craig.topper added a comment. In D110869#3296031 <https://reviews.llvm.org/D110869#3296031>, @void wrote:
> 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. :-) xorq is still useful when the registers are different. 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
