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

[zhi chen via llvm-dev]

That makes great sense. It would be great if we have profitability mode to
see the necessity to use gathers. Or it also would be good if there is a
compiler option for the users to enable LLVM to generate the gather
instructions no matter it is faster or slow.

Best,
Zhi

On Fri, Feb 26, 2016 at 12:49 PM, Sanjay Patel <spatel at rotateright.com>
wrote:

> If I'm understanding correctly, you're saying that vgather* is slow on all
> of Excavator, Haswell, Broadwell, and Skylake (client). Therefore, we will
> not generate it for any of those machines.
>
> Even if that's true, we should not define "gatherIsSlow()" as "hasAVX2()
> && !hasAVX512()". It could break for some hypothetical future processor
> that manages to implement it properly. The AVX2 spec includes gather;
> whether it's slow or fast is an implementation detail. We need a feature
> bit / cost model entry somewhere to signify this, so we're not overloading
> the meaning of the architectural features with that implementation detail.
>
> On Fri, Feb 26, 2016 at 12:23 PM, Demikhovsky, Elena <
> elena.demikhovsky at intel.com> wrote:
>
>> No. Gather operation is slow on AVX2 processors.
>>
>>
>>
>> -          * Elena*
>>
>>
>>
>> *From:* zhi chen [mailto:zchenhn at gmail.com]
>> *Sent:* Thursday, February 25, 2016 20:48
>> *To:* Sanjay Patel <spatel at rotateright.com>
>> *Cc:* Demikhovsky, Elena <elena.demikhovsky at intel.com>; Nema, Ashutosh <
>> Ashutosh.Nema at amd.com>; llvm-dev <llvm-dev at lists.llvm.org>
>>
>> *Subject:* Re: [llvm-dev] how to force llvm generate gather intrinsic
>>
>>
>>
>> It seems that http://reviews.llvm.org/D15690 only implemented
>> gather/scatter for AVX-512, but not for AVX/AVX2. Is there any plan to
>> enable gather for AVX/2? Thanks.
>>
>>
>>
>> Best,
>>
>> Zhi
>>
>>
>>
>> On Thu, Feb 25, 2016 at 8:28 AM, Sanjay Patel <spatel at rotateright.com>
>> wrote:
>>
>> I don't think gather has been enabled for AVX2 as of r261875.
>>
>> Masked load/store were enabled for AVX with:
>> http://reviews.llvm.org/D16528 / http://reviews.llvm.org/rL258675
>>
>>
>>
>> On Wed, Feb 24, 2016 at 11:39 PM, Demikhovsky, Elena <
>> elena.demikhovsky at intel.com> wrote:
>>
>> Yes, masked load/store/gather/scatter are completed.
>>
>>
>>
>> -          * Elena*
>>
>>
>>
>> *From:* zhi chen [mailto:zchenhn at gmail.com]
>> *Sent:* Thursday, February 25, 2016 01:20
>> *To:* Demikhovsky, Elena <elena.demikhovsky at intel.com>
>> *Cc:* Sanjay Patel <spatel at rotateright.com>; Nema, Ashutosh <
>> Ashutosh.Nema at amd.com>; llvm-dev <llvm-dev at lists.llvm.org>
>>
>>
>> *Subject:* Re: [llvm-dev] how to force llvm generate gather intrinsic
>>
>>
>>
>> Hi Elena,
>>
>>
>>
>> Are the masked_load and gather working now?
>>
>>
>>
>> Best,
>>
>> Zhi
>>
>>
>>
>> On Sat, Jan 23, 2016 at 12:06 PM, Demikhovsky, Elena <
>> elena.demikhovsky at intel.com> wrote:
>>
>> Ø  Can we legalize the same set of masked load/store operations for AVX1
>> as AVX2?
>>
>> Yes, of course.
>>
>>
>>
>> -          * Elena*
>>
>>
>>
>> *From:* Sanjay Patel [mailto:spatel at rotateright.com]
>> *Sent:* Saturday, January 23, 2016 18:42
>> *To:* Nema, Ashutosh <Ashutosh.Nema at amd.com>
>> *Cc:* Demikhovsky, Elena <elena.demikhovsky at intel.com>; zhi chen <
>> zchenhn at gmail.com>; llvm-dev <llvm-dev at lists.llvm.org>
>> *Subject:* Re: [llvm-dev] how to force llvm generate gather intrinsic
>>
>>
>>
>>
>>
>> On Sat, Jan 23, 2016 at 6:45 AM, Nema, Ashutosh <Ashutosh.Nema at amd.com>
>> wrote:
>>
>> Thanks Sanjay for highlighting this, few days back I also faced similar
>> problem
>>
>> while generating masked store in avx1 mode, found its only supported
>> under
>>
>> avx2 else we scalarize it.
>>
>>
>>
>> >  1)   I did not switch-on masked_load/store to AVX1, I can do this.
>>
>>
>>
>> Yes Elena, This should be supported for FP type in avx1 mode (for INT
>> type, I doubt X86 has masked_load/store instruction in avx1 mode).
>>
>>
>>
>> Thanks everyone for the answers. My immediate motivation is to improve
>> the masked load/store ops for an AVX target. If we can fix scatter/gather
>> similarly, that would be great.
>>
>> Can we legalize the same set of masked load/store operations for AVX1 as
>> AVX2? If I'm understanding them correctly, the AVX1 FP instructions
>> (vmaskmovps/pd) can be used in place of the AVX2 int instructions
>> (vpmaskmovd/q), just with domain crossing penalties thrown in. I think we
>> do this for other missing integer ops for an AVX1 target either in x86
>> lowering or in the tablegen patterns.
>>
>>  Elena - I'm not too familiar with the vectorizers or scatter/gather, but
>> I'll certainly take a look at D15690. Thanks for pointing out the patch!
>>
>>
>>
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