http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53355
Bug #: 53355
Summary: Autovectorization of a simple loop could be improved.
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: svfue...@gmail.com
The following simple function gets autovectorized by gcc-4.7 -O3
void foo(double *x)
{
int i;
for (i = 0; i < 100000; i++)
{
x[i] += 2.0;
}
}
However, the generated code is rather poor:
foo:
.LFB0:
.cfi_startproc
movq %rdi, %rax
salq $60, %rax
sarq $63, %rax
movq %rax, %rdx
andl $1, %edx
testq %rdx, %rdx
movl %edx, %ecx
je .L7
movsd .LC0(%rip), %xmm0
movl $99999, %r9d
movl $1, %r10d
addsd (%rdi), %xmm0
movsd %xmm0, (%rdi)
.L2:
movl $100000, %r8d
andl $1, %eax
subl %ecx, %r8d
movl %r8d, %esi
shrl %esi
movl %esi, %r11d
addl %r11d, %r11d
je .L3
movapd .LC1(%rip), %xmm1
leaq (%rdi,%rax,8), %rcx
xorl %edx, %edx
xorl %eax, %eax
.p2align 4,,10
.p2align 3
.L4:
movapd (%rcx,%rax), %xmm0
addl $1, %edx
addpd %xmm1, %xmm0
movapd %xmm0, (%rcx,%rax)
addq $16, %rax
cmpl %esi, %edx
jb .L4
addl %r11d, %r10d
subl %r11d, %r9d
cmpl %r11d, %r8d
je .L1
.L3:
leal -1(%r9), %edx
movslq %r10d, %r10
leaq (%rdi,%r10,8), %rax
movsd .LC0(%rip), %xmm1
addq %rdx, %r10
leaq 8(%rdi,%r10,8), %rdx
.p2align 4,,10
.p2align 3
.L6:
movsd (%rax), %xmm0
addsd %xmm1, %xmm0
movsd %xmm0, (%rax)
addq $8, %rax
cmpq %rdx, %rax
jne .L6
.L1:
rep
ret
.L7:
movl $100000, %r9d
xorl %r10d, %r10d
jmp .L2
The first thing wrong is that the alignment test is rather clunky. Instead, we
can just do a "testq $8, %rdi" followed by a jump to the miss-aligned case.
The second thing wrong is that the code instead of falling-through to the
aligned case, jumps down to L7 instead. Forward jumps are predicted as not
taken.
The inner loop is also longer than it should be. It is possible to have one
less increment statement inside.
Finally, after the main loop, the code again tests for miss-alignment. This
pessimizes the fast path. Instead, it possible to move the slow miss-aligned
case completely out with very little size penalty.
Doing the above optimizations yields:
movapd 2_s, %xmm1
testq $8, %rdi
jne 2f
lea 800000(%rdi), %rax
# The "Hot" inner loop
1: movapd (%rdi), %xmm0
addq $16, %rdi
addpd %xmm1, %xmm0
movapd %xmm0, (%rdi)
cmpq %rdi, %rax
jne 1b
rep ret
# The slow miss-aligned case
2: movsd (%rdi), %xmm0
addsd %xmm1, %xmm0
movsd %xmm0, (%rdi)
leaq (800000 - 16) (%rdi), %rax
addq $8, %rdi
3: movapd (%rdi), %xmm0
addq $16, %rdi
addpd %xmm1, %xmm0
movapd %xmm0, (%rdi)
cmpq %rdi, %rax
jne 3b
movsd (%rdi), %xmm0
addsd %xmm1, %xmm0
movsd %xmm0, (%rdi)
ret
