[Bug tree-optimization/97108] Wmaybe-uninitialized false positive

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97108

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2020-11-06 00:00:00         |2021-4-9
                 CC|                            |msebor at gcc dot gnu.org
      Known to fail|                            |10.2.0, 11.0, 4.7.0, 4.8.4,
                   |                            |4.9.4, 5.5.0, 6.4.0, 7.2.0,
                   |                            |8.3.0, 9.1.0
           Keywords|                            |diagnostic

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
Reconfirming with a slightly more simplified test case below.

$ cat pr97108.c && gcc -O2 -S -Wall -fdump-tree-uninit-details=/dev/stdout
pr97108.c
int z;

void f (int flags, int i, int j)
{
  int x;
  _Bool f = !flags;

  if (f)
    x = j;

  if (i != 0)
    z = i;

  if (f && x != 2)
    z = 0;
}

;; Function f (f, funcdef_no=0, decl_uid=1946, cgraph_uid=1, symbol_order=1)

[WORKLIST]: add to initial list: x_18 = PHI <j_9(D)(10), x_8(D)(13)>
[CHECK]: examining phi: x_18 = PHI <j_9(D)(10), x_8(D)(13)>
[CHECK]: Found unguarded use: x_19 = PHI <j_9(D)(11), x_18(4)>
[WORKLIST]: Update worklist with phi: x_19 = PHI <j_9(D)(11), x_18(4)>
[CHECK]: examining phi: x_19 = PHI <j_9(D)(11), x_18(4)>
[CHECK]: Found unguarded use: _1 = x_19 != 2;
pr97108.c: In function ‘f’:
pr97108.c:14:14: warning: ‘x’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
   14 |   if (f && x != 2)
      |            ~~^~~~
pr97108.c:5:7: note: ‘x’ was declared here
    5 |   int x;
      |       ^
void f (int flags, int i, int j)
{
  _Bool f;
  int x;
  _Bool _1;
  _Bool _2;

  <bb 2> [local count: 1073741824]:
  f_7 = flags_6(D) == 0;
  if (flags_6(D) == 0)
    goto <bb 3>; [50.00%]
  else
    goto <bb 9>; [50.00%]

  <bb 3> [local count: 536870912]:
  if (i_10(D) != 0)
    goto <bb 10>; [50.00%]
  else
    goto <bb 11>; [50.00%]

  <bb 11> [local count: 268435456]:
  goto <bb 5>; [100.00%]

  <bb 10> [local count: 268435456]:

  <bb 4> [local count: 536870913]:
  # x_18 = PHI <j_9(D)(10), x_8(D)(13)>
  z = i_10(D);

  <bb 5> [local count: 805306368]:
  # x_19 = PHI <j_9(D)(11), x_18(4)>
  _1 = x_19 != 2;
  _2 = _1 & f_7;
  if (_2 != 0)
    goto <bb 7>; [66.67%]
  else
    goto <bb 12>; [33.33%]

  <bb 12> [local count: 268435458]:

  <bb 6> [local count: 536870914]:
  goto <bb 8>; [100.00%]

  <bb 7> [local count: 536870913]:
  z = 0;

  <bb 8> [local count: 1073741824]:
  return;

  <bb 9> [local count: 536870912]:
  if (i_10(D) != 0)
    goto <bb 13>; [50.00%]
  else
    goto <bb 14>; [50.00%]

  <bb 14> [local count: 268435456]:
  goto <bb 6>; [100.00%]

  <bb 13> [local count: 268435456]:
  goto <bb 4>; [100.00%]

}


Bisection indicates either r180295 or r180298 introduced the warning.  Before
then the dump was:

;; Function f (f, funcdef_no=0, decl_uid=1598, cgraph_uid=0)

[WORKLIST]: add to initial list: x_1 = PHI <x_4(D)(3), j_5(D)(2)>
[CHECK]: examining phi: x_1 = PHI <x_4(D)(3), j_5(D)(2)>

Use in stmt if (x_1 != 2)
is guarded by :
if (f_3 != 0)

[CHECK] Found def edge 1 in x_1 = PHI <x_4(D)(3), j_5(D)(2)>
Operand defs of phi x_1 = PHI <x_4(D)(3), j_5(D)(2)>
is guarded by :
if (f_3 != 0)
f (int flags, int i, int j)
{
  _Bool f;
  int x;

<bb 2>:
  f_3 = flags_2(D) == 0;
  if (f_3 != 0)
    goto <bb 4>;
  else
    goto <bb 3>;

<bb 3>:

<bb 4>:
  # x_1 = PHI <x_4(D)(3), j_5(D)(2)>
  if (i_7(D) != 0)
    goto <bb 5>;
  else
    goto <bb 6>;

<bb 5>:
  z = i_7(D);

<bb 6>:
  if (f_3 != 0)
    goto <bb 7>;
  else
    goto <bb 9>;

<bb 7>:
  if (x_1 != 2)
    goto <bb 8>;
  else
    goto <bb 9>;

<bb 8>:
  z = 0;

<bb 9>:
  return;

}