pengfei added a comment. In D94268#2487765 <https://reviews.llvm.org/D94268#2487765>, @jyknight wrote:
> In D94268#2485958 <https://reviews.llvm.org/D94268#2485958>, @pengfei wrote: > >> Is inline assembly the only case `emms` instruction will be needed? But >> inline assembly doesn't enable `mmx` attribute automatically, right? E.g. >> https://godbolt.org/z/43ases > > Yes, inline or external asm should be the only reason there should be any MMX > register usage when all is done here. After this patch, the default is still > to have mmx enabled by default with sse, despite that clang won't use it. But > users can pass -mno-mmx if they like. And, yes, clang only requires -mmmx if > you use the "y" asm constraint, not if you use mmx instructions inside the > asm string. > > I wrote this patch because making it a no-op is the same behavior GCC has. > However, I'm not sure this is necessarily the right way to go. On the plus > side for this patch, it allows intrinsic-using code to stop emitting spurious > emms instructions, if compiled with -mno-mmx. However, the negative is that > inline-asm code which _doesn't_ use the "y" constraint might still be using > MMX within an asm blob, and be depending on calls to _mm_empty() outside of > the asm, and such code would be silently broken when compiled with -mno-mmx. > > At first, I thought that most uses of inline-asm would be using constraints, > but after looking around at existing MMX asm, it seems that nearly all of it > does _not_ use a "y" constraint or even clobber any fpu or mmx registers. And > they do also depend on _mm_empty() in combination with their unmarked inline > asm. Which...now that I think about it more, makes passing -mno-mmx to the > compiler almost entirely pointless. > > So, now I'm thinking I'll just drop this change, actually. Yeah, I forgot the external asm case. Compiler has no way to know if MMX instructions are used by external code. So I'm in favour of keeping `_mm_empty` with `mmx` attribute and adding comments it is only used for context switch from inline or external code. Then if user is passing -mno-mmx and sure of it, they should remove the intrinsic from their code. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D94268/new/ https://reviews.llvm.org/D94268 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
