[Bug tree-optimization/69359] New: Warn about constant comparisons between pointers and arrays

[fw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69359

            Bug ID: 69359
           Summary: Warn about constant comparisons between pointers and
                    arrays
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fw at gcc dot gnu.org
  Target Milestone: ---

This code fragment should result in a warning because p < a can never be true.

int *f (int *);

int g (void)
{
  int a[3];
  int *p = f (a);
  return p < a;
}

This variant is even more clear:

int f (void);

int g (void)
{
  int a[3];
  int *p = a + f ();
  return p < a;
}

Even for the following test case, it makes sense to warn because p <= a is
equivalent to p == a:

int f (void);

int g (void)
{
  int a[3];
  int *p = a + f ();
  return p <= a;
}

(Observed with GCC 5.3 and trunk from a week ago.)

I set the component to tree-optimization; a purely type-based implementation in
the C/C++ front ends could make sense as well (although it would miss cases
where the array variable has already decayed to a pointer).