From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <steffen.hirschmann@ipvs.uni-stuttgart.de>
Received: from mxex1.tik.uni-stuttgart.de (mxex1.tik.uni-stuttgart.de
 [IPv6:2001:7c0:2041:24::a:1])
 by sourceware.org (Postfix) with ESMTPS id F127C3858D34
 for <gcc-help@gcc.gnu.org>; Tue, 26 May 2020 13:07:13 +0000 (GMT)
DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org F127C3858D34
Authentication-Results: sourceware.org; dmarc=none (p=none dis=none)
 header.from=ipvs.uni-stuttgart.de
Authentication-Results: sourceware.org; spf=none
 smtp.mailfrom=steffen.hirschmann@ipvs.uni-stuttgart.de
Received: from localhost (localhost [127.0.0.1])
 by mxex1.tik.uni-stuttgart.de (Postfix) with ESMTP id B1836608AB
 for <gcc-help@gcc.gnu.org>; Tue, 26 May 2020 15:07:12 +0200 (CEST)
Authentication-Results: mxex1.tik.uni-stuttgart.de (amavisd-new);
 dkim=pass (2048-bit key) reason="pass (just generated, assumed good)"
 header.d=uni-stuttgart.de
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=uni-stuttgart.de;
 h=content-disposition:content-type:content-type:mime-version
 :message-id:subject:subject:from:from:date:date:received
 :received; s=dkim; i=@ipvs.uni-stuttgart.de; t=1590498430; x=
 1592237231; bh=XkQpFU+3Tyd2movJj0zI7FgLB0hCb2WGnCrqLv+5zW4=; b=b
 R2Se3rB9v9WSU+7yXSwf1EbDAMMp7wjgaKuNSgFJE7OVB919CETwOhy/rXUCHxrp
 bCLz7LLxdUgUudMzE0DhzetMgikO6OQcrFSegrFSHMVSJfXE696PZZ0xmR8vZqHx
 tp2k7apKnn/6WEP90wrHZXIhXway6CJ4OC2TdMvAXyJEjqInXZafv2qzYbSfOTRV
 LZn1DY0zOg5bxuLHOima2g4rSCzrRyJW07HPMPVwAYuXR4Np9dqZw8j3yNjOg9Yo
 B/6sYpuQ9IeQfjk46BwqulktDFC+y/3YAgvGL49sOVaWgUjCx+9aKPWN7ajbuOlN
 6ZtoUj9HfL7vKPjOeey3A==
X-Virus-Scanned: USTUTT mailrelay AV services at mxex1.tik.uni-stuttgart.de
Received: from mxex1.tik.uni-stuttgart.de ([127.0.0.1])
 by localhost (mxex1.tik.uni-stuttgart.de [127.0.0.1]) (amavisd-new, port 10031)
 with ESMTP id OBI7zFCL_5Jb; Tue, 26 May 2020 15:07:10 +0200 (CEST)
Received: from localhost (lapsgs17.informatik.uni-stuttgart.de
 [129.69.213.160])
 (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
 (No client certificate requested)
 by mxex1.tik.uni-stuttgart.de (Postfix) with ESMTPSA;
 Tue, 26 May 2020 15:07:10 +0200 (CEST)
Date: Tue, 26 May 2020 15:07:10 +0200
From: Steffen Hirschmann <steffen.hirschmann@ipvs.uni-stuttgart.de>
To: gcc-help@gcc.gnu.org
Subject: Unnecessary stores with std::optional?
Message-ID: <20200526130710.GB362962@lapsgs17>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
X-Spam-Status: No, score=-1.8 required=5.0 tests=BAYES_00, DKIM_SIGNED,
 DKIM_VALID, DKIM_VALID_EF, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_NONE,
 TXREP autolearn=ham autolearn_force=no version=3.4.2
X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on
 server2.sourceware.org
X-BeenThere: gcc-help@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-help mailing list <gcc-help.gcc.gnu.org>
List-Unsubscribe: <http://gcc.gnu.org/mailman/options/gcc-help>,
 <mailto:gcc-help-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-help/>
List-Post: <mailto:gcc-help@gcc.gnu.org>
List-Help: <mailto:gcc-help-request@gcc.gnu.org?subject=help>
List-Subscribe: <http://gcc.gnu.org/mailman/listinfo/gcc-help>,
 <mailto:gcc-help-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Tue, 26 May 2020 13:07:15 -0000

Dear GCC community,

I was testing std::optional when I noticed that gcc produces stores that
don't seem to be required.

Code:
--------
#include <optional>
std::optional<long> foo();
long bar()
{
    auto r = foo();
    if (r)
        return *r;
    else
        return 0L;
}
--------

What gcc 10.1 with -std=c++17 -O3 produces is:
bar():
        sub     rsp, 24
        call    foo()
        mov     QWORD PTR [rsp+8], rdx
        cmp     BYTE PTR [rsp+8], 0
        mov     QWORD PTR [rsp], rax
        mov     rax, QWORD PTR [rsp]
        jne     .L1
        xor     eax, eax
.L1:
        add     rsp, 24
        ret

(see: https://godbolt.org/z/uHE6QB)

I don't understand the stores (and loads) after the call to foo. They
don't seem necessary to me. Can anyone explain them to me?

Greetings,
Steffen