[Bug middle-end/89230] Bogus uninited usage warning

[lavr at ncbi dot nlm.nih.gov]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89230

--- Comment #5 from lavr at ncbi dot nlm.nih.gov ---
Thank you Martin, for giving me the idea of where the problem might be stemming
from!  It does look like *printf() is not recognized by GCC as not modifying
the local memory.  But in your example GCC _might_ think that "p" and "q" can
be aliased (being the result of the same function).

Below is a mockup of the code that I was dealing with, clearly showing that "s"
and "h" cannot be aliased!  "s" is being on the local frame all alone.  I get
the same "uninitialized" warning if BUG is defined:

$ cat bogus2.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct S {
    int a;
    int b;
};

struct H {
    int c;
    int d;
};

void getblk(void* blk)
{
    struct S* s = (struct S*) blk;
    memset(blk, 0, 512);
    s->a = rand() & 1;
}

struct H* gethdr(void* blk)
{
    memset(blk, 0, 512);
    return rand() & 1 ? (struct H*) blk : 0;
}

int main(void)
{
    char blk[512], tmp[512];
    struct S *s = (struct S*) blk;
    struct H *h;

    getblk(blk);

    if (s->a  ||  !(h = gethdr(tmp))  ||  s->a != h->d) {
#ifdef BUG
        printf("%d\n", s->b);
#endif
        if (s->a)
            printf("s->a = %d\n", s->a);
        else if (!h)
            printf("!h\n");
        else
            printf("h->d = %d\n", h->d);
    }
}

$ gcc -O6 -Wall -c bogus2.c

$ gcc -DBUG -O6 -Wall -c bogus2.c
bogus2.c: In function ‘main’:
bogus2.c:42:17: warning: ‘h’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
         else if (!h)
                 ^