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 176D83AAB038; Thu,  6 May 2021 17:50:45 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 176D83AAB038
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: Thu, 06 May 2021 17:50:44 +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: cc
Message-ID: <bug-80878-4-UmRxGaRqAj@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: Thu, 06 May 2021 17:50:45 -0000

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

Niall Douglas <s_gccbugzilla at nedprod dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |s_gccbugzilla at nedprod d=
ot com

--- Comment #30 from Niall Douglas <s_gccbugzilla at nedprod dot com> ---
I got bit by this GCC regression today at work. Consider
https://godbolt.org/z/M9fd7nhdh where std::atomic<__int128> is compare
exchanged with -march=3Dsandybridge:

- On GCC 6.4 and earlier, this emits lock cmpxchg16b, as you would expect.

- From GCC 7 up to trunk (12?), this emits __atomic_compare_exchange_16.

- On clang, this emits lock cmpxchg16b, as you would expect.

This is clearly a regression. GCCs before 7 did the right thing. GCCs from 7
onwards do not. clangs with libstdc++ do do the right thing.

This isn't just an x64 thing, either. Consider https://godbolt.org/z/x6d5GE=
4o6
where GCC on ARM64 emits __atomic_compare_exchange_16, whereas clang on ARM=
64
emits ldaxp/stlxp, as you would expect.

Please mark this bug as a regression affecting all versions of GCC from 7 to
trunk, and affecting all 128 bit atomic capable architectures not just x64.=