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

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

On Mon, Jun 28, 2021 at 8:07 PM Martin Sebor <msebor@gmail.com> wrote:
>
> On 6/28/21 2:07 AM, Richard Biener wrote:
> > 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<>&?
>
> 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<T> 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
> >>
>