[all-commits] [llvm/llvm-project] f6703a: [ctxprof] don't inline weak symbols after instrume...

[Mircea Trofin via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: f6703a4ff56972ed6bd1693cdb51cc3bd5848582
      https://github.com/llvm/llvm-project/commit/f6703a4ff56972ed6bd1693cdb51cc3bd5848582
  Author: Mircea Trofin <mtrofin at google.com>
  Date:   2025-02-26 (Wed, 26 Feb 2025)

  Changed paths:
    M llvm/include/llvm/Transforms/Instrumentation/PGOCtxProfLowering.h
    M llvm/lib/Passes/PassBuilderPipelines.cpp
    M llvm/lib/Passes/PassRegistry.def
    M llvm/lib/Transforms/Instrumentation/PGOCtxProfLowering.cpp
    A llvm/test/Transforms/PGOProfile/ctx-instrumentation-block-inline.ll

  Log Message:
  -----------
  [ctxprof] don't inline weak symbols after instrumentation (#128811)

Contextual profiling identifies functions by GUID. Functions that may get overridden by the linker with a prevailing copy may have, during instrumentation, different variants in different modules. If these variants get inlined before linking (here I assume thinlto), they will identify themselves to the ctxprof runtime as their GUID, leading to issues - they may have different counter counts, for instance.

If we block their inlining in the pre-thinlink compilation, only the prevailing copy will survive post-thinlink and the confusion is avoided.

The change introduces a small pass just for this purpose, which marks any symbols that could be affected by the above as `noinline` (even if they were `alwaysinline`). We already carried out some inlining (via the preinliner), before instrumenting, so technically the `alwaysinline` directives were honored.

We could later (different patch) choose to mark them back to their original attribute (none or `alwaysinline`) post-thinlink, if we want to - but experimentally that doesn't really change much of the performance of the instrumented binary.



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