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

Jason Merrill jason@redhat.com
Tue Jun 29 14:43:22 GMT 2021

On 6/28/21 2:07 PM, Martin Sebor 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.

The c_parser_declaration_or_fndef case is rather ugly: the vec is passed 
by value, but then the modifications in c_finish_omp_declare_simd modify 
the original vec.

We could keep the same semantic problem and make it more blatant by 
changing to const vec& and doing a const_cast in 
c_finish_omp_declare_simd before modifying the vec.

Do the other two have the same problem?


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