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

[Martin Sebor <msebor@gmail.com>]

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.

Martin

> 
> Jason
>