[PATCH] D67561: [SampleFDO] minimize performance impact when profile-sample-accurate is enabled

[Wei Mi via Phabricator via llvm-commits]

wmi created this revision.
wmi added reviewers: davidxl, mtrofin.
Herald added subscribers: kristof.beyls, aprantl.
Herald added a project: LLVM.

We can save memory and reduce binary size significantly by enabling ProfileSampleAccurate. However when ProfileSampleAccurate is true, function without sample will be regarded as cold and this could potentially cause performance regression.

To minimize the potential negative performance impact, we want to be a little conservative here saying if a function shows up in the profile, no matter as outline function, inline instance or call targets, treat the function as not being cold. This will handle the cases such as most callsites of a function are inlined in sampled binary (thus outline copy don't get any sample) but not inlined in current build (because of source code drift, imprecise debug information, or the callsites are all cold individually but not cold accumulatively...), so that the outline function showing up as cold in sampled binary will actually not be cold after current build. After the change, such function will be treated as not cold even profile-sample-accurate is enabled.

At the same time we lower the hot criteria of callsiteIsHot check when profile-sample-accurate is enabled. callsiteIsHot is used to determined whether a callsite is hot and qualified for early inlining. When profile-sample-accurate is enabled, functions without profile will be regarded as cold and much less inlining will happen in CGSCC inlining pass, so we can worry less about size increase and be aggressive to allow more early inlining to happen for warm callsites and it is helpful for performance overall.


Repository:
  rL LLVM

https://reviews.llvm.org/D67561

Files:
  include/llvm/ProfileData/SampleProfReader.h
  lib/Transforms/IPO/SampleProfile.cpp
  test/Transforms/SampleProfile/Inputs/profsampleacc.extbinary.afdo
  test/Transforms/SampleProfile/profile-sample-accurate.ll

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