[llvm-dev] how to force llvm generate gather intrinsic

[Sanjay Patel via llvm-dev]

I was just looking at the related masked load/store operations, and I think
there are at least 2 bugs:

1. X86TTIImpl::isLegalMaskedLoad/Store() should be legal for FP types with
AVX1 (not just AVX2).
2. X86TTIImpl::isLegalMaskedGather/Scatter() should be legal for 128/256
bit vectors with AVX2 (not just AVX512).

I looked at this for the first time today, so I may be missing something...

So for the moment, the answer to your question is 'no'; there's no generic
way to produce these instructions. You should be able to use the _mm_*
intrinsics in C though.




On Fri, Jan 22, 2016 at 5:00 PM, zhi chen via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> I used clang -O3 -c -emit-llvm on the follow code to generate a bitcode,
> say a.bc. I read the .ll file and didn't see any gather intrinsic. Also, I
> used opt -O3 -mcpu=core-avx2/-mcpu=skx, but there is still no gather
> intrinsic generated.
>
> int foo(int A[800], int B[800], int C[800]) {
>     for (int i = 0; i < 800; i++) {
>         A[B[i]] = i + 5;
>     }
>
>     for (int i = 0; i < 800; i++) {
>         A[B[i]]++;
>     }
>
>     for (int i = 0; i < 800; i++) {
>         A[i] = B[C[i]];
>     }
>     return 0;
> }
>
> Could some give me an example that will generate gather intrinsic for
> AVX2? I tried to used the masked_gather intrinsic provided in the language
> ref. But it seemed that it only generates gather intrinsic for AVX-512 but
> for AVX-2. I found that there are 16 gather intrinsic versions depending on
> the data types provided for AVX-2. Do I have to check the data type before
> calling them specifically? or is there a generic way to use AVX-2 gather
> intrinsic?
>
> Best,
> Zhi
>
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