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

[Martin Sebor <msebor@gmail.com>]

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().

Martin