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

[Demikhovsky, Elena via llvm-dev]

It is not supported on codegen level. You can’t just switch it on.
But you can use compiler intrinsics instead. See examples in
test/CodeGen/X86/avx2-intrinsics-x86.ll

-           Elena

From: zhi chen [mailto:zchenhn at gmail.com]
Sent: Saturday, January 23, 2016 10:21
To: Demikhovsky, Elena <elena.demikhovsky at intel.com>
Cc: Sanjay Patel <spatel at rotateright.com>; LLVM Developers Mailing List <llvm-dev at lists.llvm.org>; Zaks, Ayal <ayal.zaks at intel.com>
Subject: Re: [llvm-dev] how to force llvm generate gather intrinsic

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<mailto: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<mailto:zchenhn at gmail.com>]
Sent: Saturday, January 23, 2016 02:58
To: Sanjay Patel <spatel at rotateright.com<mailto:spatel at rotateright.com>>
Cc: Demikhovsky, Elena <elena.demikhovsky at intel.com<mailto:elena.demikhovsky at intel.com>>; LLVM Developers Mailing List <llvm-dev at lists.llvm.org<mailto: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<mailto: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<mailto: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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