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

[Bill Wendling via Phabricator via cfe-commits]

void added a comment.

In D110869#3295912 <https://reviews.llvm.org/D110869#3295912>, @craig.topper wrote:

> In D110869#3295906 <https://reviews.llvm.org/D110869#3295906>, @void wrote:
>
>> In D110869#3295578 <https://reviews.llvm.org/D110869#3295578>, @nickdesaulniers wrote:
>>
>>> In D110869#3295559 <https://reviews.llvm.org/D110869#3295559>, @void wrote:
>>>
>>>> Weird. We generate similar code to GCC:
>>>>
>>>>   Clang:
>>>>   _paravirt_ident_64:                     # @_paravirt_ident_64
>>>>           movq    %rdi, %rax
>>>>           xorq    %rdi, %rdi
>>>>           retq
>>>>   
>>>>   GCC:
>>>>   _paravirt_ident_64:
>>>>           movq    %rdi, %rax      # tmp85, x
>>>>           xorl    %edi, %edi      #
>>>>           ret
>>>
>>> Does `xorl` not zero the upper 32b?
>>
>> I'm thinking no. But it's odd, because both are using `%rdi` but GCC is only zeroing out the bottom 32-bits. Seems a bit counter-intuitive.
>
> Every write to a 32-bit register on x86-64 zeros bits 63:32 of the register. `xorl %edi, %edi` has the same behavior as `xorq %rdi, %rdi`, but is 1 byte shorter to encode.

Oh, interesting! TIL. So it's really not profitable to use `xorq` at all here...Though it does beg the question of why `xorq` exists. :-)


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