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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/51392] Wrong code with -Os when __attribute__((__const__)) function returns structure
Date: Sun, 04 Dec 2011 15:51:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51392

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-12-04 15:50:40 UTC ---
__attribute__((__const__))
Vector3f pie_SurfaceNormal3fv(Vector3f a, Vector3f b)
{
    return Vector3f(a.y*b.z - a.z*b.y, a.z*b.x - a.x*b.z, a.x*b.y - a.y*b.x);
}

I think it is illegal to use the const attribute when the C++ frontend
may elide the return value copy to being passed by invisible reference.
That way the middle-end sees a store to global memory which is a side-effect
that the const attribute does not allow (similar for pure btw.).

Note that the Vector3f() constructors are not const either but they are
both called from pie_SurfaceNormal3fv.