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 D31F93858C30; Thu,  2 Feb 2023 10:15:14 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org D31F93858C30
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1675332914;
	bh=wvUpQtaMYaAvgGq0AiqkLj8hyep5wSHXtqQUmm/gRhA=;
	h=From:To:Subject:Date:In-Reply-To:References:From;
	b=lgyWU27EUWMrpE8u7luH6RwCC0/yAG5egdZpMPkymV3FnV684qf2KCyUDnRrISGoZ
	 D5RFBvaZ//S7/tTF2Yzbi3t0Z9mJZeHY0KzpoXwUPWnpa5pZitrJyo2o2rTSRVtt/c
	 39mNG0fAZg85maeToVkAtKVB3VN3n95JyF2pDZDI=
From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/45115] pure functions returning structs are
 not optimized.
Date: Thu, 02 Feb 2023 10:15:13 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: unknown
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: enhancement
X-Bugzilla-Who: redi at gcc dot gnu.org
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: 
Message-ID: <bug-45115-4-lie8K9EvT6@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-45115-4@http.gcc.gnu.org/bugzilla/>
References: <bug-45115-4@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=3D45115

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This affects C++20 three-way comparisons, which return trivial structs wrap=
ping
an integer.

>From PR 108635:

    #include <compare>

    struct S
    {
        std::weak_ordering operator<=3D>(const S&) const __attribute__((con=
st));
    };

    int compare3way(S& a, S& b)
    {
        return (a < b) ? -1 : (a > b) ? 1 : 0;
    }

I expect operator<=3D> to be called once, but it is called twice.  This can=
 be a
major missed optimization if operator<=3D> is expensive.  It happens regard=
less
of:

 1. Using attribute((const)) or attribute((pure)).
 2. Making operator<=3D> a free function or a member.
 3. Comparing (a > b) or (a < b) in the second ternary expression.  This is
especially strange, because it's really calling the same pure function twic=
e,
and that's optimized correctly when the function being called is operator<
instead of operator<=3D>.

Clang optimizes it as expected.

Demo: https://godbolt.org/z/jP51E6xaz=