https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112629
Bug ID: 112629
Summary: Missed-optimization: constant << (b + signed_constant)
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: ---
The following C code:
```
int add_shift(signed shift) {
return 1 << (shift + 4);
}
add_shift:
lea ecx, [rdi+4]
mov eax, 1
sal eax, cl
ret
```
Could be optimized to:
```
int optimized_shift(signed shift) {
return 16 << shift;
}
optimized_shift:
mov ecx, edi
mov eax, 16
sal eax, cl
ret
```
We always know we are shifting by at least 4, since the added value is a signed
integer and overflow is undefined. Also note that this optimization works the
same for right shifts.
