[llvm-dev] RFC: Reducing Instr PGO size overhead

[Xinliang David Li via llvm-dev]

>
> I think it is reasonable to simply replace the key we currently use with
> MD5(key) for getting a size reduction.  In practice for my use cases, I have
> not observed any of the issues you mentioned under "Large size of overhead
> can limit the usability of PGO greatly", but I can understand that some of
> these issues could become problems in Google's use case. I would personally
> prefer to keep the existing behavior as the default (see below), and have
> MD5(key) as an option.

The problem is that this requires profile format changes. It will be
very messy to support multiple formats in instr-codegen and
instr-runtime.  For compatibility concerns, the reader is taught to
support previous format, but the changes there are isolated (also
expected to be removed in the future).

>
> My primary concern is that if the function name are not kept at all stages,
> then it becomes difficult to analyze the profile data in a standalone way.
> Many times, I have used `llvm-profdata show -all-functions foo.profdata` on
> the resulting profile data and then imported that data into Mathematica for
> analysis.

This is certainly a very valid use case.

>My understanding of your proposal is that `llvm-profdata show
> -all-functions foo.profdata` will not show the actual function names but
> instead MD5 hashes,

Yes.

To support your use case, there are two solutions:

1) user can add -fcoverage-mapping option in the build
2) introduce a new option -fprofile-instr-names that force the
emission of the name sections in the .o file. This is similar to 1),
but no covmap section is needed.

llvm-profdata tool  will be taught to read the name section and attach
function names to the profile records.

Note that with 1) or 2), the user can still benefit from the reduced
profile size.

thanks,

David




>which will make it more difficult for me to do this kind
> of analysis (would require using nm on the original binary, hashing
> everything, etc.).
>
> btw, feel free to attach the patch even if it in a rough state. It can still
> help to clarify the proposal and be a good talking point. Fine-grained patch
> review for caring about the rough parts will happen on llvm-commits; the
> rough parts will not distract the discussion here on llvm-dev.
>
> -- Sean Silva
>
>>
>>
>> thanks,
>>
>> David
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>
>