| Issue |
130510
|
| Summary |
Optimization: (x != C) comparison can utilize (x ^ C) result
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
Explorer09
|
This is an optimization opportunity that can benefit RISC-V, MIPS and other processors that don't use condition codes for their compare-and-branch instructions.
```c
#include <stdlib.h>
unsigned int func1_a(unsigned int x) {
if (x != 0x555)
return (x ^ 0x555);
abort();
}
unsigned int func1_c(unsigned int x) {
unsigned int tmp = x ^ 0x555;
__asm__ ("" : "+r" (tmp));
if (tmp == 0)
abort();
return tmp;
}
unsigned int func2_a(unsigned int x) {
if (x != 0x555)
return (x - 0x555);
abort();
}
unsigned int func2_c(unsigned int x) {
unsigned int tmp = x - 0x555;
__asm__ ("" : "+r" (tmp));
if (tmp == 0)
abort();
return tmp;
}
```
The example code above can be tested in Compiler Explorer (godbolt.org). (The target architecture I mostly concern here is `riscv32`.)
`func1_c()` and `func2_c()` are the results I expected for `func1_a()` and `func2_a()` to optimize to when using the `-Os` option. Clang 19 is already able to recognize the `func2` case (`x != 0x555` and `x - 0x555 != 0`) but misses on the `func1` case (`(x ^ 0x555) != 0`).
(Note: I also [reported this in GCC](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119178).)
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