From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
 id 63BDF3A600EF; Fri,  7 May 2021 12:27:39 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 63BDF3A600EF
From: "s_gccbugzilla at nedprod 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: Fri, 07 May 2021 12:27:38 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Version: 7.1.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: s_gccbugzilla at nedprod dot com
X-Bugzilla-Status: NEW
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: 
Message-ID: <bug-80878-4-noI9B3X1Vy@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-80878-4@http.gcc.gnu.org/bugzilla/>
References: <bug-80878-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Fri, 07 May 2021 12:27:39 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D80878

--- Comment #33 from Niall Douglas <s_gccbugzilla at nedprod dot com> ---
(In reply to Andrew Pinski from comment #31)
>=20
> 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).

I think we are talking about different things here. You are talking about
calling convention. I'm talking about
std::atomic<__int128>::compare_exchange_weak() i.e. that the specific member
function compare_exchange_weak() is not generating cmpxchg16b if compiled w=
ith
GCC, but does with clang.

Re: your original point, I cannot say anything about _Atomic. However, for
std::atomic<__int128>, as __int128 is not an integral type it seems reasona=
ble
to me that its specialisation tell the compiler to not store it in read only
memory. Mark it with attribute section, give it a non-trivial destructor, or
whatever it needs.

Perhaps I ought to open a separate issue here, as my specific problem is th=
at
std::atomic<__int128>::compare_exchange_weak() is not using cmpxchg16b? Mr.
Wakely your thoughts?=