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

[zhi chen via llvm-dev]

I don't need scatter but only gather on AVX-2, and performance is not the
biggest concern. Could you please kindly suggest me how to switch masked
gather on?

Best,
Zhi

On Sat, Jan 23, 2016 at 12:01 AM, Demikhovsky, Elena <
elena.demikhovsky at intel.com> wrote:

> 1)      I did not switch-on masked_load/store to AVX1, I can do this.
>
> 2)      I did not switch-on masked gather on AVX2 because the instruction
> is slow. There is no scatter on AVX2.
>
> 3)      Currently, gather/scatter does not work on SKX because the patch
> is still under review http://reviews.llvm.org/D15690. I’d be happy if you
> agree to review it.
>
>
>
> -          * Elena*
>
>
>
> *From:* zhi chen [mailto:zchenhn at gmail.com]
> *Sent:* Saturday, January 23, 2016 02:58
> *To:* Sanjay Patel <spatel at rotateright.com>
> *Cc:* Demikhovsky, Elena <elena.demikhovsky at intel.com>; LLVM Developers
> Mailing List <llvm-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] how to force llvm generate gather intrinsic
>
>
>
> Thanks for your response, Sanjay. I know there are intrinsics available in
> C/C++. But the problem is that I want to instrument my code at the IR level
> and generate those instructions. I don't want to touch the source code.
>
>
>
> Best,
>
> Zhi
>
>
>
> On Fri, Jan 22, 2016 at 4:54 PM, Sanjay Patel <spatel at rotateright.com>
> wrote:
>
> 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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