[PATCH] D103372: [InstrProfiling] If no value profiling, make data variable private and (for Windows) use one comdat

[Fangrui Song via Phabricator via llvm-commits]

MaskRay created this revision.
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`__profd_*` variables are referenced by code only when value profiling is
enabled. If disabled (e.g. default -fprofile-instr-generate), the symbols just
waste space on ELF/Mach-O.

(In -DLLVM_BUILD_INSTRUMENTED_COVERAGE=on build, there is now no `__profd_*` symbols.)

On Windows this enables further optimization. We are no longer affected by the
link.exe limitation: an external symbol in IMAGE_COMDAT_SELECT_ASSOCIATIVE can
cause duplicate definition error.
https://lists.llvm.org/pipermail/llvm-dev/2021-May/150758.html
We can thus use llvm.compiler.used instead of llvm.used like ELF (D97585 <https://reviews.llvm.org/D97585>).
This avoids `/INCLUDE:` directives in `.drectve`.

Hope someone can test this on Windows. I am speculating with theories and don't
have a testing setup:/


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D103372

Files:
  llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp
  llvm/test/Instrumentation/InstrProfiling/comdat.ll
  llvm/test/Instrumentation/InstrProfiling/icall.ll
  llvm/test/Instrumentation/InstrProfiling/linkage.ll
  llvm/test/Instrumentation/InstrProfiling/platform.ll
  llvm/test/Instrumentation/InstrProfiling/profiling.ll
  llvm/test/Transforms/PGOProfile/comdat_internal.ll

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