On Tue, Jun 29, 2021 at 7:18 PM Martin Sebor <mse...@gmail.com> wrote: > > On 6/29/21 8:43 AM, Jason Merrill wrote: > > 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 <mse...@gmail.com> 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 <mse...@gmail.com> > >>>>>>>>>>> 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 > >>>>>>>>>>>>> <gcc-patches@gcc.gnu.org> 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? > > Yes, the functions that take a vec by value and are passed an auto_vec > "by reference" (the result of to_vec()) modify the auto_vec. This is > the "bug" this patch is designed to keep from happening by accident, > while letting the API clients do it intentionally. > > Changing these APIs to take a const vec& while still letting them > modify the argument by casting away the constness seems even more > surprising to me than the current by-value style. > > I do think it should be fixed but I'd have been more comfortable > handling that separately. Attached is a (near) minimal change > along these lines to c_parser_declaration_or_fndef and its callers. > The logic isn't exactly the same as the original but no tests fail. > If this is the direction we want to go in I can see about making > an analogous change to the other two similar functions in the patch. > Let me know.
Note there's also array_slice<> which could be used to pass non-const vec<>s that are never resized but modified - the only "valid" case of passing a non-const vec<> by value. But as noted array_slice<> lacks most of the vec<> API so I'm not sure how awkward that option would be. We of course can amend its API as well. Richard. > Martin
