https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110240
Bug ID: 110240
Summary: Unnecessary register move in indexed swap routine
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: tkoenig at gcc dot gnu.org
Target Milestone: ---
void swap (unsigned int * restrict a, unsigned int * restrict b)
{
if (a[b[0]] > a[b[1]])
{
unsigned int tmp = b[0];
b[0] = b[1];
b[1] = tmp;
}
}
$ gcc -O3 -S swap.c
gets me
swap:
.LFB0:
.cfi_startproc
movl (%rsi), %ecx
movl 4(%rsi), %r8d
movq %rcx, %rax
movl (%rdi,%rcx,4), %ecx
cmpl %ecx, (%rdi,%r8,4)
jnb .L1
movl %r8d, (%rsi)
movl %eax, 4(%rsi)
.L1:
ret
.cfi_endproc
where the
movq %rcx, %rax
is unneeded, because rcs is not overwritten.
(It is probably also a zero-latency operation due to register renaming,
but still).
