Re: [PATCH] define auto_vec copy ctor and assignment (PR 90904)

[Richard Biener <richard.guenther@gmail.com>]

On Sat, Jun 26, 2021 at 12:36 AM Martin Sebor <msebor@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 <msebor@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
>