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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/94836] Failure to optimize condition based on
 known value of static variable
Date: Thu, 30 Apr 2020 08:09:10 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94836

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
OK, so it's not that easy to do.  Consider

int f(int x)
{
    static int s;

    if (!s)
        s =3D x;

    return s;
}

which we cannot optimize in this way.  VN would need to work in a way
assigning the value 0 to 's' optimistically and if it eventually
arrives at s =3D x it would need to invalidate that optimistic assumption
and iterate.  That is it would need to see the function as

int f(int x)
{
    static int s;
 # s =3D PHI <0, s'>

    if (!s)
        s =3D x;

    return s;
 s' =3D s;
 goto start;
}

with the goto of course not explicit.  I suppose rather than integrating
the feature into their value-numbering scheme compilers implement special
purpose optimization (IIRC this pattern appears in SPEC).=