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<>&? Richard. > > Martin >
