin (i.e. LLVMgold.so)
> • Using LLVM’s compiler-rt as the runtime library
> • On a Linux / BSD platform
> This problem has been confirmed *not* to happen with gold and LLD
> linkers. Impacts on usages on Windows platform remain unknown.
>
> I just cooked a patch for this bug: https://reviews.llvm.org/D83967
> Basically the solution (and the root cause) is similar to this patch
> several years ago: https://reviews.llvm.org/D34797
>
> The intriguing (or annoying) part of this bug is that you’ll never find
> the issue (since it neither crashes nor causes significant performance
> regression) unless you take a look into the generated profile files. So I
> just want to put a heads up here, though I know most of the folks here are
> moving away from using BFD linkers.
>
> Best,
> -Min
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