[PATCH] D83906: [CodeGen] Emit a call instruction instead of an invoke if the called llvm function is marked nounwind

[Lei Wang via Phabricator via cfe-commits]

wlei added subscribers: modimo, hoy, wlei.
wlei added a comment.

Hi @ahatanak

We recently hit an issue of inconsistent codegen related with this optimization. In one build, Clang frontend generates different llvm IRs for the same function that is originally from one header file. It turned out this optimization gives different results for different function definition order which is naturally unstable.

See this two repro programs:

p1.cpp: https://godbolt.org/z/bavTYEG1x

  void foo() {};
  void bar() noexcept {foo();};


p2.cpp: https://godbolt.org/z/zfsnzPrE6

  void foo();
  void bar() noexcept {foo();};
  void foo(){};

See the codegens of bar are different, for p2.cpp, the callee(foo)’s definition is after the caller(bar), it's unknown to be marked `nounwind` before it see foo's definition, so it still generates the `invoke` things.

This inconsistency affected the AutoFDO, one of our work assigns consecutive number IDs to the BBs of CFG, the unstable CFGs causes the BB ID mismatched and a lot of samples are lost.

Would like to hear from your feedback. Wondering if FE can handle this perfectly or perhaps we can just leave it for BE. Thank you in advance!

cc @hoy @modimo @wenlei


Repository:
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https://reviews.llvm.org/D83906