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

[Graham Yiu via llvm-dev]

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.

Thanks,

Graham Yiu
LLVM Compiler Development
IBM Toronto Software Lab
Office: (905) 413-4077      C2-707/8200/Markham
Email: gyiu at ca.ibm.com



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> 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 C2-707/8200/Markham
  Email: gyiu at ca.ibm.com

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--
 ----------------------------------------------------------- 
 Teresa       Software                                       
 Johnson |   Engineer |    tejohnson at google.com 408-460-2413 
                           |                                 
                                                             



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