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 F34743858D37; Sun, 16 Aug 2020 20:36:42 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org F34743858D37
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
 s=default; t=1597610203;
 bh=yqjYnWHHIjDJLJ/106EIovmbaB/576WNCXaEDSCkttI=;
 h=From:To:Subject:Date:From;
 b=RACk/E3ykzBsiJqQzguDqzd3tvjp2Fgq+jCFEBVvqjIe+fYDMZBVnpvXyfMo971Fc
 Mio08WLVQoVAzz3luvfTz1Vl1ClDLQipl73NCDLgdqHuWr0HdDMhFtfK753ug4Au2T
 y8+Zi2fzy7Vj4mdHJtt5HBmKw5jRKv9MlyD3boe0=
From: "goldstein.w.n at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/96632] New: missed-optimization with conditionally unsetting
 bits in memory.
Date: Sun, 16 Aug 2020 20:36:42 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Version: 10.2.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: goldstein.w.n at gmail dot com
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-96632-4@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: Sun, 16 Aug 2020 20:36:43 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96632

            Bug ID: 96632
           Summary: missed-optimization with conditionally unsetting bits
                    in memory.
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: goldstein.w.n at gmail dot com
  Target Milestone: ---

https://stackoverflow.com/questions/63432157/why-does-gcc-use-btq-in-conjun=
ction-with-btcq-when-conditionally-set-a-bit-in-a

Basically:

void __attribute__((noinline))
cond_unset_bit(uint64_t * v, uint32_t b) {
    if(__builtin_expect(!!(*v & ((1UL) << b)), 1)) {
        *v ^=3D ((1UL) << b);
    }
}

Compiles to:

cond_unset_bit(unsigned long*, unsigned int):
        movq    (%rdi), %rax
        btq     %rsi, %rax
        jnc     .L6
        btcq    %rsi, %rax
        movq    %rax, (%rdi)
.L6:
        ret

The btq instruction is unnecessary.

cond_unset_bit(unsigned long*, unsigned int):
        movq    (%rdi), %rax
        btcq    %rsi, %rax
        jnc     .L6
        movq    %rax, (%rdi)
.L6:
        ret


Accomplishes the same thing without the btq instruction.=