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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/106187] armhf: Miscompilation at O2 level (O0 / O1 are
 working)
Date: Mon, 25 Jul 2022 09:50:16 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106187

--- Comment #36 from rguenther at suse dot de <rguenther at suse dot de> ---
On Mon, 25 Jul 2022, rearnsha at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106187
>=20
> --- Comment #35 from Richard Earnshaw <rearnsha at gcc dot gnu.org> ---
> > There's no union involved here though but a memcpy used in BitCast.
> Agreed, but by creating a shared stack slot, the compiler is effectively
> creating a union of its own, and I think that needs to be accounted for.=
=20
> update_alias_info_with_stack_vars handles the cases where we have pointer=
s (at
> the gimple level) into a shared stack slot, but doesn't (AFAICT) cater fo=
r RTL
> lowering creating additional pointers (as it must since all objects on the
> stack ultimately have to be addressed).
>=20
> So if we create a shared stack slot for objects of different types, why d=
o we
> not also create an alias set for the combination of such types, much as we
> would do for a union?

Note that the only thing we have to do is fix points-to info, the TBAA
info should be correct and OK even when objects share location, so there's
nothing we can do at RTL expansion time.=