[Bug c/99159] New: Confusing -Warray-bounds warning

[sirl at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99159

            Bug ID: 99159
           Summary: Confusing -Warray-bounds warning
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sirl at gcc dot gnu.org
  Target Milestone: ---

Hi,

with this minimized testcase, compiled with -O2 -Warray-bounds:

struct s1
{
  char b[12];
};

struct s2
{
  int x;
  struct s1 y;
} *pb, c;

extern struct s2 *es;

void test1 (int f)
{
  struct s2 *p = f ? 0 : es;
  __builtin_memcpy (&pb->y, p->y.b, sizeof(struct s1));
}

void test2 ()
{
  struct s2 *p = 0;
  __builtin_memcpy (&pb->y, p->y.b, sizeof(struct s1));
}

trunk@r11-7270 warns like this:
testcase.c: In function 'test2':
testcase.c:23:3: warning: '__builtin_memcpy' offset [0, 11] is out of the
bounds [0, 0] [-Warray-bounds=]
   23 |   __builtin_memcpy (&pb->y, p->y.b, sizeof(struct s1));
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I believe -Warray-bounds shouldn't warn here (or at least not with this
message) and gcc-10.2 and also trunk@r11-3693 don't.
Note that the real code is more like test1(), where it's not clear that 'p' is
always zero.