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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 16:39:36 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102586

--- Comment #24 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I bet with the default -flifetime-dse=3D2 we would then have a
  var =3D {};
  var =3D{v} {CLOBBER}; // Start of ctor clobber
  var.whatever =3D ...; // Ctor content
and the zero initialization would be optimized away then.
I guess ideal would be to emit such zero initialization in the complete obj=
ect
ctors right after the -flifetime-dse=3D2 beginning clobber and ideally one =
that
clearly says to the gimplifier and rest of middle-end that it must initiali=
ze
even padding bits.
Because right now gimplification of var =3D {} will often happily lower tha=
t to
  var.a =3D 0; var.b =3D 0; var.c =3D 0;
even when there are padding bits, which is ok for most of ctors, but not ok=
 for
D and not ok for C++ zero initialization.
So, I think we need some flag on CONSTRUCTORS requesting the initialize also
padding bits behavior, set it for D ctors and for C++ zero initialization,
properly propagate it through the C++ FE and honor it during gimplification
(and wherever else it is needed).=