[llvm-dev] enabling interleaved access loop vectorization
Demikhovsky, Elena via llvm-dev
llvm-dev at lists.llvm.org
Fri Aug 5 13:00:56 PDT 2016
Ayal tried to enable interleave access, just by switching “false” to “true” and measured performance. He got significant performance degradation on several benchmarks. Some benchmarks looked better, of course.
Right now we concluded that interleaved access is not always beneficial for X86 or it requires additional target specific optimizations at least.
First of all, we need more precise cost model that can estimate a real number of shuffles in each case. We also need to understand what to do with edge elements in the vector if their loading is not required. We, probably, should issue a masked load in this case.
As far as I remember, may be I’m wrong, vectorizer does not generate shuffles for interleave access. It generates a bunch of extracts and inserts that ought to be coupled into shuffles after wise.
It adds uncertainty to the cost modeling.
- Elena
From: Michael Kuperstein [mailto:mkuper at google.com]
Sent: Friday, August 05, 2016 20:05
To: Matthew Simpson <mssimpso at codeaurora.org>
Cc: Nema, Ashutosh <Ashutosh.Nema at amd.com>; Demikhovsky, Elena <elena.demikhovsky at intel.com>; Renato Golin <renato.golin at linaro.org>; Sanjay Patel <spatel at rotateright.com>; llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] enabling interleaved access loop vectorization
Regarding InterleavedAccessPass - sure, but proper strided/interleaved access optimization ought to have a positive impact even without target support.
Case in point - Hal enabled it on PPC last September. An important difference vs. x86 seems to be that arbitrary shuffles are cheap on PPC, but, as I said below, I hope we can enable it on x86 with a conservative cost function, and still get improvement.
On Fri, Aug 5, 2016 at 7:02 AM, Matthew Simpson <mssimpso at codeaurora.org<mailto:mssimpso at codeaurora.org>> wrote:
Isn't our current interleaved access vectorization just a special case of the more general strided access proposal? If so, from a development perspective, it might make sense to begin incorporating some of that work into the existing framework (with appropriate target hooks and costs). This could probably be done piecemeal rather than all at once.
Also, keep in mind that ARM/Aarch64 run an additional IR pass (InterleavedAccessPass) that matches the load/store plus shuffle sequences that the vectorizer generates to target-specific instrinsics.
-- Matt
From: Nema, Ashutosh [mailto:Ashutosh.Nema at amd.com<mailto:Ashutosh.Nema at amd.com>]
Sent: Friday, August 05, 2016 7:21 AM
To: Michael Kuperstein <mkuper at google.com<mailto:mkuper at google.com>>; Demikhovsky, Elena <elena.demikhovsky at intel.com<mailto:elena.demikhovsky at intel.com>>
Cc: Renato Golin <renato.golin at linaro.org<mailto:renato.golin at linaro.org>>; Sanjay Patel <spatel at rotateright.com<mailto:spatel at rotateright.com>>; Matthew Simpson <mssimpso at codeaurora.org<mailto:mssimpso at codeaurora.org>>; llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject: RE: [llvm-dev] enabling interleaved access loop vectorization
Hi Michael,
Sometime back I did some experiments with interleave vectorizer and did not found any degrade,
probably my tests/benchmarks are not extensive enough to cover much.
Elina is the right person to comment on it as she already experienced cases where it hinders performance.
For interleave vectorizer on X86 we do not have any specific costing, it goes to BasicTTI where the costing is not appropriate(WRT X86).
It consider cost of extracts & inserts for extracting elements from a wide vector, which is really expensive.
i.e. in your test case the cost of load associated with “in[i * 2]” is 10 (for VF4).
Interleave vectorize will generate following instructions for it:
%wide.vec = load <8 x i32>, <8 x i32>* %14, align 4, !tbaa !1, !alias.scope !5
%strided.vec = shufflevector <8 x i32> %wide.vec, <8 x i32> undef, <4 x i32> <i32 0, i32 2, i32 4, i32 6>
For wide load it get cost as 2(as it has to generate 2 loads) but for extracting elements (shuffle operation) it get cost as 8 (4 for extract + 4 for insert).
The cost should be 3 here, 2 for loads & 1 for shuffle.
To enable Interleave vectorizer on X86 we should implement a proper cost estimation.
Test you mentioned is indeed a candidate for Stride memory vectorization.
Regards,
Ashutosh
From: Michael Kuperstein [mailto:mkuper at google.com]
Sent: Friday, August 5, 2016 4:53 AM
To: Demikhovsky, Elena <elena.demikhovsky at intel.com<mailto:elena.demikhovsky at intel.com>>
Cc: Renato Golin <renato.golin at linaro.org<mailto:renato.golin at linaro.org>>; Sanjay Patel <spatel at rotateright.com<mailto:spatel at rotateright.com>>; Nema, Ashutosh <Ashutosh.Nema at amd.com<mailto:Ashutosh.Nema at amd.com>>; Matthew Simpson <mssimpso at codeaurora.org<mailto:mssimpso at codeaurora.org>>; llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject: Re: [llvm-dev] enabling interleaved access loop vectorization
Hi Elena,
Circling back to this, do you know of any concrete cases where enabling interleaved access on x86 is unprofitable?
Right now, there are some cases where we lose significantly, because (a) we consider gathers (on architectures that don't have them) extremely expensive, so we won't vectorize them at all without interleaved access, and (b) we have interleaved access turned off.
Consider something like this:
void foo(int *in, int *out) {
int i = 0;
for (i = 0; i < 256; ++i) {
out[i] = in[i] + in[i + 1] + in[i + 2] + in[i * 2];
}
}
We don't vectorize this loop at all, because we calculate the cost of the in[i * 2] gather to be 14 cycles per lane (!).
This is an overestimate we need to fix, since the vectorized code is actually fairly decent - e.g. forcing vectorization, with SSE4.2, we get:
.LBB0_3: # %vector.body
# =>This Inner Loop Header: Depth=1
movdqu (%rdi,%rax,4), %xmm3
movd %xmm0, %rcx
movdqu 4(%rdi,%rcx,4), %xmm4
paddd %xmm3, %xmm4
movdqu 8(%rdi,%rcx,4), %xmm3
paddd %xmm4, %xmm3
movdqa %xmm1, %xmm4
paddq %xmm4, %xmm4
movdqa %xmm0, %xmm5
paddq %xmm5, %xmm5
movd %xmm5, %rcx
pextrq $1, %xmm5, %rdx
movd %xmm4, %r8
pextrq $1, %xmm4, %r9
movd (%rdi,%rcx,4), %xmm4 # xmm4 = mem[0],zero,zero,zero
pinsrd $1, (%rdi,%rdx,4), %xmm4
pinsrd $2, (%rdi,%r8,4), %xmm4
pinsrd $3, (%rdi,%r9,4), %xmm4
paddd %xmm3, %xmm4
movdqu %xmm4, (%rsi,%rax,4)
addq $4, %rax
paddq %xmm2, %xmm0
paddq %xmm2, %xmm1
cmpq $256, %rax # imm = 0x100
jne .LBB0_3
But the real point is that with interleaved access enabled, we vectorize, and get:
.LBB0_3: # %vector.body
# =>This Inner Loop Header: Depth=1
movdqu (%rdi,%rcx), %xmm0
movdqu 4(%rdi,%rcx), %xmm1
movdqu 8(%rdi,%rcx), %xmm2
paddd %xmm0, %xmm1
paddd %xmm2, %xmm1
movdqu (%rdi,%rcx,2), %xmm0
movdqu 16(%rdi,%rcx,2), %xmm2
pshufd $132, %xmm2, %xmm2 # xmm2 = xmm2[0,1,0,2]
pshufd $232, %xmm0, %xmm0 # xmm0 = xmm0[0,2,2,3]
pblendw $240, %xmm2, %xmm0 # xmm0 = xmm0[0,1,2,3],xmm2[4,5,6,7]
paddd %xmm1, %xmm0
movdqu %xmm0, (%rsi,%rcx)
cmpq $992, %rcx # imm = 0x3E0
jne .LBB0_7
The performance I see out of the 3 versions (with a 500K-iteration outer loop):
Scalar: 0m10.320s
Vector (Non-interleaved): 0m8.054s
Vector (Interleaved): 0m3.541s
This is far from being the perfect use case for interleaved access:
1) There's no real interleaving, just one strided gather, so this would be better served by Ashutosh's full "strided access" proposal.
2) It looks like the actual move + shuffle sequence is not better, and even probably worse, than just inserting directly from memory - but it's still worthwhile because of how much we save on the index computations.
Regardless of all that, the fact of the matter is that we get much better code by treating it as interleaved, and I think this may be a good enough motivation to enable it, unless we significantly regress in other cases.
I was going to look at benchmarks to see if we get any regressions, but if you already have examples you're aware of, that would be great.
Thanks,
Michael
On Thu, May 26, 2016 at 12:35 PM, Demikhovsky, Elena via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Interleaved access is not enabled on X86 yet.
We looked at this feature and got into conclusion that interleaving (as loads + shuffles) is not always profitable on X86. We should provide the right cost which depends on number of shuffles. Number of shuffles depends on permutations (shuffle mask). And even if we estimate the number of shuffles, the shuffles are not generated in-place. Vectorizer produces a long queue of "extracts" and "inserts" that hopefully will be coupled into shuffles on a later instcombine pass.
- Elena
>-----Original Message-----
>From: Renato Golin [mailto:renato.golin at linaro.org<mailto:renato.golin at linaro.org>]
>Sent: Thursday, May 26, 2016 21:25
>To: Sanjay Patel <spatel at rotateright.com<mailto:spatel at rotateright.com>>; Demikhovsky, Elena
><elena.demikhovsky at intel.com<mailto:elena.demikhovsky at intel.com>>
>Cc: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
>Subject: Re: [llvm-dev] enabling interleaved access loop vectorization
>
>On 26 May 2016 at 19:12, Sanjay Patel via llvm-dev <llvm-
>dev at lists.llvm.org<mailto:dev at lists.llvm.org>> wrote:
>> Is there a compile-time and/or potential runtime cost that makes
>> enableInterleavedAccessVectorization() default to 'false'?
>>
>> I notice that this is set to true for ARM, AArch64, and PPC.
>>
>> In particular, I'm wondering if there's a reason it's not enabled for
>> x86 in relation to PR27881:
>> https://llvm.org/bugs/show_bug.cgi?id=27881
>
>Hi Sanjay,
>
>The feature was originally developed for ARM's VLDn/VSTn instructions
>and then extended to AArch64 and PPC, but not x86/64 yet.
>
>I believe Elena was working on that, but needed to get the scatter/gather
>intrinsics working first. I just copied her in case I'm wrong. :)
>
>cheers,
>--renato
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