[PATCH] D152399: [CodeGen] Fine tune MachineFunctionSplitPass (MFS) for FSAFDO.

[Han Shen via Phabricator via llvm-commits]

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The original MFS work D85368 <https://reviews.llvm.org/D85368> shows good performance improvement with Instrumented FDO. However, AutoFDO or Flow-Sensitive AutoFDO (FSAFDO) does not show performance gain. This is mainly caused by a less accurate profile compared to the iFDO profile.

For the past few months, we have been working to improve FSAFDO quality, like in  D145171 <https://reviews.llvm.org/D145171>. Taking advantage of this improvement, MFS now shows performance improvements over FSAFDO profiles.

That being said, 2 minor changes need to be made, 1) An FS-AutoFDO profile generation pass needs to be added right before MFS pass and an FSAFDO profile load pass is needed when FS-AutoFDO is enabled and the MFS flag is present. 2) MFS only applies to hot functions, because we believe (and experiment also shows) FS-AutoFDO is more accurate about functions that have plenty of samples than those with no or very few samples.

With this patch, we see a 1.2% performance improvement in clang benchmark, 0.9% QPS improvement in our internal search benchmark, and 3%-5% improvement in internal storage benchmark.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D152399

Files:
  llvm/include/llvm/CodeGen/Passes.h
  llvm/lib/CodeGen/MachineFunctionSplitter.cpp
  llvm/lib/CodeGen/TargetPassConfig.cpp
  llvm/test/CodeGen/X86/fsafdo_mfs_test.ll

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