* RE: How to avoid auto-vectorization for this loop (rolls at most 3 times)
[not found] ` <680044E4997F5343A2C58032DDD099161733F9@ZIPPY.Emea.Arm.com>
@ 2010-09-09 23:13 ` Fang, Changpeng
0 siblings, 0 replies; 2+ messages in thread
From: Fang, Changpeng @ 2010-09-09 23:13 UTC (permalink / raw)
To: Ian Bolton; +Cc: gcc
[-- Attachment #1: Type: text/plain, Size: 986 bytes --]
>> It seems the auto-vectorizer could not recognize that this loop will
>> roll at most 3 times.
>> And it will generate quite messy code.
>>
>> int a[1024], b[1024];
>> void foo (int n)
>> {
>> int i;
>> for (i = (n/4)*4; i< n; i++)
>> a[i] = a[i] + b[i];
>> }
>>
>> How can we correctly estimate the number of iterations for this case
>> and use this info for the vectorizer?
>Does it recognise it if you rewrite the loop as follows:
>for (i = n&~0x3; i< n; i++)
> a[i] = a[i] + b[i];
NO.
But it is OK for the following case:
for (i = n-3; i< n; i++)
a[i] = a[i] + b[i];
It seems it fails at the case of "unknown but small". Anyway, this mostly
affects compilation time and code size, and has limited impact on
performance.
For
for (i = n&~0x3; i< n; i++)
a[i] = a[i] + b[i];
The attached foo-O3-no-tree-vectorize.s is what we expect from the optimizer.
foo-O3.s is too bad.
Thanks,
Changpeng
[-- Attachment #2: foo-O3-no-tree-vectorize.s --]
[-- Type: application/octet-stream, Size: 500 bytes --]
.file "foo1.c"
.text
.p2align 4,,15
.globl foo
.type foo, @function
foo:
.LFB0:
.cfi_startproc
movl %edi, %eax
andl $-4, %eax
cmpl %eax, %edi
jle .L1
.p2align 4,,10
.p2align 3
.L4:
movslq %eax, %rdx
addl $1, %eax
movl a(,%rdx,4), %ecx
addl b(,%rdx,4), %ecx
cmpl %edi, %eax
movl %ecx, a(,%rdx,4)
jne .L4
.L1:
rep
ret
.cfi_endproc
.LFE0:
.size foo, .-foo
.comm b,4096,32
.comm a,4096,32
.ident "GCC: (GNU) 4.6.0 20100831 (experimental)"
.section .note.GNU-stack,"",@progbits
[-- Attachment #3: foo-O3.s --]
[-- Type: application/octet-stream, Size: 1634 bytes --]
.file "foo1.c"
.text
.p2align 4,,15
.globl foo
.type foo, @function
foo:
.LFB0:
.cfi_startproc
movl %edi, %esi
pushq %rbp
.cfi_def_cfa_offset 16
andl $-4, %esi
cmpl %esi, %edi
pushq %rbx
.cfi_def_cfa_offset 24
jle .L1
.cfi_offset 3, -24
.cfi_offset 6, -16
movslq %esi, %rbx
movl %edi, %r9d
leaq a(,%rbx,4), %r8
subl %esi, %r9d
andl $15, %r8d
shrq $2, %r8
negl %r8d
andl $3, %r8d
cmpl %r9d, %r8d
cmova %r9d, %r8d
testl %r8d, %r8d
mov %r8d, %ebp
je .L8
movl %esi, %eax
.p2align 4,,10
.p2align 3
.L4:
movslq %eax, %rdx
addl $1, %eax
movl a(,%rdx,4), %ecx
addl b(,%rdx,4), %ecx
movl %ecx, a(,%rdx,4)
movl %eax, %edx
subl %esi, %edx
cmpl %edx, %r8d
ja .L4
cmpl %r8d, %r9d
je .L1
.L3:
movl %r9d, %r11d
subl %r8d, %r11d
movl %r11d, %r8d
shrl $2, %r8d
leal 0(,%r8,4), %r10d
testl %r10d, %r10d
je .L9
leaq (%rbx,%rbp), %r9
xorl %edx, %edx
xorl %ecx, %ecx
salq $2, %r9
leaq a(%r9), %rsi
addq $b, %r9
.p2align 4,,10
.p2align 3
.L6:
movdqu (%r9,%rdx), %xmm0
addl $1, %ecx
paddd (%rsi,%rdx), %xmm0
movdqa %xmm0, (%rsi,%rdx)
addq $16, %rdx
cmpl %r8d, %ecx
jb .L6
addl %r10d, %eax
cmpl %r10d, %r11d
je .L1
.p2align 4,,10
.p2align 3
.L9:
movslq %eax, %rdx
addl $1, %eax
movl a(,%rdx,4), %ecx
addl b(,%rdx,4), %ecx
cmpl %eax, %edi
movl %ecx, a(,%rdx,4)
jg .L9
.L1:
popq %rbx
.cfi_remember_state
.cfi_def_cfa_offset 16
popq %rbp
.cfi_def_cfa_offset 8
ret
.L8:
.cfi_restore_state
movl %esi, %eax
jmp .L3
.cfi_endproc
.LFE0:
.size foo, .-foo
.comm b,4096,32
.comm a,4096,32
.ident "GCC: (GNU) 4.6.0 20100831 (experimental)"
.section .note.GNU-stack,"",@progbits
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