From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi1-x22a.google.com (mail-oi1-x22a.google.com [IPv6:2607:f8b0:4864:20::22a]) by sourceware.org (Postfix) with ESMTPS id 9C2213857401 for ; Fri, 25 Jun 2021 22:36:17 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 9C2213857401 Received: by mail-oi1-x22a.google.com with SMTP id s23so13053388oiw.9 for ; Fri, 25 Jun 2021 15:36:17 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=ZMn6zm0d/hxcDV17AI1AeVavWW/TVii30LM6mH6aFvE=; b=NzbyerpK7ryWDthIXEX3OHn5AWJZvePB4WepCFsKZnn6Q1D0MUXL91fW61lU+pWZVP fJdjKcg20hk+RILFpYfZHto+FhNSwNMswcqAPscGEcQz/6LiYNe11Bf0P7DhdWA5OL+m 6joRrEZbG7rUZ4R2SghFujN1hRwre3di9YwNdk92lLfPrIkVjpOLcrO0BvRFDuIKVVH5 zjFfzDK+jH/jv5UsqBJQsa25Ijl1cON2fT+/RNeadgAq7zh0ZFIr5JmzEbE7ZM5rCdC7 TkuxaWxYqOaeYAqeu13K5F7EaRFGENif3Yh4bj5iDTKYVYlXBN4kG/WXS3V09+AXhRbR oe4g== X-Gm-Message-State: AOAM5312cf+25/uTMSHuv37vdDRVCIOaWGPlLBtFUybng72Ek1Xy6cTE uVHzxitCjvkSK5kHLQQ6JK0= X-Google-Smtp-Source: ABdhPJzfepQP+BWKKMMqENq4kCJmOgf2sFhcVNm3F2unEppbEePzAqyjuCAiugW60ZVdIcUMF8sPXg== X-Received: by 2002:aca:df83:: with SMTP id w125mr11109490oig.33.1624660577099; Fri, 25 Jun 2021 15:36:17 -0700 (PDT) Received: from [192.168.0.41] (97-118-105-195.hlrn.qwest.net. [97.118.105.195]) by smtp.gmail.com with ESMTPSA id t21sm1708623otd.35.2021.06.25.15.36.15 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 25 Jun 2021 15:36:16 -0700 (PDT) Subject: Re: [PATCH] define auto_vec copy ctor and assignment (PR 90904) To: Jason Merrill , Richard Biener Cc: gcc-patches , Jonathan Wakely References: <91545a73-12af-33b2-c6e7-119b5a21de60@gmail.com> <4d503394-4e82-1d36-41ca-34315042775b@redhat.com> <49569f1d-9856-55c7-b9e9-578bbd7c7b7a@gmail.com> From: Martin Sebor Message-ID: Date: Fri, 25 Jun 2021 16:36:15 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-4.0 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, NICE_REPLY_A, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) 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-List-Received-Date: Fri, 25 Jun 2021 22:36:20 -0000 On 6/25/21 4:11 PM, Jason Merrill wrote: > On 6/25/21 4:51 PM, Martin Sebor wrote: >> On 6/1/21 3:38 PM, Jason Merrill wrote: >>> On 6/1/21 3:56 PM, Martin Sebor wrote: >>>> On 5/27/21 2:53 PM, Jason Merrill wrote: >>>>> On 4/27/21 11:52 AM, Martin Sebor via Gcc-patches wrote: >>>>>> On 4/27/21 8:04 AM, Richard Biener wrote: >>>>>>> On Tue, Apr 27, 2021 at 3:59 PM Martin Sebor >>>>>>> wrote: >>>>>>>> >>>>>>>> On 4/27/21 1:58 AM, Richard Biener wrote: >>>>>>>>> On Tue, Apr 27, 2021 at 2:46 AM Martin Sebor via Gcc-patches >>>>>>>>> wrote: >>>>>>>>>> >>>>>>>>>> PR 90904 notes that auto_vec is unsafe to copy and assign because >>>>>>>>>> the class manages its own memory but doesn't define (or delete) >>>>>>>>>> either special function.  Since I first ran into the problem, >>>>>>>>>> auto_vec has grown a move ctor and move assignment from >>>>>>>>>> a dynamically-allocated vec but still no copy ctor or copy >>>>>>>>>> assignment operator. >>>>>>>>>> >>>>>>>>>> The attached patch adds the two special functions to auto_vec >>>>>>>>>> along >>>>>>>>>> with a few simple tests.  It makes auto_vec safe to use in >>>>>>>>>> containers >>>>>>>>>> that expect copyable and assignable element types and passes >>>>>>>>>> bootstrap >>>>>>>>>> and regression testing on x86_64-linux. >>>>>>>>> >>>>>>>>> The question is whether we want such uses to appear since those >>>>>>>>> can be quite inefficient?  Thus the option is to delete those >>>>>>>>> operators? >>>>>>>> >>>>>>>> I would strongly prefer the generic vector class to have the >>>>>>>> properties >>>>>>>> expected of any other generic container: copyable and >>>>>>>> assignable.  If >>>>>>>> we also want another vector type with this restriction I suggest >>>>>>>> to add >>>>>>>> another "noncopyable" type and make that property explicit in >>>>>>>> its name. >>>>>>>> I can submit one in a followup patch if you think we need one. >>>>>>> >>>>>>> I'm not sure (and not strictly against the copy and assign). >>>>>>> Looking around >>>>>>> I see that vec<> does not do deep copying.  Making auto_vec<> do it >>>>>>> might be surprising (I added the move capability to match how vec<> >>>>>>> is used - as "reference" to a vector) >>>>>> >>>>>> The vec base classes are special: they have no ctors at all (because >>>>>> of their use in unions).  That's something we might have to live with >>>>>> but it's not a model to follow in ordinary containers. >>>>> >>>>> I don't think we have to live with it anymore, now that we're >>>>> writing C++11. >>>>> >>>>>> The auto_vec class was introduced to fill the need for a conventional >>>>>> sequence container with a ctor and dtor.  The missing copy ctor and >>>>>> assignment operators were an oversight, not a deliberate feature. >>>>>> This change fixes that oversight. >>>>>> >>>>>> The revised patch also adds a copy ctor/assignment to the auto_vec >>>>>> primary template (that's also missing it).  In addition, it adds >>>>>> a new class called auto_vec_ncopy that disables copying and >>>>>> assignment as you prefer. >>>>> >>>>> Hmm, adding another class doesn't really help with the confusion >>>>> richi mentions.  And many uses of auto_vec will pass them as vec, >>>>> which will still do a shallow copy.  I think it's probably better >>>>> to disable the copy special members for auto_vec until we fix vec<>. >>>> >>>> There are at least a couple of problems that get in the way of fixing >>>> all of vec to act like a well-behaved C++ container: >>>> >>>> 1) The embedded vec has a trailing "flexible" array member with its >>>> instances having different size.  They're initialized by memset and >>>> copied by memcpy.  The class can't have copy ctors or assignments >>>> but it should disable/delete them instead. >>>> >>>> 2) The heap-based vec is used throughout GCC with the assumption of >>>> shallow copy semantics (not just as function arguments but also as >>>> members of other such POD classes).  This can be changed by providing >>>> copy and move ctors and assignment operators for it, and also for >>>> some of the classes in which it's a member and that are used with >>>> the same assumption. >>>> >>>> 3) The heap-based vec::block_remove() assumes its elements are PODs. >>>> That breaks in VEC_ORDERED_REMOVE_IF (used in gcc/dwarf2cfi.c:2862 >>>> and tree-vect-patterns.c). >>>> >>>> I took a stab at both and while (1) is easy, (2) is shaping up to >>>> be a big and tricky project.  Tricky because it involves using >>>> std::move in places where what's moved is subsequently still used. >>>> I can keep plugging away at it but it won't change the fact that >>>> the embedded and heap-based vecs have different requirements. >>>> >>>> It doesn't seem to me that having a safely copyable auto_vec needs >>>> to be put on hold until the rats nest above is untangled.  It won't >>>> make anything worse than it is.  (I have a project that depends on >>>> a sane auto_vec working). >>>> >>>> A couple of alternatives to solving this are to use std::vector or >>>> write an equivalent vector class just for GCC. >>> >>> It occurs to me that another way to work around the issue of passing >>> an auto_vec by value as a vec, and thus doing a shallow copy, would >>> be to add a vec ctor taking an auto_vec, and delete that.  This would >>> mean if you want to pass an auto_vec to a vec interface, it needs to >>> be by reference.  We might as well do the same for operator=, though >>> that isn't as important. >> >> Thanks, that sounds like a good idea.  Attached is an implementation >> of this change.  Since the auto_vec copy ctor and assignment have >> been deleted by someone else in the interim, this patch doesn't >> reverse that.  I will propose it separately after these changes >> are finalized. >> >> My approach was to 1) disable the auto_vec to vec conversion, >> 2) introduce an auto_vec::to_vec() to make the conversion possible >> explicitly, and 3) resolve compilation errors by either changing >> APIs to take a vec by reference or callers to convert auto_vec to >> vec explicitly by to_vec().  In (3) I tried to minimize churn while >> improving the const-correctness of the APIs. > > What did you base the choice between reference or to_vec on?  For > instance, it seems like c_parser_declaration_or_fndef could use a > reference, but you changed the callers instead. I went with a reference whenever I could. That doesn't work when there are callers that pass in a vNULL, so there, and in assignments, I used to_vec(). Martin