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

[Trevor Saunders]

On Wed, Jun 02, 2021 at 10:04:03AM -0600, Martin Sebor via Gcc-patches wrote:
> On 6/2/21 12:55 AM, Richard Biener wrote:
> > On Tue, Jun 1, 2021 at 9:56 PM Martin Sebor <msebor@gmail.com> 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.

I've been away a while, but trying to get back into this, sorry.  It was
definitely an oversight to leave these undefined for the compiler to
provide a default definition of, but I agree with Richi, the better
thing to have done, or do now would be to mark them as deleted and make
auto_vec move only (with copy() for when you really need a deep copy.
> > > > > 
> > > > > 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.
> > 
> > So you figured that neither vec<> nor auto_vec<> are a container like
> > std::vector.
> 
> That's obvious from glancing at their definitions.  I didn't go
> through the exercise to figure that out.
> 
> > 
> > I'm not sure it makes sense to try to make it so since obviously vec<>
> > was designed to match the actual needs of GCC.  auto_vec<> was added
> > to make a RAII (like auto_bitmap, etc.) wrapper, plus it got the ability
> > to provide initial stack storage.
> 
> The goal was to see if the two vec instances could be made safer
> to use but taking advantage of C++ 11 features.  As I mentioned
> recently, creating a copy of a vec and modifying it changes it as
> well as the original (e.g., by changing a vec argument passed to
> it by value a function changes the actual argument in the caller).
> That's surprising to most C++ programmers.

It can probably be improved now with c++11, but while very unfortunate
There is hard requirements on how vec works from existing code using it.

> My conclusion from the exercise is that although some of the problems
> with vec can, and IMO should, be solved, making the heap-based one
> a well-behaved C++ 11 container will take considerable effort and
> is impossible for the embedded vec.

Yes, fortunately things using embedded vec do not at all expect a c++
container, and so don't really mismatch it.  You probably should not be
creating them yourself unless you are creating a new object with an
embedded vector, and you probably don't want to do that.

> > 
> > > 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).
> > 
> > So how does your usage look like?  I can't really figure who'd need
> > deep copying of a container - note there's vec<>::copy at your
> > discretion.
> > 
> > > A couple of alternatives to solving this are to use std::vector or
> > > write an equivalent vector class just for GCC.

imho one of the significant advantages to having our own datastructures
rather than using the standard library is the ability to have a
different API that is less constrained by history, and can make better
choices than standard containers like deleting operators that would
otherwise require deep coppies.  Though certainly they don't always live
up to that like the oversight here of not defining the copy / assignment
operators at all.  Perhaps there's an argument to be made for the
standard containers doing deep coppies that it makes the language easier
to use, but its not all that much easier than .copy(), if that's your
priority c++ probably isn't the right tool for the job, and I doubt it
makes sense for gcc in particular.

> > As said, can you show the usage that's impossible to do with
> > the current vec<>/auto_vec<>?
> 
> The test case in PR 90904 shows a trivial example.  More generally,
> using an auto_vec that depends on it being assignable (e.g., storing
> an auto_vec in another container like hash_map or auto_vec itself)
> is impossible.  Using a plain vec requires manual memory management
> and so is error-prone.

Certainly deleting the copy constructor and assignment operator means
that you can't use them,  but can you show real code where it is a
significant imposition to have to call .copy() rather than using them?
Certainly its a little longer, but deep copies are a bit of a
performance footgun, especially when you have vectors linear in the size
of the function all over, and your goal is to be no worse than
O(N log(N)), meaning you can copy the vector at most log(N) times at
worst.

I would think storing move only objects in auto_vec and hash_* should
work, and if it doesn't should be fixable without introducing overly
easy ways to make deep coppies.

> But more important, as a C++ code base, GCC should follow the best
> practices for the language.  Among the essential ones are using RAII
> to manage resources and the Rule of Three (or Five in C++ 11): a class
> that defines a dtor should also define a copy ctor and copy assignment
> (and move ctor and move assignment in C++).

When discussing the rule of 3/5 at least
https://en.cppreference.com/w/cpp/language/rule_of_three considers
deleting the member to be a form of definition, see the part about non
copiable members and deleting both copy constructor and assignment, in
this case to make the class move only.  Strictly speaking, I suppose its
true that an array of 10k items is copiable, but its also very likely
something to be avoided if at all possible, and doesn't need to be made
easy.

Trev

> 
> Martin