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

Min-Yih Hsu via llvm-dev llvm-dev at lists.llvm.org
Thu Jul 16 11:44:59 PDT 2020

On Jul 16, 2020, at 11:04 AM, David Major <dmajor at mozilla.com> wrote:

This sounds a lot like https://bugs.llvm.org/showbug.cgi?id=45050 <https://bugs.llvm.org/showbug.cgi?id=45050>. Yeah I think that bug is the same as this one. I’ve also left a comment there to point out my patch review.

I'm glad to hear you figured out the root cause, thank you!

Best -Min

On Thu, Jul 16, 2020 at 1:55 PM Min-Yih Hsu via llvm-dev <llvm-dev at lists.llvm.org <mailto: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 sampleexe -fprofile-generate_ _ The resulting sampleexe 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 <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 <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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