[all-commits] [llvm/llvm-project] 10629b: Reland "[X86][ABI] Don't preserve return regs for ...

[Anton Bikineev via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 10629bb96aebfefaadfb0069bc7e276b868fcef9
      https://github.com/llvm/llvm-project/commit/10629bb96aebfefaadfb0069bc7e276b868fcef9
  Author: Anton Bikineev <bikineev at chromium.org>
  Date:   2023-02-09 (Thu, 09 Feb 2023)

  Changed paths:
    M llvm/docs/LangRef.rst
    M llvm/lib/Target/X86/X86CallingConv.td
    M llvm/lib/Target/X86/X86ISelLowering.cpp
    A llvm/test/CodeGen/X86/dynamic-regmask-preserve-all.ll
    A llvm/test/CodeGen/X86/dynamic-regmask-preserve-most.ll
    A llvm/test/CodeGen/X86/preserve_allcc64-ret-double.ll
    M llvm/test/CodeGen/X86/preserve_allcc64.ll
    A llvm/test/CodeGen/X86/preserve_mostcc64-ret-double.ll
    A llvm/test/CodeGen/X86/preserve_mostcc64-sret.ll
    M llvm/test/CodeGen/X86/preserve_mostcc64.ll

  Log Message:
  -----------
  Reland "[X86][ABI] Don't preserve return regs for preserve_all/preserve_most CCs""

The original change mistakenly excluded parameter registers from the
list of callee-saved-registers. This reland fixes it - it only excludes
the return registers for preserve_all/preserve_most CCs.

Original description:
> Currently both calling conventions preserve registers that are used to
> store a return value. This causes the returned value to be lost:
>
>   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 change makes CSRs be preserved iff they are not used to store a
> return value (e.g.  %rax for scalars, {%rax:%rdx} for __int128, %xmm0
> for double).  For void functions no additional registers are
> preserved, i.e.  the behaviour is backward compatible with existing
> code.

Differential Revision: https://reviews.llvm.org/D143425