[LLVMdev] RFC - A tool to convert profiles from external profilers

[Justin Bogner]

Diego Novillo <dnovillo at google.com> writes:
> A few weeks ago, I announced the availability of a conversion tool that
> converts Linux Perf sample profiles to LLVM's sample profiler (https://
> github.com/google/autofdo).
>
> I have now ported this tool to the LLVM tree, so it can be made available as
> part of LLVM. I've got a working version, but I still need to massage the code
> to use LLVM's own libraries (logging, flags, etc) and adapt it to LLVM's
> coding guidelines.
>
> I expect to have an initial patch ready in a few days. In the meantime, I
> would like to open it up for some bike shedding on how this tool will
> integrate with LLVM. I cannot guarantee that I'll agree to address all
> feedback myself, but I do want to make sure that the major issues and
> direction are addressed.

Sounds great! Thanks for working on this.

> The tool receives two inputs: a profile file and an ELF executable (built with
> -gmlt). It produces as output a profile in the format expected by
> -fprofile-sample-use.

Do you have a plan in mind on how this will decide between input profile
file formats once there are options other than perf? The two obvious
approaches are autodetection/a format flag or having a different
profile-converter tool per format. I think I'm in favour of the former.

> The requirements on the input is that we must be able to map counts to
> specific source file locations. So, the profiling source must be some kind of
> sample-based instruction execution counter. Anything that keeps track of how
> frequently a specific instruction has been executed.
>
> Using the executable's line table information, the tool maps instruction
> locations back to source locations.
>
> Currently, the native profile reader for Linux Perf and the writer for LLVM's
> profile are part of the same tree. They both reside in llvm/tools/
> llvm-prof-converter.
>
> I would like to support more than Linux Perf, eventually. So, I'm thinking
> that I want to move the LLVM profile writer to llvm/lib/ProfileData and only
> have the various readers under llvm/tools/llvm-prof-converter.

This sounds like the right direction to me. Arguably, the readers could
go into lib/ProfileData as well, leaving only the tool logic under
llvm/tools. I think either split would be fine.

> I don't think I will be working on supporting anything other than Linux Perf
> for now. But if anyone is interested in supporting profilers in other
> platforms, please let me know. I want to make sure the implementation doesn't
> tie itself too much to Linux Perf.
>
> Thanks. Diego.