[llvm-dev] [RFC] Vector Predication (original) (raw)

Simon Moll via llvm-dev llvm-dev at lists.llvm.org
Mon Feb 4 12:13:57 PST 2019

On 2/4/19 6:15 PM, David Greene wrote:

Simon Moll <moll at cs.uni-saarland.de> writes:

You are referring to the sub-vector sizes, if i am understanding correctly. I'd assume that the mask sub-vector length always has to be either 1 or the same as the data sub-vector length. For example, this is ok:

%result = call <scalable 3 x float> @llvm.evl.fsub.v4f32(<scalable 3 x_ _float> %x, <scalable 3 x float> %y, <scalable 1 x i1> %M, i32 %L) What does <scalable 1 x i1> applied to <scalable 3 x float> mean? I would expect a requirement of <scalable 3 x i1>. At least that's how I understood the SVE proposal [1]. The n's in have to match.

It would mean that the each mask bit M[i] applies to data lanes D[3i] to D[3i+2]. It has applications in graphics codes where the vector element type would be a short vector as in [2]:

for(int i = 0; i < 1000; i++) { vec4 color = colors[i]; // <scalable 4 x float>  vec3 normal = normals[i]; // <scalable 3 x float> color.rgb *= fmax(0.0, dot(normal, light_dir)); colors[i] = color; }

I don't see any direct conflict with LLVM-SVE [1] but it will add complexity to the vectorizer, legal and TTI to choose a good sub-vector size/strategy for each target.

[2] https://lists.llvm.org/pipermail/llvm-dev/2019-January/129822.html

-David [1] http://lists.llvm.org/pipermail/llvm-dev/2016-November/106819.html

--

Simon Moll Researcher / PhD Student

Compiler Design Lab (Prof. Hack) Saarland University, Computer Science Building E1.3, Room 4.31

Tel. +49 (0)681 302-57521 : moll at cs.uni-saarland.de Fax. +49 (0)681 302-3065 : http://compilers.cs.uni-saarland.de/people/moll

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