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 CCC2F3858D33; Thu,  1 Feb 2024 12:08:23 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org CCC2F3858D33
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1706789303;
	bh=wgauaVj8eFcfrSasH/ccBIe+4/KYgobNqODBdfY+X4w=;
	h=From:To:Subject:Date:From;
	b=LOV20ij+j4smWfTmPWUmHyE0RM9YO7OfpHwIzz1epwL6chunxTIwqTJkQeeF1AjRX
	 nM3lZF7fd7L2pz0axWikiB03ToV1v948FRjoGVmeWJWoS6Rcsv3sK6ly3LhwPfC2fj
	 sG2E+cfvxER/c/u0m0XdS+Ak6sWpORZOKc1pQ9DY=
From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/113704] New: std::locale::locale<Facet>(const
 locale&, Facet*) is inefficient
Date: Thu, 01 Feb 2024 12:08:22 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 11.4.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: redi at gcc dot gnu.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
 keywords bug_severity priority component assigned_to reporter
 target_milestone
Message-ID: <bug-113704-4@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=3D113704

            Bug ID: 113704
           Summary: std::locale::locale<Facet>(const locale&, Facet*) is
                    inefficient
           Product: gcc
           Version: 11.4.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

template<typename _Facet>
    locale::
    locale(const locale& __other, _Facet* __f)
    {
      _M_impl =3D new _Impl(*__other._M_impl, 1);

      __try
        { _M_impl->_M_install_facet(&_Facet::id, __f); }
      __catch(...)
        {
          _M_impl->_M_remove_reference();
          __throw_exception_again;
        }
      delete [] _M_impl->_M_names[0];
      _M_impl->_M_names[0] =3D 0;   // Unnamed.
    }

This is very wasteful if the facet isn't already present in __other. First =
the
_Impl constructor allocates new copies of all the arrays in *__other._M_imp=
l,
then _M_install_facets reallocates them with additional space for the new
facet.

We should check whether _Facet::id can be stored without reallocation. If it
can't, we should avoid allocating and copying all the arrays twice.

This matters for custom user facets which will always have a higher ID and
require reallocation.=