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

[Diego Novillo]

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.

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.

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.

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.
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