https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82224
Alexander Cherepanov <ch3root at openwall dot com> changed:
What |Removed |Added
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CC| |ch3root at openwall dot com
--- Comment #6 from Alexander Cherepanov <ch3root at openwall dot com> ---
Here are simplified testcases. With a union (C and C++):
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#include <stdio.h>
union u {
long x;
long long y;
};
static long test(long *px, long long *py, union u *pu)
{
pu->x = 0; // make .x active member (for C++)
*px = 0; // access .x via pointer
pu->y = pu->x; // switch active member to .y (for C++)
*py = 1; // access .y via pointer
pu->x = pu->y; // switch active member back to .x
return *px; // access .x via pointer
}
int main(void)
{
union u u;
printf("%ld\n", test(&u.x, &u.y, &u));
}
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Results:
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$ gcc -std=c11 -pedantic -Wall -Wextra test.c && ./a.out
1
$ gcc -std=c11 -pedantic -Wall -Wextra -O3 test.c && ./a.out
0
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And with allocated memory (C; add placement new's for C++):
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#include <stdlib.h>
#include <string.h>
#include <stdio.h>
static long test(long *px, long long *py, void *pu)
{
*px = 0;
*py = 1;
// change effective type from long long to long
long tmp;
memcpy(&tmp, pu, sizeof(tmp));
memcpy(pu, &tmp, sizeof(tmp));
return *px;
}
int main(void)
{
void *p = malloc(10);
printf("%ld\n", test(p, p, p));
}
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Results:
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$ gcc -std=c11 -pedantic -Wall -Wextra test.c && ./a.out
1
$ gcc -std=c11 -pedantic -Wall -Wextra -O3 test.c && ./a.out
0
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gcc version: gcc (GCC) 8.0.0 20171023 (experimental)
