On 13/05/2024 14:42, Rademacher Lars via Gcc-help wrote:
> Dear all,
>
> I am having the problem, that since GCC 8 in the following code the if-section is optimized away for any optimization levels above 0.
> But actually for a negative value of 'a', the value of 'ret' will overflow and thus 'ret' will be < 0, so the if-section cannot be optimized away.
>
> #include <stdint.h>
> int32_t foo(int32_t a)
> {
> int32_t ret = INT32_MAX - a;
>
> if (ret < 0) {
> ret -= 50;
> }
>
> return ret;
> }
>
> I am guessing, this is due to the fact, that signed integer overflow is undefined behavior in C/C++ and thus the compiler assumes this is prevented. Can you confirm this?
Correct.
> Can you give a hint, if this is a correct assumption and why this was introduced?
>
> Which part of GCC 8 release notes represents this optimization?
https://gcc.gnu.org/gcc-8/changes.html
Just look for 'signed integer overflow'.
R.
