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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/105030] store motion if-change flag causes
 if-conversion optimization can't be taken.
Date: Fri, 08 Apr 2022 08:07:24 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105030

--- Comment #14 from rguenther at suse dot de <rguenther at suse dot de> ---
On Fri, 8 Apr 2022, guihaoc at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105030
>=20
> --- Comment #13 from HaoChen Gui <guihaoc at gcc dot gnu.org> ---
> Could we use the original alias set if the tree code of 'atemp' is var_de=
cl? Is
> it safe? In which situation we shall use alias-set zero? Thanks.

No, just being a VAR_DECL is not good enough.  Basically you have to
somehow have knowledge about the dynamic type of the object we store
to before the store and see whether the conditional store might
change that.  If so the store needs to use a type that allows accesses
to both the original and the possibly changed dynamic type from
existing accesses following the now unconditional store.

When the access is to a VAR_DECL we can look for the case where
the access is to the full variable (partial stores are always difficult
to judge wrt dynamic type changes) and whether there is a dominating
load (also to the full variable) that either uses the same type
or allows us to find a common type to use.=