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

[Teresa Johnson via llvm-dev]

On Tue, Oct 3, 2017 at 1:38 PM, Graham Yiu <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. 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 C2-707/8200/Markham
> Email: 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> wrote:
>
> From: Teresa Johnson <tejohnson at google.com>
> To: Graham Yiu <gyiu at ca.ibm.com>
> Cc: llvm-dev <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 |  408-460-2413
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