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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/106389] [11/12/13 Regression] IPA modref breaks Safe
 Bitfields in C++
Date: Mon, 25 Jul 2022 06:20:35 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106389

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Marek Polacek from comment #4)
> (In reply to Richard Biener from comment #3)
> > For the rule to take effect the frontend needs to mark each access that
> > falls under the rule as to have alias-set zero.
>=20
> Something like: make c_common_get_alias_set return -1 for those
> COMPONENT_REFs in a standard-layout union that share a common initial
> sequence, and are standard-layout structs?

return 0, not -1 for them.  But note to be reliable you'd have to do this
for the actual aggregate type, not just COMPONENT_REFs since we have to
be able to re-query TBAA from the types (and COMPONENT_REFs might get lost).
See also the response from Honza.

union U { struct A { int i; } a; struct B { int i; int f; } b; } u;

is 'u.a' accessing the common initial sequence of A and B btw.?=