https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66673
Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|RESOLVED |REOPENED
Last reconfirmed| |2015-06-25
CC| |manu at gcc dot gnu.org
Resolution|INVALID |---
Summary|swapping variables via |warning missing for
|chained xor fails |undefined behavior when
| |swapping variables via
| |chained xor
Ever confirmed|0 |1
--- Comment #7 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Marek Polacek from comment #6)
> (In reply to joe.carnuccio from comment #2)
> > -Wall produces no warnings...
>
> Oh, you're using too old GCC. Please try a newer version.
To be fair, GCC 5.1 does not generate a warning either with -Wall -Wextra.
Neither does clang 3.7.
For this testcase:
void swap(int *a, int *b) {
*a ^= *b ^= *a ^= *b;
}
int main() {
int a = 5;
int b = 8;
printf("%d, %d\n", a, b);
a ^= b ^= a ^= b;
__builtin_printf("%d, %d\n", a, b);
swap(&a, &b);
__builtin_printf("%d, %d\n", a, b);
}
Clang warns:
20 : warning: unsequenced modification and access to 'a' [-Wunsequenced]
a ^= b ^= a ^= b;
but gcc is silent.
