From patchwork Fri Jan 21 13:49:33 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Jonathan Wakely X-Patchwork-Id: 50308 Return-Path: X-Original-To: patchwork@sourceware.org Delivered-To: patchwork@sourceware.org Received: from server2.sourceware.org (localhost [IPv6:::1]) by sourceware.org (Postfix) with ESMTP id 988D23858C60 for ; Fri, 21 Jan 2022 13:51:00 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 988D23858C60 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org; s=default; t=1642773060; bh=oMsp7vEPt8mexOQWFi1dPEo/SGJL+6MiuOPDb6lGzsY=; h=To:Subject:Date:List-Id:List-Unsubscribe:List-Archive:List-Post: List-Help:List-Subscribe:From:Reply-To:Cc:From; b=K0OSBX0IYh3HPhY7BR1KjxyjRmlPPBNxJZFGMUuKUPgJ6ynM1jbjZjd5fZaSnbWWf SEZhEuIvdgQfQJoB6XzFM7+bZ70QGspAGV7ccYRcoG3Kos738gw3IIBgyvd+pAIsOb aR9MN+BPhmEaJOjHtClOR0JRhF6mAUwmAVCY+szE= X-Original-To: gcc-patches@gcc.gnu.org Delivered-To: gcc-patches@gcc.gnu.org Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by sourceware.org (Postfix) with ESMTPS id 621B73858411 for ; Fri, 21 Jan 2022 13:49:39 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 621B73858411 Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-78-BsTVAaCfNg25RpbbrElpag-1; Fri, 21 Jan 2022 08:49:35 -0500 X-MC-Unique: BsTVAaCfNg25RpbbrElpag-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 8B71710B7440; Fri, 21 Jan 2022 13:49:34 +0000 (UTC) Received: from localhost (unknown [10.33.36.2]) by smtp.corp.redhat.com (Postfix) with ESMTP id 3B02A7E21D; Fri, 21 Jan 2022 13:49:34 +0000 (UTC) To: libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org Subject: [PATCH] libstdc++: Fix aliasing violation in std::shared_ptr Date: Fri, 21 Jan 2022 13:49:33 +0000 Message-Id: <20220121134933.172441-1-jwakely@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com X-Spam-Status: No, score=-15.0 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_H3, RCVD_IN_MSPIKE_WL, 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: gcc-patches@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-Patchwork-Original-From: Jonathan Wakely via Gcc-patches From: Jonathan Wakely Reply-To: Jonathan Wakely Cc: Maged Michael Errors-To: gcc-patches-bounces+patchwork=sourceware.org@gcc.gnu.org Sender: "Gcc-patches" Tested powerpc64le-linux. Does anybody see a problem with this change? The non-atomic store that sets both reference counts to zero uses a type-punned pointer, which has undefined behaviour. We could use memset to write 8 bytes, but we don't actually need it to be a single store anyway. No other thread can observe the values, that's why it's safe to use non-atomic stores in the first place. So we can just set each count to zero. With -fstore-merging (which is enabled by default at -O2) GCC produces the same code for this as for memset or the type punned store. Clang does that store merging even at -O1. libstdc++-v3/ChangeLog: * include/bits/shared_ptr_base.h (_Sp_counted_base<>::_M_release): Set members to zero without type punning. --- libstdc++-v3/include/bits/shared_ptr_base.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h b/libstdc++-v3/include/bits/shared_ptr_base.h index 5b8f84b65be..b2f955b41f7 100644 --- a/libstdc++-v3/include/bits/shared_ptr_base.h +++ b/libstdc++-v3/include/bits/shared_ptr_base.h @@ -340,7 +340,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION // we are releasing the last strong reference. No other // threads can observe the effects of this _M_release() // call (e.g. calling use_count()) without a data race. - *(long long*)(&_M_use_count) = 0; + _M_weak_count = _M_use_count = 0; _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count); _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count); _M_dispose();