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From: "ch3root at openwall dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/92486] Wrong optimization: padding in structs
 is not copied even with memcpy
Date: Fri, 20 Mar 2020 11:23:50 +0000
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X-Bugzilla-Component: tree-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D92486

--- Comment #18 from Alexander Cherepanov <ch3root at openwall dot com> ---
Adding a `memset` makes trunk fail too:

----------------------------------------------------------------------
#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 =3D *q;
    memcpy(&w, q, sizeof(struct s));

    *p =3D w;
    memcpy(p, &w, sizeof(struct s));
}

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 =3D (unsigned char *)&y; p < (unsigned char *)&y +
sizeof(struct s); p++)
        printf("%d", *p);
    printf("\n");
}
----------------------------------------------------------------------
$ gcc -std=3Dc11 -pedantic -Wall -Wextra test.c && ./a.out
11111111
$ gcc -std=3Dc11 -pedantic -Wall -Wextra -O3 test.c && ./a.out
12221111
----------------------------------------------------------------------
gcc x86-64 version: gcc (GCC) 10.0.1 20200320 (experimental)
----------------------------------------------------------------------=