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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/102586] [12 Regression] ICE in
 clear_padding_type, at gimple-fold.c:4798 since r12-3433-ga25e0b5e6ac8a77a
Date: Fri, 11 Feb 2022 17:47:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102586

--- Comment #26 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Though, I must say I don't understand how that works currently at all.
For say
struct C0 {};
struct C1 {};
struct C2 : C1, virtual C0 {};
struct C3 : virtual C2, C1 {};
struct C6 { char c; };
struct C7 : virtual C6, virtual C3, C1 {};
struct C8 : C7 {};
void bar (C8 *);

void
foo ()
{
  C8 c;
  bar (&c);
}
I see in gimple dump:
      c =3D .DEFERRED_INIT (32, 1, &"c"[0]);
      __builtin_clear_padding (&c, 0B, 1);
      C8::C8 (&c);
      bar (&c);
and it does something only because C8::C8 doesn't get the -flifetime-dse=3D2
CLOBBER at the start.

But if I do:
struct C { char a; int b; char c; long d; C () : a (42), b (42), c (42), d =
(42)
{} };
void bar (C *);
void
foo ()
{
  C c;
  bar (&c);
}
then *.gimple is:
      c =3D .DEFERRED_INIT (24, 1, &"c"[0]);
      __builtin_clear_padding (&c, 0B, 1);
      C::C (&c);
      bar (&c);
...
void C::C (struct C * const this)
{
  *this =3D {CLOBBER};
  {
    this->a =3D 42;
    this->b =3D 42;
    this->c =3D 42;
    this->d =3D 42;
  }
}
After einline this is:
  c =3D .DEFERRED_INIT (24, 1, &"c"[0]);
  MEM <char[3]> [(struct C *)&c + 1B] =3D {};
  MEM <char[7]> [(struct C *)&c + 9B] =3D {};
  c =3D{v} {CLOBBER};
  c.a =3D 42;
  c.b =3D 42;
  c.c =3D 42;
  c.d =3D 42;
  bar (&c);
and that keeps until dse1 which optimizes that out:
  c =3D{v} {CLOBBER};
  c.a =3D 42;
  c.b =3D 42;
  c.c =3D 42;
  c.d =3D 42;
  bar (&c);
so there is no zero padding initialization at all.=