craig.topper added a comment. 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. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D110869/new/ https://reviews.llvm.org/D110869 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
