https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79201
Bug ID: 79201
Summary: issed optimization: gcc fails to cut out unnecessary
loop.
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: drraph at gmail dot com
Target Milestone: ---
Consider this code:
int f(int n) {
int i,j=0;
for (i = 0; i < 32; i++) {
j = __builtin_ffs(i);
}
return j;
}
With gcc -O3 you get
f:
xor eax, eax
xor edx, edx
mov ecx, -1
jmp .L3
.L5:
bsf eax, edx
cmove eax, ecx
add eax, 1
.L3:
add edx, 1
cmp edx, 32
jne .L5
rep ret
However with clang you get
f: # @f
mov eax, 1
ret
A similar difference occurs if you replace the limit 32 with the variable n.
gcc is unable to detect that the loop can be omitted.
