[all-commits] [llvm/llvm-project] 18cdfa: [SampleFDO] Stale profile call-graph matching (#95...

[Lei Wang via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 18cdfa72e046a40deeee4372ee98602fd1a65a94
      https://github.com/llvm/llvm-project/commit/18cdfa72e046a40deeee4372ee98602fd1a65a94
  Author: Lei Wang <wlei at fb.com>
  Date:   2024-07-17 (Wed, 17 Jul 2024)

  Changed paths:
    M llvm/include/llvm/ProfileData/SampleProf.h
    M llvm/include/llvm/Transforms/IPO/SampleProfileMatcher.h
    M llvm/include/llvm/Transforms/Utils/SampleProfileLoaderBaseImpl.h
    M llvm/lib/ProfileData/SampleProf.cpp
    M llvm/lib/Transforms/IPO/SampleProfile.cpp
    M llvm/lib/Transforms/IPO/SampleProfileMatcher.cpp
    M llvm/test/Other/new-pm-thinlto-postlink-samplepgo-defaults.ll
    M llvm/test/Other/new-pm-thinlto-prelink-samplepgo-defaults.ll
    A llvm/test/Transforms/SampleProfile/Inputs/pseudo-probe-stale-profile-renaming-recursive.prof
    A llvm/test/Transforms/SampleProfile/Inputs/pseudo-probe-stale-profile-renaming.prof
    M llvm/test/Transforms/SampleProfile/non-probe-stale-profile-matching.ll
    M llvm/test/Transforms/SampleProfile/pseudo-probe-stale-profile-matching-LCS.ll
    A llvm/test/Transforms/SampleProfile/pseudo-probe-stale-profile-renaming-recursive.ll
    A llvm/test/Transforms/SampleProfile/pseudo-probe-stale-profile-renaming.ll

  Log Message:
  -----------
  [SampleFDO] Stale profile call-graph matching (#95135)

Profile staleness could be due to function renaming. Given that sample
profile loader relies on exact string matching, a trivial change in the
function signature( such as `int foo()` --> `long foo()` ) can make the
mangled name different, the function profile(including all nested
children profile) becomes unavailable.

This patch introduces stale profile call-graph level matching, targeting
at identifying the trivial function renaming and reusing the old
function profile.

Some noteworthy details:

1. Extend the LCS based CFG level matching to identify new function. 
- Extend to match function and profile have different name instead of
the exact function name matching. This leverages LCS, i.e during the
finding of callsite anchor matching, when two function name are
different, try matching the functions instead of return.
- In LCS, the equal function check is replaced by
`functionMatchesProfile`.
- Only try matching functions that are new functions(neither appears on
each side). This reduces the matching scope as we don't need to match
the originally matched function.
2.  Determine the matching by call-site anchor similarity check.
- A new function `functionMatchesProfile(IRFunc, ProfFunc)` is used to
check the renaming for the possible <IRFunc, ProfFunc> pair, use the
LCS(diff) matching to compute the equal set and we define: `Similarity =
|equalSet * 2| / (|A| + |B|)`. The profile name is marked as renamed if
the similarity is above a
threshold(`-func-profile-similarity-threshold`)

3.  Process the matching in top-down function order 
- when a caller's is done matching, the new function names are saved for
later use, using top-down order will maximize the reused results.
- `ProfileNameToFuncMap` is used to save or cache the matching result.
4. Update the original profile at the end using `ProfileNameToFuncMap`.

5. Added a new switch --salvage-unused-profile to control this, default
is false.

Verified on one Meta's internal big service, confirmed 90%+ of the found
renaming pair is good. (There could be incorrect renaming pair if the
num of the anchor is small, but checked that those functions are simple
cold function)



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