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

[Nick Desaulniers via Phabricator via cfe-commits]

nickdesaulniers added a comment.

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?

Definitely something magical about this function. Perhaps it should have the function attribute to disable zeroing added to the kernel sources?

tools/objtool/objtool.c mentions something about `_paravirt_ident_64` and `paravirt_patch`.

I'm curious @nathanchance if you move the definition of `_paravirt_ident_64` to an external assembler file, and see whether the mere `xorl` vs `xorq` makes a difference for some reason for boot?


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