[llvm-dev] enabling interleaved access loop vectorization

Michael Kuperstein via llvm-dev llvm-dev at lists.llvm.org
Thu Aug 4 16:22:58 PDT 2016

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

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.


On Thu, May 26, 2016 at 12:35 PM, Demikhovsky, Elena via llvm-dev <
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]
>    >Sent: Thursday, May 26, 2016 21:25
>    >To: Sanjay Patel <spatel at rotateright.com>; Demikhovsky, Elena
>    ><elena.demikhovsky at intel.com>
>    >Cc: llvm-dev <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> 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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