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

Sun Feb 28 00:29:22 PST 2016

I, myself, do not plan this work right now. But I’m ready to assist and review if somebody will take it.

-           Elena

From: zhi chen [mailto:zchenhn at gmail.com]
Sent: Friday, February 26, 2016 23:47
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

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<mailto: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<mailto:elena.demikhovsky at intel.com>> wrote:
No. Gather operation is slow on AVX2 processors.

-           Elena

From: zhi chen [mailto:zchenhn at gmail.com<mailto:zchenhn at gmail.com>]
Sent: Thursday, February 25, 2016 20:48
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>>; Nema, Ashutosh <Ashutosh.Nema at amd.com<mailto:Ashutosh.Nema at amd.com>>; llvm-dev <llvm-dev at lists.llvm.org<mailto: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<mailto: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<mailto:elena.demikhovsky at intel.com>> wrote:
Yes, masked load/store/gather/scatter are completed.

-           Elena

From: zhi chen [mailto:zchenhn at gmail.com<mailto:zchenhn at gmail.com>]
Sent: Thursday, February 25, 2016 01:20
To: Demikhovsky, Elena <elena.demikhovsky at intel.com<mailto:elena.demikhovsky at intel.com>>
Cc: Sanjay Patel <spatel at rotateright.com<mailto:spatel at rotateright.com>>; Nema, Ashutosh <Ashutosh.Nema at amd.com<mailto:Ashutosh.Nema at amd.com>>; llvm-dev <llvm-dev at lists.llvm.org<mailto: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<mailto:elena.demikhovsky at intel.com>> wrote:

>  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<mailto:spatel at rotateright.com>]
Sent: Saturday, January 23, 2016 18:42
To: Nema, Ashutosh <Ashutosh.Nema at amd.com<mailto:Ashutosh.Nema at amd.com>>
Cc: Demikhovsky, Elena <elena.demikhovsky at intel.com<mailto:elena.demikhovsky at intel.com>>; zhi chen <zchenhn at gmail.com<mailto:zchenhn at gmail.com>>; llvm-dev <llvm-dev at lists.llvm.org<mailto: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<mailto: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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