From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ej1-x631.google.com (mail-ej1-x631.google.com [IPv6:2a00:1450:4864:20::631]) by sourceware.org (Postfix) with ESMTPS id BB6543857807 for ; Wed, 7 Jul 2021 07:28:20 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org BB6543857807 Received: by mail-ej1-x631.google.com with SMTP id bg14so1707216ejb.9 for ; Wed, 07 Jul 2021 00:28:20 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=hZ5Nz24LNDNPPTRdLx1fpHIthN/R4b1BcRbs9zXusms=; b=Fj7JErUhNyfTE+XOCXIu8Iy79BvcIU2/rm+FitLPSY6o3c6vwJX6BvmEkOohVTsCRb ZtkrmQ0VSm8gPLb1rfhYWckjix+MO5hKRUYGAJuXhKkJm0s0Yl9ZAmgyEoQXDwmPQDSV omubkFjJC8dN4r6ZDQ1KvoO85C1I+TTydWAdCF6AQBkbe2m6vkHJf4+5VGjfewMj7cGY ThvUmcDPOkhGrQyrl/Nfdnf+D0Yn3xGviLPROa89gHhY7+/HoTOWLtMCW8PtrY4uS1u8 VGvtC/VLyp6/G91IVdJK8UxEpfkXKQ5hB19Eh0gx9mWZghQKEiRM7PT7VU4+19b+WO6p Qamg== X-Gm-Message-State: AOAM532OBxlXQ+VZ+jeaDrw7naYj43B8Jj5iAhx9csd7CyNlBl5h9K/L 41sa7lsOuPCIvpbOqqGvblDmueClpeOwj6vOFHc= X-Google-Smtp-Source: ABdhPJzkZvgB29LmEVQ7j3CF51YsvvPPDo/iHExijpN+6LMV9Tp/Lqfb895etWPhqZn+atF1Ct7jD3x443y1Yr9/Lmg= X-Received: by 2002:a17:907:2d2a:: with SMTP id gs42mr17901074ejc.371.1625642899713; Wed, 07 Jul 2021 00:28:19 -0700 (PDT) MIME-Version: 1.0 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> In-Reply-To: From: Richard Biener Date: Wed, 7 Jul 2021 09:28:08 +0200 Message-ID: Subject: Re: [PING][PATCH] define auto_vec copy ctor and assignment (PR 90904) To: Martin Sebor Cc: Jason Merrill , gcc-patches , Jonathan Wakely Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-2.8 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: Wed, 07 Jul 2021 07:28:23 -0000 On Tue, Jul 6, 2021 at 5:06 PM Martin Sebor wrote: > > 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? I don't remember seeing one (aka saying "bootstrapped/tested, OK to commit?" or so) - and the link above doesn't have one. So, can you re-post it please? Thanks, Richard. > 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 > >>>>> > >>> > > >