From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 102566 invoked by alias); 4 May 2017 21:49:05 -0000 Mailing-List: contact gdb-patches-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-patches-owner@sourceware.org Received: (qmail 102545 invoked by uid 89); 4 May 2017 21:49:04 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-1.1 required=5.0 tests=BAYES_00,KAM_ASCII_DIVIDERS,RP_MATCHES_RCVD,SPF_HELO_PASS autolearn=no version=3.3.2 spammy=dear X-HELO: mx1.redhat.com Received: from mx1.redhat.com (HELO mx1.redhat.com) (209.132.183.28) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Thu, 04 May 2017 21:49:02 +0000 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 08F038048E; Thu, 4 May 2017 21:49:04 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com 08F038048E Authentication-Results: ext-mx04.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx04.extmail.prod.ext.phx2.redhat.com; spf=pass smtp.mailfrom=palves@redhat.com DKIM-Filter: OpenDKIM Filter v2.11.0 mx1.redhat.com 08F038048E Received: from [127.0.0.1] (ovpn04.gateway.prod.ext.ams2.redhat.com [10.39.146.4]) by smtp.corp.redhat.com (Postfix) with ESMTP id 6292217CE5; Thu, 4 May 2017 21:49:03 +0000 (UTC) Subject: Re: Oh dear. I regret to inform you that commit 'RAII-fy make_cleanup_restore_current_thread & friends' might be unfortunate To: Simon Marchi References: <34792dd0-088c-a1d1-9125-70c8585c21bd@redhat.com> <6bf88edee0fb17451d44b85bb00fb0d0@polymtl.ca> <79511435-a3a8-48f9-2e16-bca8adb1909d@redhat.com> <1fd4bf31-b0cd-3746-36d0-5f9f9e9dce4c@redhat.com> Cc: gdb-patches@sourceware.org From: Pedro Alves Message-ID: Date: Thu, 04 May 2017 21:49:00 -0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 MIME-Version: 1.0 In-Reply-To: <1fd4bf31-b0cd-3746-36d0-5f9f9e9dce4c@redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit X-SW-Source: 2017-05/txt/msg00128.txt.bz2 On 05/04/2017 07:28 PM, Pedro Alves wrote: > On 05/04/2017 07:22 PM, Pedro Alves wrote: > >> AFAICS so far, this is a false positive. >> >> Not sure what to do. I wouldn't want to force-memset >> the optional's storage to work around it, which would be >> a pessimization to quiet a warning. From above, we see that >> that wouldn't work when we later start using std::optional. >> >> There's a bug open about this (for boost::optional, but most >> probably the exact same): >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78044 > > I've tried the reproducer there with the obvious change to use > std::optional and that does not warn. So this is a different, > though related issue. > > Or maybe there's really something wrong with the gdb code > that is escaping me. OK, here's smallest, self-contained reproducer I managed to come up with: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ //#include #include template struct optional { optional () : m_dummy () {} ~optional () { m_item.~T (); } void emplace () { new (&m_item) T (); } union { int m_dummy; T m_item; }; }; template using Optional = optional; // warns //using Optional = std::optional; // warns too extern int get (); extern void set (int); struct A { A () : m (get ()) {} ~A () { set (m); } int m; }; struct B { B (); ~B (); }; void func () { Optional maybe_a; Optional maybe_b; maybe_a.emplace (); maybe_b.emplace (); } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ $ /opt/gcc/bin/g++ optional.cc -g3 -O2 -Wmaybe-uninitialized -std=gnu++17 -c optional.cc: In function ‘void func()’: optional.cc:28:15: warning: ‘maybe_a.optional::.optional::::m_dummy’ may be used uninitialized in this function [-Wmaybe-uninitialized] ~A () { set (m); } ~~~~^~~ optional.cc:41:15: note: ‘maybe_a.optional::.optional::::m_dummy’ was declared here Optional maybe_a; ^~~~~~~ Looks like a compiler bug to me. Thanks, Pedro Alves