From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 42192 invoked by alias); 8 Aug 2019 15:06:24 -0000 Mailing-List: contact gcc-patches-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-patches-owner@gcc.gnu.org Received: (qmail 42178 invoked by uid 89); 8 Aug 2019 15:06:24 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-1.9 required=5.0 tests=AWL,BAYES_00,RCVD_IN_DNSWL_NONE autolearn=ham version=3.3.1 spammy=MODIFY_EXPR, modify_expr, H*RU:209.85.160.193, HX-Spam-Relays-External:209.85.160.193 X-HELO: mail-qt1-f193.google.com Received: from mail-qt1-f193.google.com (HELO mail-qt1-f193.google.com) (209.85.160.193) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Thu, 08 Aug 2019 15:06:22 +0000 Received: by mail-qt1-f193.google.com with SMTP id a15so92430427qtn.7 for ; Thu, 08 Aug 2019 08:06:22 -0700 (PDT) Return-Path: Received: from [192.168.1.5] (pool-71-168-71-123.cncdnh.fast.myfairpoint.net. [71.168.71.123]) by smtp.gmail.com with ESMTPSA id q73sm27684934qke.90.2019.08.08.08.06.18 (version=TLS1_3 cipher=AEAD-AES128-GCM-SHA256 bits=128/128); Thu, 08 Aug 2019 08:06:18 -0700 (PDT) Subject: Re: C++ PATCH for c++/91264 - detect modifying const objects in constexpr To: Marek Polacek Cc: GCC Patches References: <20190731192659.GP32749@redhat.com> <902366c6-754a-de65-f78e-25834263ac8a@redhat.com> <20190806192021.GL28284@redhat.com> From: Jason Merrill Message-ID: <68bb270b-fa29-972d-7cc3-790dbcf02767@redhat.com> Date: Thu, 08 Aug 2019 15:18:00 -0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190806192021.GL28284@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-IsSubscribed: yes X-SW-Source: 2019-08/txt/msg00559.txt.bz2 On 8/6/19 3:20 PM, Marek Polacek wrote: > On Mon, Aug 05, 2019 at 03:54:19PM -0400, Jason Merrill wrote: >> On 7/31/19 3:26 PM, Marek Polacek wrote: >>> One of the features of constexpr is that it doesn't allow UB; and such UB must >>> be detected at compile-time. So running your code in a context that requires >>> a constant expression should ensure that the code in question is free of UB. >>> In effect, constexpr can serve as a sanitizer. E.g. this article describes in >>> in more detail: >>> >>> >>> [dcl.type.cv]p4 says "Any attempt to modify a const object during its lifetime >>> results in undefined behavior." However, as the article above points out, we >>> aren't detecting that case in constexpr evaluation. >>> >>> This patch fixes that. It's not that easy, though, because we have to keep in >>> mind [class.ctor]p5: >>> "A constructor can be invoked for a const, volatile or const volatile object. >>> const and volatile semantics are not applied on an object under construction. >>> They come into effect when the constructor for the most derived object ends." >>> >>> I handled this by keeping a hash set which tracks objects under construction. >>> I considered other options, such as going up call_stack, but that wouldn't >>> work with trivial constructor/op=. It was also interesting to find out that >>> the definition of TREE_HAS_CONSTRUCTOR says "When appearing in a FIELD_DECL, >>> it means that this field has been duly initialized in its constructor" though >>> nowhere in the codebase do we set TREE_HAS_CONSTRUCTOR on a FIELD_DECL as far >>> as I can see. Unfortunately, using this bit proved useless for my needs here. >> >>> Also, be mindful of mutable subobjects. >>> >>> Does this approach look like an appropriate strategy for tracking objects' >>> construction? >> >> For scalar objects, we should be able to rely on INIT_EXPR vs. MODIFY_EXPR >> to distinguish between initialization and modification; for class objects, I > > This is already true: only class object go into the hash set. > >> wonder about setting a flag on the CONSTRUCTOR after initialization is >> complete to indicate that the value is now constant. > > But here we're not dealing with CONSTRUCTORs in the gcc sense (i.e. exprs with > TREE_CODE == CONSTRUCTOR). We have a CALL_EXPR like Y::Y ((struct Y *) &y), > which initializes the object "y". Setting a flag on the CALL_EXPR or its underlying > function decl wouldn't help. > > Am I missing something? I was thinking that where in your current patch you call remove_object_under_construction, we could instead mark the object's value CONSTRUCTOR as immutable. > (Also, all 6 TREE_LANG_FLAGs for a CONSTRUCTOR are used.) TREE_READONLY seems suitable. Jason