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From: "hubicka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/103168] Value numbering for PRE of pure
 functions can be improved
Date: Wed, 17 Nov 2021 20:28:28 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103168

--- Comment #3 from Jan Hubicka <hubicka at gcc dot gnu.org> ---
This is simple (and fairly common) case we don't optimize
struct a {
        int a;
        static __attribute__ ((noinline))
        int ret (int v) {return v;}

        __attribute__ ((noinline))
        int inca () {return a++;}
};
int
test()
{
        struct a av;
        av.a=3D1;
        int val =3D av.ret (0) + av.inca();
        av.a=3D2;
        return val + av.ret(0) + av.inca();
}

what ret is const however becaue it is in COMDAT group we only detect it as
pure which makes GVN to give up on proving that its value did not change ov=
er
av.a=3D2 store.  We could easily work this out from modref summary (which s=
ays
that function makes no memory load)=