[llvm-dev] PGO information at LTO/thinLTO link step

Tue Oct 3 14:34:42 PDT 2017

On Tue, Oct 3, 2017 at 2:15 PM, Teresa Johnson via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
>
> On Tue, Oct 3, 2017 at 2:10 PM, Graham Yiu <gyiu at ca.ibm.com> wrote:
>
>> Thanks Easwaran. This is what we've observed as well, where the old PM
>> inliner was only looking hot/cold callee information, which have
>> signficantly smaller boosts/penalties compared to callsite information.
>>
>> Teresa, do you know if there is some documentation/video/presentation on
>> how PGO information is represented in LLVM and what information is passed
>> via the IR? I'm finding some difficulty in getting the big picture via the
>> code.
>>
>
In a nutshell, there are two main types of profile data (BB/edge related).

1) branch probability
2) function entry count.

PGO instrumentation actually collects BB count. During profile annotation
pass, the branch probability (weights) profile data is computed and
annotated to the IR (the branch instructions). After profile annotation,
the block count information is dropped except for the function entry count
which is annotated to the function entry.

The BPI/BFI pass can recompute the block frequency data from the branch
probability info. The BB frequency is intra-function and does not have
global meaning. To recompute the profile count for a BB, function entry
profile count is used, which is multiplied by the ratio of the BB freq and
entry freq.

This scheme works really well except for CFG with irreducible loops, for
which BFI's frequency propagation pass can not reconstruct the
frequency/count properly.  Hiroshi is working on a patch to fix the problem.

For AutoFDO, there is also a sample count profile for callsites which is
needed because function entry may not have any samples.

The value profiler also has its meta data for value histograms of a given
value site.    At module level, while program profile summary data is also
represented so that optimization passes can query for global hotness info.

David

>
> The documentation I am aware of is in the Language Ref and a subpage
> linked from here:
> https://llvm.org/docs/LangRef.html#prof-metadata
>
> If that doesn't help let me know and I can point you to someone who would
> know (if I can't answer it myself).
>
> Teresa
>
>>
>>
>> Graham Yiu
>> LLVM Compiler Development
>> IBM Toronto Software Lab
>> Office: (905) 413-4077 C2-707/8200/Markham
>> Email: gyiu at ca.ibm.com
>>
>> [image: Inactive hide details for Teresa Johnson ---10/03/2017 05:00:11
>> PM---On Tue, Oct 3, 2017 at 1:55 PM, Easwaran Raman <eraman at goo]Teresa
>> Johnson ---10/03/2017 05:00:11 PM---On Tue, Oct 3, 2017 at 1:55 PM,
>> Easwaran Raman <eraman at google.com> wrote: >
>>
>> From: Teresa Johnson <tejohnson at google.com>
>> To: Easwaran Raman <eraman at google.com>
>> Cc: Graham Yiu <gyiu at ca.ibm.com>, llvm-dev <llvm-dev at lists.llvm.org>
>> Date: 10/03/2017 05:00 PM
>>
>> Subject: Re: [llvm-dev] PGO information at LTO/thinLTO link step
>> ------------------------------
>>
>>
>>
>>
>>
>> On Tue, Oct 3, 2017 at 1:55 PM, Easwaran Raman <*eraman at google.com*
>> <eraman at google.com>> wrote:
>>
>>
>>
>>    On Tue, Oct 3, 2017 at 1:46 PM, Teresa Johnson via llvm-dev <
>>    *llvm-dev at lists.llvm.org* <llvm-dev at lists.llvm.org>> wrote:
>>
>>
>>    On Tue, Oct 3, 2017 at 1:38 PM, Graham Yiu <*gyiu at ca.ibm.com*
>>    <gyiu at ca.ibm.com>> wrote:
>>       Hi Teresa,
>>
>>       Actually, enabling the new pass manager manually seems to have
>>       solved this issue, so this problem is only valid for the old pass manager.
>>
>>
>>    It should not be an issue in the old PM either - the callsite hotness
>>    is passed via IR.
>>    More precisely, the function entry counts are passed via IR. With the
>>    old PM,  we don't have callsite hotness information, but callee's entry
>>    count is used to boost the threshold.
>>
>>
>> Thanks for the clarification. (But essentially there should be no
>> difference between the profile info in the IR in the compile step vs the
>> link aka ThinLTO backend steps - the inliner in both cases is working off
>> the same profile info in the IR)
>>
>>
>>    As David mentioned, the new PM inliner does a better job of updating
>>    call hotness after inlining, but it should be there (some things might look
>>    hotter than then should, which seems to be the opposite of the problem you
>>    are hitting). Can you send me a reproducer with the old PM?
>>
>>    Teresa
>>
>>
>>       Thanks,
>>
>>       Graham Yiu
>>       LLVM Compiler Development
>>       IBM Toronto Software Lab
>>       Office: *(905) 413-4077* <(905)%20413-4077> C2-707/8200/Markham
>>       Email: *gyiu at ca.ibm.com* <gyiu at ca.ibm.com>
>>
>>       [image: Inactive hide details for Teresa Johnson ---10/03/2017
>>       04:18:17 PM---On Tue, Oct 3, 2017 at 12:54 PM, Graham Yiu via llvm-dev <]Teresa
>>       Johnson ---10/03/2017 04:18:17 PM---On Tue, Oct 3, 2017 at 12:54 PM, Graham
>>       Yiu via llvm-dev < *llvm-dev at lists.llvm.org*
>>       <llvm-dev at lists.llvm.org>> wrote:
>>
>>       From: Teresa Johnson <*tejohnson at google.com* <tejohnson at google.com>
>>       >
>>       To: Graham Yiu <*gyiu at ca.ibm.com* <gyiu at ca.ibm.com>>
>>       Cc: llvm-dev <*llvm-dev at lists.llvm.org* <llvm-dev at lists.llvm.org>>
>>       Date: 10/03/2017 04:18 PM
>>       Subject: Re: [llvm-dev] PGO information at LTO/thinLTO link step
>>       ------------------------------
>>
>>
>>
>>
>>
>>
>>       On Tue, Oct 3, 2017 at 12:54 PM, Graham Yiu via llvm-dev <
>>       *llvm-dev at lists.llvm.org* <llvm-dev at lists.llvm.org>> wrote:
>>          Hello,
>>
>>             My team and I noticed that callsite hotness information is
>>             not preserved from compile to link step with LTO/thinLTO enabled. As a
>>             result, the link step inlining pass remains conservative when inlining
>>             callsites known to be hot (ie. without the 'HotCallSiteThreshold' which is
>>             set at 3000 by default). There are likely many cross-module inlining
>>             opportunities lost this way, and diminishes the benefit of using
>>             LTO/thinLTO+PGO together.
>>
>>
>>       The callsite hotness is passed via the IR, so it should be there
>>       in the LTO/ThinLTO backends (during the link step). Can you provide a
>>       reproducer where that isn't happening?
>>       Teresa
>>
>>
>>             In general, does LLVM pass profiling information through the
>>             IR to the link step other than branch probabilities and function entry
>>             counts? If not, are there plans to do so in the future? For inlining
>>             specifically, perhaps we can mark callsites with hot/cold attributes during
>>             compile step to ensure LTO inlining will give appropriate threshold
>>             bonuses/penalties.
>>
>>             Any thoughts/insights/comments would be appreciated.
>>
>>             Cheers,
>>
>>             Graham Yiu
>>             LLVM Compiler Development
>>             IBM Toronto Software Lab
>>             Office: *(905) 413-4077* <(905)%20413-4077>
>>             C2-707/8200/Markham
>>             Email: *gyiu at ca.ibm.com* <gyiu at ca.ibm.com>
>>
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>>
>>
>>       --
>>       Teresa Johnson |  Software Engineer |  *tejohnson at google.com*
>>       <tejohnson at google.com> |  *408-460-2413* <(408)%20460-2413>
>>
>>
>>
>>
>>
>>    --
>>    Teresa Johnson |  Software Engineer |  *tejohnson at google.com*
>>    <tejohnson at google.com> |  *408-460-2413* <(408)%20460-2413>
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>> --
>> Teresa Johnson |  Software Engineer |  *tejohnson at google.com*
>> <tejohnson at google.com> |  408-460-2413 <(408)%20460-2413>
>>
>>
>>
>
>
> --
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
> 408-460-2413 <(408)%20460-2413>
>
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