[Bug c/105294] restrict pointer - disagreement with specification

[mattlloydhouse at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105294

Matthew House <mattlloydhouse at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |mattlloydhouse at gmail dot com

--- Comment #2 from Matthew House <mattlloydhouse at gmail dot com> ---
Isn't this example invalid per the spec? At `*r = 42;`, `&*r` is based on `s`.
However, `*p = 13;` modifies the same object that `*r` refers to, and `&*p` is
not based on `s`, so the behavior is undefined.

I've recently found another example which is valid under the current spec as
well as the two new proposed definitions in N3025 and N3058. 

https://godbolt.org/z/ezvMfPd78

static int x;

__attribute__ ((noinline))
int f(int * restrict p) {
  *p = 1;
  if (p == &x) {
    *p = 2;
  }
  return *p;
}

int main(void) {
  return f(&x);
}

At `*p = 2;`, `&*p` is based on `p` under every definition. However, both GCC
and Clang incorrectly assume that the write to `*p` inside the `if` block
cannot affect the return value.