[llvm-dev] [RFC] Profile guided section layout

Michael Spencer via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 15 10:55:42 PDT 2017

On Thu, Jun 15, 2017 at 10:08 AM, Tobias Edler von Koch <
tobias at codeaurora.org> wrote:

> Hi Michael,
> This is cool stuff, thanks for sharing!
> On 06/15/2017 11:51 AM, Michael Spencer via llvm-dev wrote:
>> The first is a new llvm pass which uses branch frequency info to get
>> counts for each call instruction and then adds a module flags metatdata
>> table of function -> function edges along with their counts.
>> The second takes the module flags metadata and writes it into a
>> .note.llvm.callgraph section in the object file. This currently just dumps
>> it as text, but could save space by reusing the string table.
> Have you considered reading the profile in the linker and extracting that
> information directly from the profile? The profile should contain call
> sites and their sample counts and you could match these up with relocations
> (calls) in the section?

I did this using IR PGO instead of sample PGO so the profile data can only
be applied in the same place in the pipeline it is generated. Even for
sample based this would be complicated as the linker would actually need to
generate machine basic blocks from sections to be able to accurately match
sample counts to relocations, as there may be cold calls in hot functions.

It may be useful however for the linker to directly accept an externally
generated call graph profile. The current approach can actually do this by
embedding it into an extra object file.

> It doesn't currently work for LTO as the llvm pass needs to be run after
>> all inlining decisions have been made and LTO codegen has to be done with
>> -ffunction-sections.
> So this is just an implementation issue, right? You can make LTO run with
> -ffunction-sections (by setting TargetOptions.FunctionSections=true) and
> insert your pass in the appropriate place in the pipeline.

Yeah, just an implementation issue. Just need to build the pass pipeline
differently for LTO and add a way to do -ffunction-sections in lld.

- Michael Spencer

> Thanks,
> Tobias
> --
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> a Linux Foundation Collaborative Project.
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