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

[Teresa Johnson via llvm-dev]

Great! Cc'ing Sriraman who implemented something like this several years
back on our gcc google branch. We may want to utilize your work in the
gold-plugin as well.


On Thu, Jun 15, 2017 at 10:55 AM, Michael Spencer via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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.
>

Also, doing this in the LLVM IR is best IMO because we can share the work
between different linkers.


>
>
>>
>>
>> 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.
>

Yeah I implemented support to pass that down for the gold-plugin (r284140),
I guess you need to do something similar for 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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>


-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |  408-460-2413
<(408)%20460-2413>
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