[Bug tree-optimization/67196] [9/10/11 Regression] loop-induced false positive from -Wmaybe-uninitialized

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67196

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2015-08-12 00:00:00         |2021-3-31
      Known to fail|                            |10.2.0, 11.0, 5.5.0, 6.4.0,
                   |                            |7.2.0, 8.3.0, 9.1.0
                 CC|                            |msebor at gcc dot gnu.org
            Summary|Another false positive from |[9/10/11 Regression]
                   |-Wmaybe-uninitialized       |loop-induced false positive
                   |                            |from -Wmaybe-uninitialized
           Keywords|                            |diagnostic

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
Reconfirmed with GCC 11 as a regression introduced in r185913 (4.8.0 20120328).

Nothing in the IL jumps out at me that could be used by the predicate analysis
to rule out the false positive: the use in the return statement in bb 8 looks
unconditional.

pr67196.c: In function ‘test’:
pr67196.c:7:7: warning: ‘first_caption_idx’ may be used uninitialized in this
function [-Wmaybe-uninitialized]
    7 |   int first_caption_idx;
      |       ^~~~~~~~~~~~~~~~~
int test (int n)
{
  int i;
  int first_caption_idx;
  int first_caption;
  int num_captions_in_row;
  int _1;
  _Bool _2;
  _Bool _3;
  _Bool _4;
  int _12;

  <bb 2> [local count: 118111600]:
  if (n_16(D) > 0)
    goto <bb 9>; [89.00%]
  else
    goto <bb 11>; [11.00%]

  <bb 11> [local count: 12992276]:
  goto <bb 8>; [100.00%]

  <bb 9> [local count: 105119324]:

  <bb 3> [local count: 955630225]:
  # num_captions_in_row_25 = PHI <num_captions_in_row_5(10), 0(9)>
  # first_caption_27 = PHI <first_caption_7(10), 0(9)>
  # first_caption_idx_29 = PHI <first_caption_idx_9(10),
                               first_caption_idx_14(D)(9)>          <<< (3)
first_caption_idx_14(D) is uninitialized
  # i_31 = PHI <i_19(10), 0(9)>
  _1 = some_test (i_31);
  if (_1 != 0)
    goto <bb 4>; [50.00%]
  else
    goto <bb 12>; [50.00%]

  <bb 12> [local count: 477815113]:
  goto <bb 5>; [100.00%]

  <bb 4> [local count: 477815112]:
  num_captions_in_row_18 = num_captions_in_row_25 + 1;

  <bb 5> [local count: 955630225]:
  # num_captions_in_row_5 = PHI <num_captions_in_row_25(12),
num_captions_in_row_18(4)>
  # first_caption_7 = PHI <first_caption_27(12), 1(4)>
  # first_caption_idx_9 = PHI <first_caption_idx_29(12), i_31(4)>   <<< (2)
  i_19 = i_31 + 1;
  if (n_16(D) != i_19)
    goto <bb 10>; [89.00%]
  else
    goto <bb 6>; [11.00%]

  <bb 10> [local count: 850510901]:
  goto <bb 3>; [100.00%]

  <bb 6> [local count: 105119324]:
  _2 = first_caption_7 != 0;
  _3 = num_captions_in_row_5 == 1;
  _4 = _2 & _3;
  if (_4 != 0)
    goto <bb 13>; [38.20%]
  else
    goto <bb 7>; [61.80%]

  <bb 13> [local count: 40157944]:
  goto <bb 8>; [100.00%]

  <bb 7> [local count: 64961380]:

  <bb 8> [local count: 118111600]:
  # _12 = PHI <first_caption_idx_9(13), 0(7), 0(11)>                <<< (1)
  return _12;                                                       <<< use

}