[Bug tree-optimization/61502] == comparison on "one-past" pointer gives wrong result

[ch3root at openwall dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61502

--- Comment #43 from Alexander Cherepanov <ch3root at openwall dot com> ---
The following example demonstrates that the instability taints the surrounding
code even if it's in dead code itself. In particular, this shows that even
making comparisons like `&x + 1 == &y` undefined will not help.

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#include <stdlib.h>
#include <stdio.h>

__attribute__((noipa)) // imagine it in a separate TU
static void *opaque(void *p) { return p; }

__attribute__((noipa)) // imagine it in a separate TU
static void g(int a)
{
    printf("%d\n", a);
    exit(0);
}

static void f(int c, void *p, void *q, void *r)
{
    while (c) {
        g(p == r);

        if (p != q && q == r)
            puts("unreachable");
    }
}

int main(int c, char **v)
{
    (void)v;

    int x[5];
    int y[2];

    void *p = &x;
    void *q = &y + 1;

    opaque(q); // escaped
    void *r = opaque(p); // hide the provenance of p

    f(c, p, q, r);
}
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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 x86-64 version: gcc (GCC) 10.0.1 20200120 (experimental)
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