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 116CE3858D28; Fri, 31 Mar 2023 14:01:17 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 116CE3858D28
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1680271277;
	bh=IphLnAcZttXcpGyib1me0b48GRrCga2c7sWUeMy5ng0=;
	h=From:To:Subject:Date:From;
	b=VVSShb4q576Pk/YI+JElOSVyz8FfcF9ir1q6vyYAjZBT30Wn2FvJqt0udbha96VVs
	 /B9hHhg86IG03OyergBMRAe5L1S/WoeCeA1b4lT2/9JUHfUTzNJB14VSBghoYHBz8f
	 18iZMirg0gxLdOuOWQ+nMMRgA1sElp+OGiTPKDVs=
From: "barry.revzin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/109362] New: codegen adds unnecessary extra add when
 reading atomic member
Date: Fri, 31 Mar 2023 14:01:16 +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: 12.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: barry.revzin 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-109362-4@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
List-Id: <gcc-bugs.sourceware.org>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109362

            Bug ID: 109362
           Summary: codegen adds unnecessary extra add when reading atomic
                    member
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

This program:

#include <atomic>

struct S {
    long size;
    std::atomic<char*> read_ptr;

    auto peek() const -> const char* {=20=20=20=20
        return read_ptr.load(std::memory_order_acquire);
    }
};

auto with_atomic(S const& v) {
    while (true) {
        if (v.peek()) {
            return true;
        }
    }
}

emits (on gcc 12.2 -O3):

with_atomic(S const&):
        add     rdi, 8
.L2:
        mov     rax, QWORD PTR [rdi]
        test    rax, rax
        je      .L2
        mov     eax, 1
        ret

But that add is completely necessary, the mov could just be:

        mov     rax, QWORD PTR [rdi + 8]

which is what clang (16.0 -O3) generates:

with_atomic(S const&):                    # @with_atomic(S const&)
.LBB0_1:                                # =3D>This Inner Loop Header: Depth=
=3D1
        mov     rax, qword ptr [rdi + 8]
        test    rax, rax
        je      .LBB0_1
        mov     al, 1
        ret

It's not just an extra add, it's consuming an extra register - which has mo=
re
downstream optimization effects.=