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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/112414] Does gcc need __builtin_assume_separate_storage?
Date: Tue, 07 Nov 2023 08:42:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112414

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu.org
         Resolution|---                         |WONTFIX
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
__builtin_assume_separate_storage

__builtin_assume_separate_storage is used to provide the optimizer with the
knowledge that its two arguments point to separately allocated objects.

Syntax:

__builtin_assume_separate_storage(const volatile void *, const volatile voi=
d *)

Example of Use:

int foo(int *x, int *y) {
    __builtin_assume_separate_storage(x, y);
    *x =3D 0;
    *y =3D 1;
    // The optimizer may optimize this to return 0 without reloading from *=
x.
    return *x;
}

Description:

The arguments to this function are assumed to point into separately allocat=
ed
storage (either different variable definitions or different dynamic storage
allocations). The optimizer may use this fact to aid in alias analysis. If =
the
arguments point into the same storage, the behavior is undefined. Note that=
 the
definition of =E2=80=9Cstorage=E2=80=9D here refers to the outermost enclos=
ing allocation of
any particular object (so for example, it=E2=80=99s never correct to call t=
his function
passing the addresses of fields in the same struct, elements of the same ar=
ray,
etc.).

Query for this feature with __has_builtin(__builtin_assume_separate_storage=
).


---

The given syntax makes it very unuseful given there's no data dependence
involved.  I assume it is supposed to work like

 typeof (x) __restrict xr =3D x;
 typeof (y) __restrict yr =3D y;
 *xr =3D 0;
 *yr =3D 1;

not sure why clang folks thought a new builtin is a great idea.  Even this
restrict form is more useful (but there's reasons we don't support that
either).

So no, RESOLVED BADFEATURE.=