| Issue |
142283
|
| Summary |
Missed optimization (unnecessary sext): y >= x should imply y - x >= 0
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
Explorer09
|
Test code
```c
#include <stdint.h>
extern void subroutine(uint64_t x);
void func1(int32_t a, int32_t b) {
if (a > b)
return;
subroutine(b - a);
}
void func2(int32_t a, int32_t b) {
if (a > b)
return;
subroutine((uint64_t)(int32_t)(b - a));
}
void func3(int32_t a, int32_t b) {
if (a > b)
return;
subroutine((uint64_t)(uint32_t)(b - a));
}
uint64_t func4(int32_t a, int32_t b) {
if (a > b)
return 0;
return (b - a);
}
uint64_t func5(int32_t a, int32_t b) {
if (a > b)
return 0;
return (uint64_t)(int32_t)(b - a);
}
uint64_t func6(int32_t a, int32_t b) {
if (a > b)
return 0;
return (uint64_t)(uint32_t)(b - a);
}
```
Actual result (x86-64 clang 20.1.0 with `-Os` option): `func1`, `func2`, `func4` and `func5` made unnecessary sign extensions (`movsxd` instruction). (https://godbolt.org/z/s3zvnET5e)
Expected result: `func1`, `func2` and `func3` compile to same code. `func4`, `func5` and `func6` compile to same code.
After the `(a > b)` conditional, Clang should be able to know that `(a <= b)`, which in turn implies `(0 <= b - a)`. In other words, `(b - a)` would be always nonnegative.
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