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

[Sanjay Patel via llvm-dev]

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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