[Bug middle-end/103292] [12 regression] xorg-server-1.20.13 -Werror=array-bounds false positive on unions

["msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103292

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|INVALID                     |DUPLICATE
          Component|c                           |middle-end
             Blocks|                            |56456
                 CC|                            |jeffreyalaw at gmail dot com

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
The originally intended purpose of this instance of -Warray-bounds was to warn
for accesses to smaller buffers by larger lvalues, like in this function:

struct A { int i; };
struct B { long j; };

struct B* make_B_from_A (const struct A *p)
{
  struct B *q = __builtin_malloc (sizeof *p);   // should be sizeof *q
  q->j = p->i;
  return q;
}

Here a warning should be issued regardless of whether -fstrict-aliasing is in
effect because the access is out of bounds.  That the warning also triggers in
instances when the problem isn't one of an out-of-bounds access but rather an
aliasing violation was incidental (i.e., I didn't set out with that as a goal),
but because -Wstrict-aliasing in GCC is very limited, seemed like a bonus.

So an argument could be (and in pr98503 in fact was) made that the instances of
-Warray-bounds where the ultimate access is strictly in bounds should be
replaced by one of -Wstrict-aliasing, which is enabled only when
-fstrict-aliasing is in effect.  I agreed and submitted a patch to do that:

  https://gcc.gnu.org/pipermail/gcc-patches/2021-January/564483.html

Regrettably, the change was rejected.  I CC the reviewer for his comments.

*** This bug has been marked as a duplicate of bug 98503 ***


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56456
[Bug 56456] [meta-bug] bogus/missing -Warray-bounds