[llvm-dev] RFC: New intrinsics masked.expandload and masked.compressstore

Demikhovsky, Elena via llvm-dev llvm-dev at lists.llvm.org
Sun Sep 18 23:37:02 PDT 2016

       Hi all,

       AVX-512 ISA introduces new vector instructions VCOMPRESS and VEXPAND in order to allow vectorization of the following loops with two specific types of cross-iteration dependencies:

       for (int i=0; i<N; ++i)
         If (t[i])
           *A++ = expr;

       for (i=0; i<N; ++i)
         If (t[i])
            X[i] = *A++;
            X[i] = PassThruV[i];

       On this poster ( http://llvm.org/devmtg/2013-11/slides/Demikhovsky-Poster.pdf ) you'll find depicted "compress" and "expand" patterns.

       The RFC proposes to support this functionality by introducing two intrinsics to LLVM IR:

       The syntax of these two intrinsics is similar to the syntax of llvm.masked.load.* and masked.store.*, respectively, but the semantics are different, matching the above patterns.

       %res = call <16 x float> @llvm.masked.expandload.v16f32.p0f32 (float* %ptr, <16 x i1>%mask, <16 x float> %passthru)
       void @llvm.masked.compressstore.v16f32.p0f32 (<16 x float> <value>, float* <ptr>, <16 x i1> <mask>)

       The arguments - %mask, %value and %passthru all have the same vector length.
       The underlying type of %ptr corresponds to the scalar type of the vector value.
       (In brief; the full syntax description will be provided in subsequent full documentation.)

       The intrinsics are planned to be target independent, similar to masked.load/store/gather/scatter. They will be lowered effectively on AVX-512 and scalarized on other targets, also akin to masked.* intrinsics.
       Loop vectorizer will query TTI about existence of effective support for these intrinsics, and if provided will be able to handle loops with such cross-iteration dependences.

       The first step will include the full documentation and implementation of CodeGen part.

       An additional information about expand load ( https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=expandload&techs=AVX_512 )  and compress store (https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=compressstore&techs=AVX_512 ) you also can find in the Intel Intrinsic Guide.

-       Elena

Intel Israel (74) Limited

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