https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105618
Bug ID: 105618
Summary: Missed loop body simplification by -O3 (trunk v.s.
10.3)
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: shaohua.li at inf dot ethz.ch
Target Milestone: ---
For the following code, gcc truck -O3 fails to optimize away the loop body,
while gcc10.3 can. You can see from the assembly code that gcc10.3 figured out
that the variable c would be a constant after the loop while gcc-trunk didn't.
$cat a.c
static int b=4;
int c;
int main() {
int e[5] = {1,1,1,1,1};
for (; b >= 0; b--) {
c = e[b];
}
return 0;
}
$
$gcc-trunk -O3 -S -o trunk.s a.c
$gcc-10-3 -O3 -S -o 10-3.s a.c
$wc trunk.s 10-3.s
66 147 1005 trunk.s
52 106 698 10-3.s
118 253 1703 total
$
$cat 10-3.s
main:
.LFB0:
.cfi_startproc
endbr64
movl b(%rip), %eax
testl %eax, %eax
js .L2
movl $1, c(%rip)
movl $-1, b(%rip)
.L2:
xorl %eax, %eax
ret
$
$cat trunk.s
main:
.LFB0:
.cfi_startproc
movdqa .LC0(%rip), %xmm0
movl b(%rip), %eax
movl $1, -24(%rsp)
movaps %xmm0, -40(%rsp)
testl %eax, %eax
js .L2
movslq %eax, %rdx
leal -1(%rax), %ecx
movl -40(%rsp,%rdx,4), %edx
je .L3
leal -2(%rax), %edx
cmpl $1, %eax
je .L10
leal -3(%rax), %ecx
cmpl $2, %eax
je .L6
leal -4(%rax), %edx
cmpl $3, %eax
je .L10
.L6:
movslq %edx, %rdx
movl -40(%rsp,%rdx,4), %edx
.L3:
movl %edx, c(%rip)
movl $-1, b(%rip)
.L2:
xorl %eax, %eax
ret
.L10:
movslq %ecx, %rcx
movl -40(%rsp,%rcx,4), %edx
jmp .L3
$
