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 9C8D73858428; Sat,  2 Oct 2021 16:08:46 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 9C8D73858428
From: "thiago at kde dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/102566] New: [i386] GCC should emit LOCK BTS for simple
 bit-test-and-set operations with std::atomic
Date: Sat, 02 Oct 2021 16:08:46 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Version: unknown
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: thiago at kde dot org
X-Bugzilla-Status: UNCONFIRMED
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: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-102566-4@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: Sat, 02 Oct 2021 16:08:46 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102566

            Bug ID: 102566
           Summary: [i386] GCC should emit LOCK BTS for simple
                    bit-test-and-set operations with std::atomic
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: thiago at kde dot org
  Target Milestone: ---

Simple test:

$ cat test.cpp
#include <atomic>
bool tbit(std::atomic<int> &i)
{
    return i.fetch_or(1, std::memory_order_relaxed) & 1;
}

The sequence x.fetch_or(singlebit_constant) & singlebit_constant can be
implemented by a LOCK BTS sequence. The above should emit:

    lock bts $1, (%rdi)
    setb %al
    ret

But instead it emits a cmpxchg loop - see https://gcc.godbolt.org/z/99enKaf=
fa.

This was found reviewing MariaDB lightweight-mutex code, which uses the sign
bit to indicate a contended mutex. See this commit[1] by one of their
maintainers for the removal of fetch_or because it emits an extra loop.

Bonus: LOCK BTR can be used in the sequence x.fetch_and(~single_bit_constan=
t) &
single_bit_constant

[1]
https://github.com/dr-m/atomic_sync/commit/d5e22b2d42cdbac7a15d242bf1446377=
555c4041=