[Bug tree-optimization/93051] Wrong optimizations for pointers: `if (p == q) use p` -> `if (p == q) use q`

[ch3root at openwall dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93051

--- Comment #2 from Alexander Cherepanov <ch3root at openwall dot com> ---
Example with a dead malloc (not in tree-opt):

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

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

int main()
{
    int *q = malloc(sizeof(int));
    uintptr_t iq = (uintptr_t)(void *)q;
    free(q);

    int *p = malloc(sizeof(int));

    uintptr_t ir = (uintptr_t)(void *)opaque(p); // hide provenance of p

    if (ir == iq) {
        *p = 1;
        *(int *)ir = 2;
        printf("result: %d\n", *p);
    }
}
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$ gcc -std=c11 -pedantic -Wall -Wextra -Wno-attributes test.c && ./a.out
result: 2
$ gcc -std=c11 -pedantic -Wall -Wextra -Wno-attributes -O3 test.c && ./a.out
result: 1
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gcc x86-64 version: gcc (GCC) 10.0.0 20191223 (experimental)
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