[PATCH] D94268: Allow _mm_empty() (via llvm.x86.mmx.emms) to be a no-op without MMX.

Fri Jan 8 18:58:45 PST 2021

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.

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https://reviews.llvm.org/D94268