https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112660
Bug ID: 112660
Summary: missed-optimization: combine shifts when shifted out
bits are known 0
Product: gcc
Version: 13.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: goon.pri.low at gmail dot com
Target Milestone: ---
This function here we know the shifted out bits have to be 0:
int unopt(int v) {
if (v & 3) return -1;
return v >> 2 << 5;
}
unopt:
test dil, 3
jne .L3
sar edi, 2
mov eax, edi
sal eax, 5
ret
.L3:
mov eax, -1
ret
Therefore we could combine the shifts:
int opt(int v) {
if (v & 3) return -1;
return v << 3;
}
opt:
test dil, 3
jne .L7
lea eax, [0+rdi*8]
ret
.L7:
mov eax, -1
ret
