[Bug middle-end/96384] New: [11 Regression] bogus -Wstringop-overflow= storing into multidimensional array with index in range

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96384

            Bug ID: 96384
           Summary: [11 Regression] bogus -Wstringop-overflow= storing
                    into multidimensional array with index in range
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The following test case triggers a false positive -Wstringop-overflow:

$ cat z.c && gcc -O2 -S -Wall -fdump-tree-strlen-all=/dev/stdout z.c
char a[2][3];

void f (unsigned i)
{
  if (i > ~0U - 1)
    i = ~0U - 1;
  a[i][0] = 0;
}

;; Function f (f, funcdef_no=0, decl_uid=1932, cgraph_uid=1, symbol_order=1)


Pass statistics of "strlen": ----------------

;; 1 loops found
;;
;; Loop 0
;;  header 0, latch 1
;;  depth 0, outer -1
;;  nodes: 0 1 2
;; 2 succs { 1 }

extract_range_from_stmt visiting:
# RANGE [0, 4294967294]
_2 = MIN_EXPR <i_1(D), 4294967294>;
Intersecting
  unsigned int [0, 4294967294]
and
  unsigned int [0, 4294967294]
to
  unsigned int [0, 4294967294]
z.c: In function ‘f’:
z.c:7:11: warning: writing 1 byte into a region of size 0
[-Wstringop-overflow=]
    7 |   a[i][0] = 0;
      |   ~~~~~~~~^~~
z.c:1:6: note: at offset 0 to object ‘a’ with size 6 declared here
    1 | char a[2][3];
      |      ^
maybe_invalidate called for a[_2][0] = 0;
maybe_invalidate returns 0

Pass statistics of "strlen": ----------------

f (unsigned intD.9 iD.1931)
{
  unsigned intD.9 i_1(D) = iD.1931;
  unsigned intD.9 _2;

;;   basic block 2, loop depth 0, count 1073741824 (estimated locally), maybe
hot
;;    prev block 0, next block 1, flags: (NEW, REACHABLE, VISITED)
;;    pred:       ENTRY [always]  count:1073741824 (estimated locally)
(FALLTHRU,EXECUTABLE)
  # RANGE [0, 4294967294]
  _2 = MIN_EXPR <i_1(D), 4294967294>;
  # .MEM_4 = VDEF <.MEM_3(D)>
  aD.1930[_2][0] = 0;
  # VUSE <.MEM_4>
  return;
;;    succ:       EXIT [always]  count:1073741824 (estimated locally)
(EXECUTABLE)

}