[PATCH] D110869: [X86] Implement -fzero-call-used-regs option

[Bill Wendling via Phabricator via llvm-commits]

void added a comment.

In D110869#3295477 <https://reviews.llvm.org/D110869#3295477>, @nathanchance wrote:

> It looks like `_paravirt_ident_64()` is the problematic function. This diff on top of v5.17-rc2 allows me to boot:
>
>   diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
>   index 4420499f7bb4..c1b68504136c 100644
>   --- a/arch/x86/kernel/paravirt.c
>   +++ b/arch/x86/kernel/paravirt.c
>   @@ -96,7 +96,7 @@ static unsigned paravirt_patch_call(void *insn_buff, const void *target,
>   
>    #ifdef CONFIG_PARAVIRT_XXL
>    /* identity function, which can be inlined */
>   -u64 notrace _paravirt_ident_64(u64 x)
>   +u64 notrace __attribute__((zero_call_used_regs("skip"))) _paravirt_ident_64(u64 x)
>    {
>           return x;
>    }
>
> Rather interesting function to have problems with as a result of this patch but it seems like this function is being used in a very specific way further down the file with the `__PV_IS_CALLEE_SAVE` macro.

Weird. We generate similar code to GCC:

  Clang:
  _paravirt_ident_64:                     # @_paravirt_ident_64
  .Lfunc_begin2:
          .loc    2 100 0 is_stmt 1               # arch/x86/kernel/paravirt.c:100:0
          .cfi_startproc
  # %bb.0:                                # %entry
          #DEBUG_VALUE: _paravirt_ident_64:x <- $rdi
          movq    %rdi, %rax
  .Ltmp21:
          .loc    2 101 2 prologue_end            # arch/x86/kernel/paravirt.c:101:2
          xorq    %rdi, %rdi
  .Ltmp22:
          #DEBUG_VALUE: _paravirt_ident_64:x <- $rax
          retq
  .Ltmp23:
  .Lfunc_end2:
          .size   _paravirt_ident_64, .Lfunc_end2-_paravirt_ident_64
          .cfi_endproc
  
  GCC:
  _paravirt_ident_64:
  # arch/x86/kernel/paravirt.c:100: {
          movq    %rdi, %rax      # tmp85, x
  # arch/x86/kernel/paravirt.c:102: }
          xorl    %edi, %edi      #
          ret
          .size   _paravirt_ident_64, .-_paravirt_ident_64

I'm a bit confused...


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