| Issue |
170634
|
| Summary |
RISC-V optimization: builtin_sub_overflow(unsigned) can make sub-optimal code
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
Explorer09
|
https://godbolt.org/z/oW3nq5aKM
```c
#include <stdbool.h>
unsigned long func1_a(unsigned long x, unsigned long y) {
if (__builtin_usubl_overflow(x, y, &x))
return 0x123;
return x;
}
unsigned long func1_b(unsigned long x, unsigned long y) {
if (x < y)
return 0x123;
return x - y;
}
unsigned long func1_c(unsigned long x, unsigned long y) {
if (x - y > x)
return 0x123;
return x - y;
}
```
rv64 clang 21.1.0 with `-Os` option:
```assembly
func1_a:
mv a2, a0
sub a0, a0, a1
bgeu a2, a0, .LBB0_2
li a0, 291
.LBB0_2:
ret
func1_b:
bgeu a0, a1, .LBB1_2
li a0, 291
ret
.LBB1_2:
sub a0, a0, a1
ret
```
A `__builtin_sub_overflow(x, y, ...)` with unsigned integers makes a slightly worse code than a simple `(x < y)` conditional. GCC documentation doesn't say that `__builtin_sub_overflow` need to be an atomic calculation, so I think the optimization from `func1_a` to `func1_b` is allowed.
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