llvm dev - Jan 2016 - how to force llvm generate gather intrinsic

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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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
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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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
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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