[PATCH] D12052: [X86][SSE] Add _mm_undefined_* intrinsics

[Kuperstein, Michael M via cfe-commits]

I’m not sure how much people actually use these, but the AVX-512 versions of these, at least, can be very useful internally to implement AVX-512 intrinsics.
For AVX-512, we use the same GCC builtin for all 3 versions of the intrinsic (pass-through masked, set to zero masked, and unmasked). This is the same implementation that’s used in GCC, and is fairly clean, since the only difference is in the desired pass-through values (actual value, zero, or undef).

However, since we don’t actually have the undef intrinsics right now, we put a zero in the unmasked version as well, which is definitely a pessimization.
The plan is to change them to use undef once the undef intrinsics are implemented.


From: Eric Christopher [mailto:echristo at gmail.com]
Sent: Monday, August 17, 2015 21:33
To: reviews+D12052+public+a6057f04f570e35c at reviews.llvm.org; llvm-dev at redking.me.uk; craig.topper at gmail.com; Kuperstein, Michael M
Cc: david.majnemer at gmail.com; Badouh, Asaf; cfe-commits at lists.llvm.org; Richard Smith
Subject: Re: [PATCH] D12052: [X86][SSE] Add _mm_undefined_* intrinsics


On Sun, Aug 16, 2015 at 3:05 AM Simon Pilgrim <llvm-dev at redking.me.uk<mailto:llvm-dev at redking.me.uk>> wrote:
RKSimon added a comment.

Yes using that uninitialized value has worried me as well. I originally set it to zero (and considered using __ LINE __ or __ COUNTER __) but both introduce defined behaviour that I could see causing all sorts of problems further down the line in debug vs release builds. How undefined do we want our undefined to be? ;-)

Yeah, this is why I hadn't implemented them yet either.

I can create __builtin_ia32_undef64mmx / __builtin_ia32_undef128 / __builtin_ia32_undef256 / __builtin_ia32_undef512 if nobody can think of a better alternative?

This seems fairly heavyweight, but I don't have any better ideas. I'll assume we don't want to try to expose undef as a value in C (making it as something we could just add), if not then this seems to make the most sense. It's pretty painful/ugly though.

Are people using these or did they just notice for completeness? We probably _could_ define them to zero and leave it at that. It's not pleasant and slower than it needs to be but not crazy.

-eric


Repository:
  rL LLVM

http://reviews.llvm.org/D12052


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