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[130.44.159.43]) by smtp.gmail.com with ESMTPSA id e1sm4928746qti.27.2021.06.25.15.11.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 25 Jun 2021 15:11:59 -0700 (PDT) Subject: Re: [PATCH] define auto_vec copy ctor and assignment (PR 90904) To: Martin Sebor , 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: Jason Merrill Message-ID: Date: Fri, 25 Jun 2021 18:11:57 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.0 MIME-Version: 1.0 In-Reply-To: <49569f1d-9856-55c7-b9e9-578bbd7c7b7a@gmail.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-7.2 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, NICE_REPLY_A, RCVD_IN_DNSWL_LOW, RCVD_IN_MSPIKE_H4, RCVD_IN_MSPIKE_WL, 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 22:12:05 -0000 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. Jason