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

[Min-Yih Hsu via llvm-dev]

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 <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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200716/e791ec4f/attachment-0001.html>