[PATCH] D141020: [X86][ABI] Clobber %RAX for preserve_allcc and preserve_mostcc CCs

[Anton Bikineev via Phabricator via llvm-commits]

AntonBikineev created this revision.
AntonBikineev added reviewers: pengfei, ributzka.
Herald added subscribers: jdoerfert, hiraditya, kristof.beyls.
Herald added a project: All.
AntonBikineev requested review of this revision.
Herald added a project: LLVM.

Currently, both calling conventions preserve %RAX. This breaks in this trivial case:

  define i32 @bar() {
    %1 = call preserve_mostcc i32 @foo()
     ret i32 %1
  }
  
  define preserve_mostcc i32 @foo() {
    ret i32 2
    ; preserve_mostcc will restore %rax, whatever it was before the call.
  }

This contradicts the current documentation (preserve_allcc "behaves
identical to the C calling conventions on how arguments and return
values are passed") and also breaks `[[clang::preserve_most]]`.

This is not a problem for AArch64, where X0-X7 are not part of `CSR_AArch64_RT_MostRegs`.

Juergen, can you please take a look, since you added the CCs originally?
Phoebe, I added you as a reviewer as well, since you touched the X86 CC code recently :)

Thanks!


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D141020

Files:
  llvm/docs/LangRef.rst
  llvm/lib/Target/X86/X86CallingConv.td
  llvm/test/CodeGen/X86/ipra-reg-usage.ll
  llvm/test/CodeGen/X86/machine-copy-prop.mir
  llvm/test/CodeGen/X86/preserve_allcc64.ll
  llvm/test/CodeGen/X86/preserve_mostcc64.ll

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