[llvm-dev] enabling interleaved access loop vectorization

Fri Aug 5 04:20:45 PDT 2016

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>
Cc: Renato Golin <renato.golin at linaro.org>; Sanjay Patel <spatel at rotateright.com>; Nema, Ashutosh <Ashutosh.Nema at amd.com>; Matthew Simpson <mssimpso at codeaurora.org>; llvm-dev <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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