On Mon, Dec 16, 2013 at 5:54 PM, Bingfeng Mei <b...@broadcom.com> wrote:
> Hi,
> I was looking at some loops that can be vectorized by LLVM, but not GCC. One
> type of loop is with store of negative step.
>
> void test1(short * __restrict__ x, short * __restrict__ y, short *
> __restrict__ z)
> {
> int i;
> for (i=127; i>=0; i--) {
> x[i] = y[127-i] + z[127-i];
> }
> }
>
> I don't know why GCC only implements negative step for load, but not store. I
> implemented a patch, very similar to code in vectorizable_load.
>
> ~/scratch/install-x86/bin/gcc ghs-dec.c -ftree-vectorize -S -O2 -mavx
>
> Without patch:
> test1:
> .LFB0:
> addq $254, %rdi
> xorl %eax, %eax
> .p2align 4,,10
> .p2align 3
> .L2:
> movzwl (%rsi,%rax), %ecx
> subq $2, %rdi
> addw (%rdx,%rax), %cx
> addq $2, %rax
> movw %cx, 2(%rdi)
> cmpq $256, %rax
> jne .L2
> rep; ret
>
> With patch:
> test1:
> .LFB0:
> vmovdqa .LC0(%rip), %xmm1
> xorl %eax, %eax
> .p2align 4,,10
> .p2align 3
> .L2:
> vmovdqu (%rsi,%rax), %xmm0
> movq %rax, %rcx
> negq %rcx
> vpaddw (%rdx,%rax), %xmm0, %xmm0
> vpshufb %xmm1, %xmm0, %xmm0
> addq $16, %rax
> cmpq $256, %rax
> vmovups %xmm0, 240(%rdi,%rcx)
> jne .L2
> rep; ret
>
> Performance is definitely improved here. It is bootstrapped for
> x86_64-unknown-linux-gnu, and has no additional regressions on my machine.
>
> For reference, LLVM seems to use different instructions and slightly worse
> code. I am not so familiar with x86 assemble code. The patch is originally
> for our private port.
> test1: # @test1
> .cfi_startproc
> # BB#0: # %entry
> addq $240, %rdi
> xorl %eax, %eax
> .align 16, 0x90
> .LBB0_1: # %vector.body
> # =>This Inner Loop Header: Depth=1
> movdqu (%rsi,%rax,2), %xmm0
> movdqu (%rdx,%rax,2), %xmm1
> paddw %xmm0, %xmm1
> shufpd $1, %xmm1, %xmm1 # xmm1 = xmm1[1,0]
> pshuflw $27, %xmm1, %xmm0 # xmm0 = xmm1[3,2,1,0,4,5,6,7]
> pshufhw $27, %xmm0, %xmm0 # xmm0 = xmm0[0,1,2,3,7,6,5,4]
> movdqu %xmm0, (%rdi)
> addq $8, %rax
> addq $-16, %rdi
> cmpq $128, %rax
> jne .LBB0_1
> # BB#2: # %for.end
> ret
>
> Any comment?
Looks good to me. One of the various TODOs in vectorizable_store I presume.
Needs a testcase and at this stage a bugreport that is fixed by it.
Thanks,
Richard.
> Bingfeng Mei
> Broadcom UK
>
>
