[Bug tree-optimization/93010] New: Wrong optimization: provenance affects comparison of saved bits of addresses of dead auto variables

[ch3root at openwall dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93010

            Bug ID: 93010
           Summary: Wrong optimization: provenance affects comparison of
                    saved bits of addresses of dead auto variables
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ch3root at openwall dot com
  Target Milestone: ---

It's known that the value of a pointer to an object becomes indeterminate after
the object is dead (C11, 6.2.4p2). Whether its representation becomes
indeterminate is up for debate but let's bypass the issue by saving the
representation while the object is still alive. For example, we can cast it to
an integer. And we'll get an ordinary integer, with a stable value etc., not
affected by changes in the life of the original object. Right?

This seems to be broken for the equality operators when the operands are
constructed from addresses of automatic variables and at least one of these
variables is dead at the time of comparison.

----------------------------------------------------------------------
#include <stdio.h>

int main()
{
    unsigned long u, v;

    {
        int x[5];
        u = (unsigned long)x;
    }

    {
        int y[5];
        v = (unsigned long)y;
    }

    printf("u = %#lx\n", u);
    printf("v = %#lx\n", v);
    printf("diff = %#lx\n", u - v);
    printf("eq = %d\n", u == v);
}
----------------------------------------------------------------------
$ gcc -std=c11 -pedantic -Wall -Wextra -O3 test.c && ./a.out
u = 0x7ffeb6326180
v = 0x7ffeb6326180
diff = 0
eq = 0
----------------------------------------------------------------------
gcc x86-64 version: gcc (GCC) 10.0.0 20191219 (experimental)

If "diff == 0" then "eq" should be 1.