[Bug tree-optimization/104069] -Werror=use-after-free false positive on elfutils-0.186

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104069

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |msebor at gcc dot gnu.org
           Assignee|unassigned at gcc dot gnu.org      |msebor at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2022-01-17
     Ever confirmed|0                           |1

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
The warning triggers at -O0 and above for an IL that looks like this (at -O1):

  <bb 6> [local count: 59126544]:
  # sb_4 = PHI <0B(4), sb_3(5)>
  b_15 = realloc (sb_4, 1);
  if (b_15 != 0B)
    goto <bb 7>; [99.96%]
  else
    goto <bb 8>; [0.04%]

  <bb 7> [local count: 59102893]:

  <bb 8> [local count: 59126544]:
  # sb_5 = PHI <sb_4(6), b_15(7)>
  return sb_5;                         <<< -Wuse-after-free 

}

All it does is check to see if the use of sb_5 in the return statement is
dominated by the realloc call without considering the data flow (i.e., that
sb_5 is equal to sb_4 when b_15 is null).

The handling of the dominance is too simplistic.  Let me look into make it more
intelligent.