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[75.166.102.22]) by smtp.gmail.com with ESMTPSA id c25sm1671293otn.47.2021.07.06.08.06.25 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 06 Jul 2021 08:06:26 -0700 (PDT) Subject: [PING][PATCH] define auto_vec copy ctor and assignment (PR 90904) From: Martin Sebor To: Richard Biener Cc: Jason Merrill , 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> <390c6652-0a1f-e8c4-d70d-56ced2f7b0fb@gmail.com> Message-ID: Date: Tue, 6 Jul 2021 09:06:25 -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: <390c6652-0a1f-e8c4-d70d-56ced2f7b0fb@gmail.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-4.6 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, KAM_SHORT, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) 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, 06 Jul 2021 15:06:30 -0000 Ping: https://gcc.gnu.org/pipermail/gcc-patches/2021-June/573968.html Any questions/suggestions on the final patch or is it okay to commit? On 6/29/21 7:46 PM, Martin Sebor wrote: > On 6/29/21 4:58 AM, Richard Biener wrote: >> On Mon, Jun 28, 2021 at 8:07 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. >>> An alternate design is to have them take a vec* and pass in >>> a plain NULL (or nullptr) instead of vNULL.  That would require >>> some surgery on the function bodies that I've been trying to >>> avoid in the first pass. >> >> But I wonder if since you now identified them they could be massaged >> prior to doing the change. >> >> I do hope we end up not needing .to_vec () after all, if no users >> remain ;) > > I'd be happy to if none remained.  I see how to eliminate those in > calls to functions like c_parser_declaration_or_fndef() (done in > the attached revision of the patch), but no easy way to get rid > of those that replace other implicit conversions, like all those > assignments to the vec members of the ipa_call_arg_values ctor. > If it's appropriate to std::move those then that would get rid > of the .to_vec () call.  I'm not familiar with the code but I > have the impression it might be meant more as a reference to > some "remote" object (an instance of ipa_auto_call_arg_values?) > If that's right then making the vec members auto_vec references > (or pointers) would be one way to "fix" this. > >>> Functions that don't leak memory now shouldn't leak with these >>> changes, and conversely, those that do will still leak.  The patch >>> doesn't change that (as far as I know). >> >> It just occurs to me those cases could pass auto_vec<>() by reference >> instead >> of vNULL?  So if the vector is modified then it's released afterwards? >> That would fix the memleak. > > I see what you mean.  A function that modified the unnamed vec > temporary constructed from vNULL then the modified vector would > leak.  I don't think the functions the patch touches do that but > I've removed the vNULL conversion from all of them.  There are > many others that pass vNULL to a vec arguments that that the patch > doesn't touch but those would be worth a closer look at some point. > > Attached is a revised patch with these changes (a superset of > those I sent in response to Jason's question), tested on x86_64. > > Martin > >> >>> Going forward I think it's possible to replace most uses of vNULL >>> in GCC with direct initialization (e.g., vec v{ }).  Those that >>> can't be readily replaced are the ones where vNULL is passed as >>> an argument to functions taking a vec by value.  Those could be >>> changed to avoid vNULL too, but it would take a different approach >>> and more effort.  I'm not against it but I'd rather decouple those >>> changes from this already sizeable patch. >>> >>> Martin >>> >>>> >>>> Richard. >>>> >>>>> >>>>> Martin >>>>> >>> >