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

[muecker at gwdg dot de]

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

            Bug ID: 105294
           Summary: restrict pointer - disagreement with specification
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: muecker at gwdg dot de
  Target Milestone: ---

Here is an example where GCC/Clang assume that variables
may not alias where the C spec seems to say otherwise.

This example is based on one found by Hubert Tong
after discussions with him (and Ralf Jung).

Probably the spec should be changed instead of the
compilers.

Note that when removing the first restrict, clang but
not GCC still does the optimization, which hints at
a missed optimization opportunity (assuming a
change in spec which generally makes this valid).

https://godbolt.org/z/K89jdd473

int f(int * q, int * restrict p, int * restrict s)
{
  int x;
  int *r = &x;
  if (p == q)
    r = s;
  *p = 13;
  *r = 42; // `&(*r)` is based on both `p`, `s`!
  return *p;
}

int main(void)
{
  int q;
  return f(&q, &q, &q);
}