[Bug tree-optimization/82581] New: missing -Warray-bounds on writing past the end of a member array

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82581

            Bug ID: 82581
           Summary: missing -Warray-bounds on writing past the end of a
                    member array
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

When the bounds of a member array are exceeded in a loop that accesses the
member directly by its name, GCC issues a -Waggressive-loop-optimizations
warning mentioning the iteration of the loop in which the undefined behavior
occurs.  But when the same access takes place indirectly through a pointer to
the member, no diagnostic is issued.  Since the number of iterations of the
loop is known I would expect a -Warray-bounds warning (ideally in both cases,
even when no aggressive loop optimizations are performed).

As an aside, since the first iteration of a loop is usually thought of as
iteration 1 (not iteration zero), the iteration number referenced in the
-Waggressive-loop-optimizations warning is off by one.  Since the array has
three elements, iteration 3 writes into the last (third) element, and it's
iteration four that has undefined behavior.

$ cat a.c && gcc -O2 -S -Wall -Warray-bounds -Wextra a.c
struct S
{
  int a[3];
  void (*pf)(void);
} x;

void f (void)
{
  for (unsigned i = 0; i != sizeof x; ++i)   // -Waggressive-loop-optimizations
    x.a[i] = i;
}

void g (void)
{
  int *p = x.a;

  for (unsigned i = 0; i != sizeof x; ++i)
    p[i] = i;                                // missing -Warray-bounds
}
a.c: In function ‘f’:
a.c:10:12: warning: iteration 3 invokes undefined behavior
[-Waggressive-loop-optimizations]
     x.a[i] = i;
     ~~~~~~~^~~
a.c:9:3: note: within this loop
   for (unsigned i = 0; i != sizeof x; ++i)
   ^~~