[PATCH] D41517: mmintrin.h documentation fixes and updates

Eli Friedman via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Mon Jan 15 13:21:11 PST 2018

efriedma added inline comments.

Comment at: lib/Headers/mmintrin.h:55
-/// This intrinsic corresponds to the <c> VMOVD / MOVD </c> instruction.
+/// This intrinsic corresponds to the <c> MOVD </c> instruction.
RKSimon wrote:
> efriedma wrote:
> > craig.topper wrote:
> > > kromanova wrote:
> > > > I tried clang on Linux, x86_64, and if -mavx option is passed, we generate VMOVD, if this option is omitted, we generate MOVD.
> > > > I think I understand the rational behind this change (namely, to keep MOVD, but remove VMOVD),
> > > > since this intrinsic should use MMX registers and shouldn't have corresponding AVX instruction(s).
> > > > 
> > > > However, that's what we generate at the moment when -mavx is passed (I suspect because our MMX support is limited)
> > > > vmovd   %edi, %xmm0
> > > > 
> > > > Since we are writing the documentation for clang compiler, we should document what clang compiler is doing, not what is should be doing.
> > > > Craig, what do you think? Should we revert back to VMOVD/MOVD?
> > > > 
> > > We can change it back to VMOVD/MOVD
> > The reference to vmovd seems confusing.  Yes, LLVM compiles `_mm_movpi64_epi64(_mm_cvtsi32_si64(i))` to vmovd, but that doesn't mean either of those intrinsics "corresponds" to vmovd; that's just the optimizer combining two operations into one.
> Should all these _mm_cvt* intrinsics be replaced with a 'this is a utility function' style comment like the _mm_set1* intrinsics?
In general, I think "corresponds to" should mean "if the inputs are produced by an inline asm, and the output is used by an inline asm, and the lowering will produce a single instruction, what instruction will we generate?".  That's unambiguous, and will usually give a useful hint to the user.  In this case, on trunk, the answer is consistently "movd".

Otherwise, it's not clear what it means for an intrinsic to correspond to anything; optimizations exist which can modify the instructions we choose for almost any intrinsic.


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