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From: "marxin at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/96722] [8/9/10/11 Regression] Clobbers on
 NULL since r8-1519
Date: Thu, 20 Aug 2020 13:59:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96722

Martin Li=C5=A1ka <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|marxin at gcc dot gnu.org          |unassigned at gcc d=
ot gnu.org
             Status|ASSIGNED                    |NEW

--- Comment #2 from Martin Li=C5=A1ka <marxin at gcc dot gnu.org> ---
So the problematic transformation happens in einline:

BEFORE:

void foo(S*) (struct S * a)
{
  <bb 2> [0.00%] [count: INV]:
  if (a_2(D) !=3D 0B)
    goto <bb 4>; [0.00%] [count: INV]
  else
    goto <bb 3>; [0.00%] [count: INV]

  <bb 3> [100.00%] [count: INV]:
  MEM[(struct  &)a_2(D)] =3D{v} {CLOBBER};

  <bb 4> [0.00%] [count: INV]:
  return;

}

after:

void foo(S*) (struct S * a)
{
  <bb 2> [0.00%] [count: INV]:
  if (a_2(D) !=3D 0B)
    goto <bb 3>; [0.00%] [count: INV]
  else
    goto <bb 4>; [0.00%] [count: INV]

  <bb 3> [0.00%] [count: INV]:
  // predicted unlikely by early return (on trees) predictor.
  goto <bb 5>; [0.00%] [count: INV]

  <bb 4> [100.00%] [count: INV]:
  MEM[(struct  &)a_2(D)] =3D{v} {CLOBBER};

  <bb 5> [0.00%] [count: INV]:
  return;

}

So as seen, the CFG is more complex as there's an extra GIMPLE_PREDICT.

main then inlines to:

Scope blocks after cleanups:

{ Scope block #0=20

  { Scope block #7 ../pr96722.C:8 Originating from :  extern void S (struct=
 S
*, int);=20

  }

  { Scope block #8 ../pr96722.C:14 Originating from :  extern void S (struc=
t S
*, int);=20

  }

}
int main() ()
{
  int _5;

  <bb 2> [100.00%] [count: INV]:
  _5 =3D 0;
  return _5;

}

No longer having address taken: s
Scope blocks after cleanups:

{ Scope block #0=20

  { Scope block #7 ../pr96722.C:8 Originating from :  extern void S (struct=
 S
*, int);=20

    { Scope block #8 Originating from :#0=20

    }

  }

  { Scope block #9 ../pr96722.C:14 Originating from :  extern void S (struc=
t S
*, int);=20

  }

}
int main() ()
{
  int _5;

  <bb 2> [100.00%] [count: INV]:
  MEM[(struct  &)0B] =3D{v} {CLOBBER};
  _5 =3D 0;
  return _5;

}

So I bet it's hidden in a CFG cleanup where it somehow removes the clobber?
I guess it was latent before my revision..=