https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92486
--- Comment #9 from Alexander Cherepanov <ch3root at openwall dot com> ---
> Now as an exercise build a complete testcase for the DSE issue above.
Source code:
----------------------------------------------------------------------
#include <string.h>
#include <stdio.h>
struct s {
char c;
int i;
};
__attribute__((noinline,noclone))
void f(struct s *p, struct s *q)
{
struct s w;
memset(&w, 0, sizeof(struct s));
w = *q;
memset(p, 0, sizeof(struct s));
*p = w;
}
int main()
{
struct s x;
memset(&x, 1, sizeof(struct s));
struct s y;
memset(&y, 2, sizeof(struct s));
f(&y, &x);
for (unsigned char *p = (unsigned char *)&y; p < (unsigned char *)&y +
sizeof(struct s); p++)
printf("%d", *p);
printf("\n");
}
----------------------------------------------------------------------
Results:
----------------------------------------------------------------------
$ gcc -std=c11 -pedantic -Wall -Wextra test.c && ./a.out
11111111
$ gcc -std=c11 -pedantic -Wall -Wextra -O3 test.c && ./a.out
12221111
----------------------------------------------------------------------
gcc x86-64 version: gcc (GCC) 10.0.0 20191114 (experimental)
But from the C standard POV this case is much more clear: there is no problem
as stores into a struct make its padding unspecified (C11, 6.2.6.1p6). OTOH
this sample demonstrates the problem with trunc, so it could be more convenient
for testing.
