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[130.44.159.43]) by smtp.gmail.com with ESMTPSA id r4sm3103719qtn.57.2021.06.29.07.43.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 29 Jun 2021 07:43:24 -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: <118e90d5-fb85-43ad-d0bc-66ac4d35225d@redhat.com> Date: Tue, 29 Jun 2021 10:43:22 -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: 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: Tue, 29 Jun 2021 14:43:31 -0000 On 6/28/21 2:07 PM, Martin Sebor wrote: > On 6/28/21 2:07 AM, Richard Biener wrote: >> On Sat, Jun 26, 2021 at 12:36 AM Martin Sebor wrote: >>> >>> 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(). >> >> Is there a way to "fix" the ugliness with vNULL?  All those functions >> should be able to use const vec<>& as otherwise they'd leak memory? >> Can't we pass vNULL to a const vec<>&? > > vNULL can bind to a const vec& (via the vec conversion ctor) but > not to vec&.  The three functions that in the patch are passed > vNULL modify the argument when it's not vNULL but not otherwise. The c_parser_declaration_or_fndef case is rather ugly: the vec is passed by value, but then the modifications in c_finish_omp_declare_simd modify the original vec. We could keep the same semantic problem and make it more blatant by changing to const vec& and doing a const_cast in c_finish_omp_declare_simd before modifying the vec. Do the other two have the same problem? Jason