From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ej1-x62a.google.com (mail-ej1-x62a.google.com [IPv6:2a00:1450:4864:20::62a]) by sourceware.org (Postfix) with ESMTPS id D90E7388CC1B for ; Tue, 29 Jun 2021 10:58:37 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org D90E7388CC1B Received: by mail-ej1-x62a.google.com with SMTP id v20so17073485eji.10 for ; Tue, 29 Jun 2021 03:58:37 -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=yvw5EfZpJlBKUFMXf/lAjssu5pqGDUKxNxMCrkhoY3Q=; b=r2Bka6Ug0rCegQXPXj8L/OuD8gueYUWbKQAk1tR/q7peH80Xesji12En+WYOZTEnN0 uz1nfBIEkeCZkCftWDo2m1ghrc70zQZDxnAUa5ygknAUwE5MdZo+MMR7y58cCkrinDnC eyyEH1V6B+/fy7hiHtUvoJRnLMxmT8BhGLDWLZoosSpXcX6JEanmPKMSU0IF/7bRgJIj La1TKQW04k/S9PCXO1A+O/NgZQkcHdqBsMgVhg4X7JVl9Bd9lWOsOvc7s5n7M9PWp6Kw 6udm/a6k2MEQ8KoBsJ1msmJi8DuriFzl4p9pS0uD+w1bLW3fkD3kHUzaUEfL4qOR09X1 aeug== X-Gm-Message-State: AOAM531UVASu/LBVwuZICy6XaJyS1f46LEtS4HZi8Uv9ZD9QuhDjPjbD qkLFchQZwsqJR3D1PZoER0OGpJaE/01d2n3BVhE= X-Google-Smtp-Source: ABdhPJzstzn4y+6QG5yOV/Sv6jpT793yoSmSLD/GYVQJyoTn7jBoIR3Od4wbGqjTS3v1bUF20cv6fYSNRcpyNfehQ+8= X-Received: by 2002:a17:906:52d9:: with SMTP id w25mr21415239ejn.138.1624964316754; Tue, 29 Jun 2021 03:58:36 -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> In-Reply-To: From: Richard Biener Date: Tue, 29 Jun 2021 12:58:25 +0200 Message-ID: Subject: Re: [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=-3.0 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, 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: Tue, 29 Jun 2021 10:58:39 -0000 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 ;) > 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. > 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 > >> >