On December 5, 2017 9:18:46 PM GMT+01:00, Jeff Law <l...@redhat.com> wrote:
>On 11/28/2017 08:15 AM, Richard Biener wrote:
>>
>> The following adds a new target hook,
>targetm.vectorize.split_reduction,
>> which allows the target to specify a preferred mode to perform the
>> final reducion on using either vector shifts or scalar extractions.
>> Up to that mode the vector reduction result is reduced by combining
>> lowparts and highparts recursively. This avoids lane-crossing
>operations
>> when doing AVX256 on Zen and Bulldozer and also speeds up things on
>> Haswell (I verified ~20% speedup on Broadwell).
>>
>> Thus the patch implements the target hook on x86 to _always_ prefer
>> SSE modes for the final reduction.
>>
>> For the testcase in the bugzilla
>>
>> int sumint(const int arr[]) {
>> arr = __builtin_assume_aligned(arr, 64);
>> int sum=0;
>> for (int i=0 ; i<1024 ; i++)
>> sum+=arr[i];
>> return sum;
>> }
>>
>> this changes -O3 -mavx512f code from
>>
>> sumint:
>> .LFB0:
>> .cfi_startproc
>> vpxord %zmm0, %zmm0, %zmm0
>> leaq 4096(%rdi), %rax
>> .p2align 4,,10
>> .p2align 3
>> .L2:
>> vpaddd (%rdi), %zmm0, %zmm0
>> addq $64, %rdi
>> cmpq %rdi, %rax
>> jne .L2
>> vpxord %zmm1, %zmm1, %zmm1
>> vshufi32x4 $78, %zmm1, %zmm0, %zmm2
>> vpaddd %zmm2, %zmm0, %zmm0
>> vmovdqa64 .LC0(%rip), %zmm2
>> vpermi2d %zmm1, %zmm0, %zmm2
>> vpaddd %zmm2, %zmm0, %zmm0
>> vmovdqa64 .LC1(%rip), %zmm2
>> vpermi2d %zmm1, %zmm0, %zmm2
>> vpaddd %zmm2, %zmm0, %zmm0
>> vmovdqa64 .LC2(%rip), %zmm2
>> vpermi2d %zmm1, %zmm0, %zmm2
>> vpaddd %zmm2, %zmm0, %zmm0
>> vmovd %xmm0, %eax
>>
>> to
>>
>> sumint:
>> .LFB0:
>> .cfi_startproc
>> vpxord %zmm0, %zmm0, %zmm0
>> leaq 4096(%rdi), %rax
>> .p2align 4,,10
>> .p2align 3
>> .L2:
>> vpaddd (%rdi), %zmm0, %zmm0
>> addq $64, %rdi
>> cmpq %rdi, %rax
>> jne .L2
>> vextracti64x4 $0x1, %zmm0, %ymm1
>> vpaddd %ymm0, %ymm1, %ymm1
>> vmovdqa %xmm1, %xmm0
>> vextracti128 $1, %ymm1, %xmm1
>> vpaddd %xmm1, %xmm0, %xmm0
>> vpsrldq $8, %xmm0, %xmm1
>> vpaddd %xmm1, %xmm0, %xmm0
>> vpsrldq $4, %xmm0, %xmm1
>> vpaddd %xmm1, %xmm0, %xmm0
>> vmovd %xmm0, %eax
>>
>> and for -O3 -mavx2 from
>>
>> sumint:
>> .LFB0:
>> .cfi_startproc
>> vpxor %xmm0, %xmm0, %xmm0
>> leaq 4096(%rdi), %rax
>> .p2align 4,,10
>> .p2align 3
>> .L2:
>> vpaddd (%rdi), %ymm0, %ymm0
>> addq $32, %rdi
>> cmpq %rdi, %rax
>> jne .L2
>> vpxor %xmm1, %xmm1, %xmm1
>> vperm2i128 $33, %ymm1, %ymm0, %ymm2
>> vpaddd %ymm2, %ymm0, %ymm0
>> vperm2i128 $33, %ymm1, %ymm0, %ymm2
>> vpalignr $8, %ymm0, %ymm2, %ymm2
>> vpaddd %ymm2, %ymm0, %ymm0
>> vperm2i128 $33, %ymm1, %ymm0, %ymm1
>> vpalignr $4, %ymm0, %ymm1, %ymm1
>> vpaddd %ymm1, %ymm0, %ymm0
>> vmovd %xmm0, %eax
>>
>> to
>>
>> sumint:
>> .LFB0:
>> .cfi_startproc
>> vpxor %xmm0, %xmm0, %xmm0
>> leaq 4096(%rdi), %rax
>> .p2align 4,,10
>> .p2align 3
>> .L2:
>> vpaddd (%rdi), %ymm0, %ymm0
>> addq $32, %rdi
>> cmpq %rdi, %rax
>> jne .L2
>> vmovdqa %xmm0, %xmm1
>> vextracti128 $1, %ymm0, %xmm0
>> vpaddd %xmm0, %xmm1, %xmm0
>> vpsrldq $8, %xmm0, %xmm1
>> vpaddd %xmm1, %xmm0, %xmm0
>> vpsrldq $4, %xmm0, %xmm1
>> vpaddd %xmm1, %xmm0, %xmm0
>> vmovd %xmm0, %eax
>> vzeroupper
>> ret
>>
>> which besides being faster is also smaller (less prefixes).
>>
>> SPEC 2k6 results on Haswell (thus AVX2) are neutral. As it merely
>> effects reduction vectorization epilogues I didn't expect big effects
>> but for loops that do not run much (more likely with AVX512).
>>
>> Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.
>>
>> Ok for trunk?
>>
>> The PR mentions some more tricks to optimize the sequence but
>> those look like backend only optimizations.
>>
>> Thanks,
>> Richard.
>>
>> 2017-11-28 Richard Biener <rguent...@suse.de>
>>
>> PR tree-optimization/80846
>> * target.def (split_reduction): New target hook.
>> * targhooks.c (default_split_reduction): New function.
>> * targhooks.h (default_split_reduction): Declare.
>> * tree-vect-loop.c (vect_create_epilog_for_reduction): If the
>> target requests first reduce vectors by combining low and high
>> parts.
>> * tree-vect-stmts.c (vect_gen_perm_mask_any): Adjust.
>> (get_vectype_for_scalar_type_and_size): Export.
>> * tree-vectorizer.h (get_vectype_for_scalar_type_and_size): Declare.
>>
>> * doc/tm.texi.in (TARGET_VECTORIZE_SPLIT_REDUCTION): Document.
>> * doc/tm.texi: Regenerate.
>>
>> i386/
>> * config/i386/i386.c (ix86_split_reduction): Implement
>> TARGET_VECTORIZE_SPLIT_REDUCTION.
>>
>> * gcc.target/i386/pr80846-1.c: New testcase.
>> * gcc.target/i386/pr80846-2.c: Likewise.
>I'm not a big fan of introducing these kinds of target queries into the
>gimple optimizers, but I think we've all agreed to allow them to
>varying
>degrees within the vectorizer.
>
>So no objections from me. You know the vectorizer bits far better than
>I :-)
I had first (ab)used the vector_sizes hook but Jakub convinced me to add a new
one. There might be non-trivial costs when moving between vector sizes.
Richard.
>
>jeff
