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From: "lh_mouse at 126 dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/80878] -mcx16 (enable 128 bit CAS) on x86_64 seems not
 to work on 7.1.0
Date: Thu, 03 Nov 2022 09:55:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D80878

LIU Hao <lh_mouse at 126 dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lh_mouse at 126 dot com

--- Comment #36 from LIU Hao <lh_mouse at 126 dot com> ---
(In reply to Andrew Pinski from comment #1)
> IIRC this was removed as the instruction cannot be used for read only mem=
ory.

That's not a valid argument. The first argument is a pointer to non-const t=
ype,
and whoever passes a read-only object bears the risk on their own.

As mention in previous posts, the double-word compare-and-swap operation is
invaluable for many algorithms. The fact that GCC does not generate it, even
when requested explicitly with `-mcx16`, is silly and unacceptable.

--- Comment #37 from LIU Hao <lh_mouse at 126 dot com> ---
(In reply to Andrew Pinski from comment #31)
> Again the problem is stuff like:
> static const _Atomic __int128_t t =3D 2000;
>=20
> __int128_t g(void)
> {
>   return t;
> }
>=20
> DOES NOT WORK if you use CAS (or ldaxp/stlxp).
>=20

Can this be made using MOVDQA instead? I haven't tested this though, just o=
ut
of curiosity.=