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From: "jason 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: Thu, 10 Feb 2022 22:07:40 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102586

--- Comment #16 from Jason Merrill <jason at gcc dot gnu.org> ---
Created attachment 52410
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D52410&action=3Dedit
sketch of vbase handling

This is roughly what I had in mind, though it's algorithmically poor becaus=
e it
walks all the bases each time.

But then it occurred to me that unless we know the complete object type, we
don't know whether we're dealing with a base subobject, and risk clearing t=
oo
much:

struct C1 { virtual void f() {} char d; }; // vfn to make non-layout-POD
struct C2: C1 { char c; };

__attribute__((noipa)) void
foo (C1 *q)
{
  __builtin_clear_padding (q);
}

int main()
{
  C2 c2;
  c2.c =3D 42;
  foo (&c2);
  if (c2.c !=3D 42)
    __builtin_abort();
}

Because of this, I think we probably want __builtin_clear_padding to reject
arguments that point to non-trivially-copyable type.  And then we don't nee=
d to
worry about vbases.=