[Bug tree-optimization/126378] [13/14/15/16/17 Regression] spurious -Wnonnull "'this' pointer is null" at -O2 for pointer guarded by preceding null checks (regression from 10.x)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126378

Drea Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic,
                   |                            |missed-optimization
   Target Milestone|---                         |13.5
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2026-07-23
            Summary|spurious -Wnonnull "'this'  |[13/14/15/16/17 Regression]
                   |pointer is null" at -O2 for |spurious -Wnonnull "'this'
                   |pointer guarded by          |pointer is null" at -O2 for
                   |preceding null checks       |pointer guarded by
                   |(regression from 10.x)      |preceding null checks
                   |                            |(regression from 10.x)

--- Comment #1 from Drea Pinski <pinskia at gcc dot gnu.org> ---
Confirmed. Looks like we are warning right after inlining which means we get
something like:
```
  <bb 2> [local count: 1073741824]:
  _6 = q_2(D) == 0B;
  if (_6 != 0)
    goto <bb 6>; [34.00%]
  else
    goto <bb 3>; [66.00%]

  <bb 3> [local count: 708669600]:
  if (q_2(D) != 0B)
    goto <bb 4>; [82.57%]
  else
    goto <bb 5>; [17.43%]
```

Which then gets optimized away and the call to `p->get()` is removed
afterwards.


But really the issue is before inlining we don't change this to the reasonible
IR:
```
  <bb 2> :
  _1 = p_6(D) == 0B;
  _2 = q_7(D) == 0B;
  _3 = _1 & _2;
  if (_3 != 0)
    goto <bb 3>; [INV]
  else
    goto <bb 4>; [INV]

  <bb 3> :
  // predicted unlikely by early return (on trees) predictor.
  goto <bb 7>; [INV]

  <bb 4> :
  if (q_7(D) != 0B)
    goto <bb 5>; [INV]
  else
    goto <bb 6>; [INV]
```