http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54116
Bug #: 54116
Summary: suboptimal code for tight loops
Classification: Unclassified
Product: gcc
Version: 4.7.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: nel...@seznam.cz
Consider following loop.
int recal(int *x){int i;
for(i=0;;i+=4){
if(__builtin_expect((x[i]|x[i+1])|(x[i+2]|x[i+3]),0))
break;
}
return (x[i]|x[i+1])*(x[i+2]|x[i+3]);
}
On x64 orl instruction is destructive. Gcc saves intermediate result to
register instead recalculating it at end of loop, making loop run slower.
Relevant assembly output is following:
gcc-4.7 -O3 -S
.file "recal.c"
.text
.p2align 4,,15
.globl recal
.type recal, @function
recal:
.LFB0:
.cfi_startproc
movl 12(%rdi), %edx
orl 8(%rdi), %edx
movl 4(%rdi), %ecx
orl (%rdi), %ecx
movl %edx, %eax
orl %ecx, %eax
jne .L2
leaq 16(%rdi), %rax
.p2align 4,,10
.p2align 3
.L3:
movl 12(%rax), %edx
orl 8(%rax), %edx
movl 4(%rax), %ecx
orl (%rax), %ecx
addq $16, %rax
movl %edx, %esi
orl %ecx, %esi
je .L3
.L2:
movl %ecx, %eax
imull %edx, %eax
ret
.cfi_endproc
.LFE0:
.size recal, .-recal
.ident "GCC: (Debian 4.7.1-2) 4.7.1"
.section .note.GNU-stack,"",@progbits
--
