From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [216.205.24.124]) by sourceware.org (Postfix) with ESMTP id 6D1C13857835 for ; Fri, 1 Oct 2021 19:41:09 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 6D1C13857835 Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-463-W0JkqQT-N86Dxsp1JHeUJQ-1; Fri, 01 Oct 2021 15:41:07 -0400 X-MC-Unique: W0JkqQT-N86Dxsp1JHeUJQ-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id E10E384A5E3; Fri, 1 Oct 2021 19:41:06 +0000 (UTC) Received: from localhost (unknown [10.33.36.47]) by smtp.corp.redhat.com (Postfix) with ESMTP id 8E9A24180; Fri, 1 Oct 2021 19:41:06 +0000 (UTC) Date: Fri, 1 Oct 2021 20:41:05 +0100 From: Jonathan Wakely To: libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org Subject: [committed] libstdc++: Make move ctor noexcept for fully-dynamic string Message-ID: MIME-Version: 1.0 X-Clacks-Overhead: GNU Terry Pratchett X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/mixed; boundary="28RY6PFMebQhfkfH" Content-Disposition: inline X-Spam-Status: No, score=-13.8 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, GIT_PATCH_0, RCVD_IN_DNSWL_LOW, RCVD_IN_MSPIKE_H2, SPF_HELO_NONE, SPF_NONE, TXREP autolearn=ham autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on server2.sourceware.org X-BeenThere: libstdc++@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libstdc++ mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 01 Oct 2021 19:41:10 -0000 --28RY6PFMebQhfkfH Content-Type: text/plain; charset=us-ascii Content-Disposition: inline The move constructor for the "fully-dynamic" COW string is not noexcept, because it allocates a new empty string rep for the moved-from string. However, there is no need to do that, because the moved-from string does not have to be left empty. Instead, implement move construction for the fully-dynamic case as a reference count increment, so the string is shared. Signed-off-by: Jonathan Wakely libstdc++-v3/ChangeLog: * include/bits/cow_string.h [_GLIBCXX_FULLY_DYNAMIC_STRING] (basic_string(basic_string&&)): Add noexcept and avoid allocation, by sharing rep with the rvalue string. Tested powerpc64le-linux. Committed to trunk. --28RY6PFMebQhfkfH Content-Type: text/plain; charset=us-ascii Content-Disposition: attachment; filename="patch.txt" commit 10b6d89baddd86139480ba902f491903fcb464a6 Author: Jonathan Wakely Date: Fri Apr 30 15:04:34 2021 libstdc++: Make move ctor noexcept for fully-dynamic string The move constructor for the "fully-dynamic" COW string is not noexcept, because it allocates a new empty string rep for the moved-from string. However, there is no need to do that, because the moved-from string does not have to be left empty. Instead, implement move construction for the fully-dynamic case as a reference count increment, so the string is shared. Signed-off-by: Jonathan Wakely libstdc++-v3/ChangeLog: * include/bits/cow_string.h [_GLIBCXX_FULLY_DYNAMIC_STRING] (basic_string(basic_string&&)): Add noexcept and avoid allocation, by sharing rep with the rvalue string. diff --git a/libstdc++-v3/include/bits/cow_string.h b/libstdc++-v3/include/bits/cow_string.h index 61edaa85484..ba4a8cc2e98 100644 --- a/libstdc++-v3/include/bits/cow_string.h +++ b/libstdc++-v3/include/bits/cow_string.h @@ -620,18 +620,25 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION * The newly-created string contains the exact contents of @a __str. * @a __str is a valid, but unspecified string. */ - basic_string(basic_string&& __str) + basic_string(basic_string&& __str) noexcept #if _GLIBCXX_FULLY_DYNAMIC_STRING == 0 - noexcept // FIXME C++11: should always be noexcept. -#endif : _M_dataplus(std::move(__str._M_dataplus)) { -#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0 __str._M_data(_S_empty_rep()._M_refdata()); -#else - __str._M_data(_S_construct(size_type(), _CharT(), get_allocator())); -#endif } +#else + : _M_dataplus(__str._M_rep()) + { + // Rather than allocate an empty string for the rvalue string, + // just share ownership with it by incrementing the reference count. + // If the rvalue string was "leaked" then it was the unique owner, + // so need an extra increment to indicate shared ownership. + if (_M_rep()->_M_is_leaked()) + __gnu_cxx::__atomic_add_dispatch(&_M_rep()->_M_refcount, 2); + else + __gnu_cxx::__atomic_add_dispatch(&_M_rep()->_M_refcount, 1); + } +#endif /** * @brief Construct string from an initializer %list. --28RY6PFMebQhfkfH--