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

[Demikhovsky, Elena via llvm-dev]

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<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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