https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105768
Bug ID: 105768
Summary: Missed optimization: shifting signed to unsigned range
before comparison not removed
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: davidfromonline at gmail dot com
Target Milestone: ---
The following translation unit:
```
#include <limits.h>
inline unsigned to_unsigned(int value) {
return (unsigned)value + (unsigned)INT_MIN;
}
bool f(int lhs, int rhs) {
return to_unsigned(lhs) < to_unsigned(rhs);
}
```
when compiled with `-O3` optimizes to
```
f(int, int):
add esi, -2147483648
add edi, -2147483648
cmp edi, esi
setb al
ret
```
I would expect this to optimize to
```
f(int, int):
cmp edi, esi
setl al
ret
```
Essentially, I want gcc to recognize that a signed value + minimum signed
value, as an unsigned, has the same comparison semantics as just comparing the
original signed value.
This code pattern comes up in implementations of radix sort (specifically,
ska_sort) when it falls back to std::sort (for instance, because the range is
small).
See it live: https://godbolt.org/z/Gn4rxr3nY
