[PATCH 5/6] make get_domminated_by_region return a auto_vec

Martin Sebor msebor@gmail.com
Wed Jun 23 23:43:32 GMT 2021

On 6/22/21 11:23 PM, Trevor Saunders wrote:
> On Tue, Jun 22, 2021 at 02:01:24PM -0600, Martin Sebor wrote:
>> On 6/21/21 1:15 AM, Richard Biener wrote:
>>> On Fri, Jun 18, 2021 at 6:03 PM Martin Sebor <msebor@gmail.com> wrote:
>>>> On 6/18/21 4:38 AM, Richard Biener wrote:
>>>>> On Thu, Jun 17, 2021 at 4:43 PM Martin Sebor <msebor@gmail.com> wrote:
>>>>>> On 6/17/21 12:03 AM, Richard Biener wrote:
>>>>>>> On Wed, Jun 16, 2021 at 6:01 PM Martin Sebor <msebor@gmail.com> wrote:
>>>>>>>> On 6/16/21 6:46 AM, Richard Sandiford via Gcc-patches wrote:
>>>>>>>>> Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>>>>>>>>>> On Tue, Jun 15, 2021 at 8:02 AM Trevor Saunders <tbsaunde@tbsaunde.org> wrote:
>>>>>>>>>>> This makes it clear the caller owns the vector, and ensures it is cleaned up.
>>>>>>>>>>> Signed-off-by: Trevor Saunders <tbsaunde@tbsaunde.org>
>>>>>>>>>>> bootstrapped and regtested on x86_64-linux-gnu, ok?
>>>>>>>>>> OK.
>>>>>>>>>> Btw, are "standard API" returns places we can use 'auto'?  That would avoid
>>>>>>>>>> excessive indent for
>>>>>>>>>> -  dom_bbs = get_dominated_by_region (CDI_DOMINATORS,
>>>>>>>>>> -                                    bbs.address (),
>>>>>>>>>> -                                    bbs.length ());
>>>>>>>>>> +  auto_vec<basic_block> dom_bbs = get_dominated_by_region (CDI_DOMINATORS,
>>>>>>>>>> +                                                          bbs.address (),
>>>>>>>>>> +                                                          bbs.length ());
>>>>>>>>>> and just uses
>>>>>>>>>>        auto dom_bbs = get_dominated_by_region (...
>>>>>>>>>> Not asking you to do this, just a question for the audience.
>>>>>>>>> Personally I think this would be surprising for something that doesn't
>>>>>>>>> have copy semantics.  (Not that I'm trying to reopen that debate here :-)
>>>>>>>>> FWIW, I agree not having copy semantics is probably the most practical
>>>>>>>>> way forward for now.)
>>>>>>>> But you did open the door for me to reiterate my strong disagreement
>>>>>>>> with that.  The best C++ practice going back to the early 1990's is
>>>>>>>> to make types safely copyable and assignable.  It is the default for
>>>>>>>> all types, in both C++ and C, and so natural and expected.
>>>>>>>> Preventing copying is appropriate in special and rare circumstances
>>>>>>>> (e.g, a mutex may not be copyable, or a file or iostream object may
>>>>>>>> not be because they represent a unique physical resource.)
>>>>>>>> In the absence of such special circumstances preventing copying is
>>>>>>>> unexpected, and in the case of an essential building block such as
>>>>>>>> a container, makes the type difficult to use.
>>>>>>>> The only argument for disabling copying that has been given is
>>>>>>>> that it could be surprising(*).  But because all types are copyable
>>>>>>>> by default the "surprise" is usually when one can't be.
>>>>>>>> I think Richi's "surprising" has to do with the fact that it lets
>>>>>>>> one inadvertently copy a large amount of data, thus leading to
>>>>>>>> an inefficiency.  But by analogy, there are infinitely many ways
>>>>>>>> to end up with inefficient code (e.g., deep recursion, or heap
>>>>>>>> allocation in a loop), and they are not a reason to ban the coding
>>>>>>>> constructs that might lead to it.
>>>>>>>> IIUC, Jason's comment about surprising effects was about implicit
>>>>>>>> conversion from auto_vec to vec.  I share that concern, and agree
>>>>>>>> that it should be addressed by preventing the conversion (as Jason
>>>>>>>> suggested).
>>>>>>> But fact is that how vec<> and auto_vec<> are used today in GCC
>>>>>>> do not favor that.  In fact your proposed vec<> would be quite radically
>>>>>>> different (and IMHO vec<> and auto_vec<> should be unified then to
>>>>>>> form your proposed new container).  auto_vec<> at the moment simply
>>>>>>> maintains ownership like a smart pointer - which is _also_ not copyable.
>>>>>> Yes, as we discussed in the review below, vec is not a good model
>>>>>> because (as you note again above) it's constrained by its legacy
>>>>>> uses.  The best I think we can do for it is to make it safer to
>>>>>> use.
>>>>>> https://gcc.gnu.org/pipermail/gcc-patches/2021-June/571622.html
>>>>> Which is what Trevors patches do by simply disallowing things
>>>>> that do not work at the moment.
>>>>>> (Smart pointers don't rule out copying.  A std::unique_ptr does
>>>>>> and std::shared_ptr doesn't.  But vec and especially auto_vec
>>>>>> are designed to be containers, not "unique pointers" so any
>>>>>> relationship there is purely superficial and a distraction.)
>>>>>> That auto_vec and vec share a name and an is-a relationship is
>>>>>> incidental, an implementation detail leaked into the API.  A better
>>>>>> name than vector is hard to come up with, but the public inheritance
>>>>>> is a design flaw, a bug waiting to be introduced due to the conversion
>>>>>> and the assumptions the base vec makes about POD-ness and shallow
>>>>>> copying.  Hindsight is always 20/20 but past mistakes should not
>>>>>> dictate the design of a general purpose vector-like container in
>>>>>> GCC.
>>>>> That auto_vec<> "decays" to vec<> was on purpose design.
>>>>> By-value passing of vec<> is also on purpose to avoid an extra
>>>>> pointer indirection on each access.
>>>> I think you may have misunderstood what I mean by is-a relationship.
>>>> It's fine to convert an auto_vec to another interface.  The danger
>>>> is in allowing that to happen implicitly because that tends to let
>>>> it happen even when it's not intended.  The usual way to avoid
>>>> that risk is to provide a conversion function, like
>>>> auto_vec::to_vec().  This is also why standard classes like
>>>> std::vector or std::string don't allow such implicit conversions
>>>> and instead provide member functions (see for example Stroustrup:
>>>> The C++ Programming Language).  So a safer auto_vec class would
>>>> not be publicly derived from vec but instead use the has-a design
>>>> (there are also ways to keep the derivation by deriving both from
>>>> from a limited, safe, interface, that each would extend as
>>>> appropriate).
>>>> To the point of by passing vec by value while allowing functions
>>>> to modify the argument: C and C++ have by-value semantics.  Every
>>>> C and C++ programmer knows and expect that.  Designing interfaces
>>>> that break this assumption is perverse, a sure recipe for bugs.
>>>> If you're concerned about intuitive semantics and surprises you
>>>> should want to avoid that.
>>>>>> I fully support fixing or at least mitigating the problems with
>>>>>> the vec base class (unsafe copying, pass-by-value etc.).  As I
>>>>>> mentioned, I already started working on this cleanup.  I also
>>>>>> have no objection to introducing a non-copyable form of a vector
>>>>>> template (I offered one in my patch), or even to making auto_vec
>>>>>> non-copyable provided a copyable and assignable one is introduced
>>>>>> at the same time, under some other name.
>>>>> Why at the same time?  I'm still not convinced we need another
>>>>> vector type here.  Yes, auto_vec<auto_vec<..> > would be convenient,
>>>>> but then auto_vec<> doesn't bother to call the DTOR on its elements
>>>>> either (it's actually vec<> again here).  So auto_vec<> is _not_
>>>>> a fancier C++ vec<>, it's still just vec<> but with RAII for the container
>>>>> itself.
>>>> I don't follow what you're saying.  Either you agree that making
>>>> auto_vec suitable as its own element would be useful.  If you do,
>>>> it needs to be safely copyable and assignable.
>>>> The basic design principle of modern C++ containers is they store
>>>> their elements by value and make no further assumptions.  This means
>>>> that an int element is treated the same as int* element as a vec<int>
>>>> element: they're copied (or moved) by their ctors on insertion,
>>>> assigned when being replaced, and destroyed on removal.  Containers
>>>> themselves don't, as a rule, manage the resources owned by
>>>> the elements (like auto_delete_vec does).  The elements are
>>>> responsible for doing that, which is why they need to be safely
>>>> copyable and assignable.  vec meets these requirements because
>>>> it doesn't manage a resource (it's not a container).  Its memory
>>>> needs to be managed some other way.  auto_vec doesn't.  It is
>>>> designed to be a container but it's broken.  It won't become one
>>>> by deleting its copy ctor and assignment.
>>>>>> Having said that, and although I don't mind the cleanup being taken
>>>>>> off my plate, I would have expected the courtesy of at least a heads
>>>>>> up first.  I do find it disrespectful for someone else involved in
>>>>>> the review of my work to at the same time submit a patch of their
>>>>>> own that goes in the opposite direction, and for you to unilaterally
>>>>>> approve it while the other review hasn't concluded yet.
>>>>> Because the changes do not change anything as far as I understand.
>>>>> They make more use of auto_vec<> ownership similar to when
>>>>> I added the move ctor and adjusted a single loop API.  At the same
>>>>> time it completes the move stuff and plugs some holes.
>>>> The vast majority of Trevor's changes are improvements and I apprciate
>>>> them.  But the change to auto_vec goes against best C++ practices and
>>>> in the opposite direction of what I have been arguing for and what
>>>> I submitted a patch for in April.  The patch is still under discussion
>>>> that both you and Trevor, as well as Jason, have been participating in.
>>>> We have not concluded that discussion and it's in bad form to simply
>>>> disregard that and proceed in a different direction.  My understanding
>>>> from it so far is that
>>>> a) you're not opposed to adding the copy ctor:
>>>>       https://gcc.gnu.org/pipermail/gcc-patches/2021-April/568827.html
>>>> b) Jason advises to prevent implicit by-value conversion from auto_vec
>>>>       to vec
>>>>       https://gcc.gnu.org/pipermail/gcc-patches/2021-June/571628.html
>>>> c) I said I was working on it (and more, some of it likely now
>>>>       obviated by Trevor's changes):
>>>>       https://gcc.gnu.org/pipermail/gcc-patches/2021-June/571622.html
>>>> So would I ask you both to respect that and refrain from approving
>>>> and committing this change (i.e., leave auto_vec as is for now)
>>>> until I've had time to finish my work.
>>>> But checking the latest sources I see Trevor already committed
>>>> the change despite this.  That's in very poor form, and quite
>>>> disrespectful of both of you.
>>> The change was needed to make the useful portions work as far
>>> as I understood Trevor.  There's also nothing that prevents you
>>> from resolving the conflicts and continue with your improvements.
>> The change disables things that were previously possible (but
>> undefined).  It isn't relied on by anything.  The change is
>> also incomplete: it disables copying and assignment for
>> the auto_vec<T, 0> specialization but not for the primary
>> template.   Neither is safe to copy or assign.
> I suppose its technically true that it disables things that were
> possible, but for those things to work you'd have to jump through some
> hoops to prevent it causing a double free, so in practice I don't think
> you can really say it disables something, it just makes explicit what
> was already the case.  Since the move constructors would have prevented
> the compiler from providing a default definition of the copy
> constructor, there deletion is essentially just documentation.  Later
> patches in the series did depend on the move constructors to work
> properly.  I would agree that the template with inline storage could use
> with adjustment in this area, but I think its fine to handle it
> separately per Richi's request, and further I suspect its much less
> useful and likely to cause trouble, the main use case would be for
> function location things, and returning such an object  or passing it by
> value seems odd (not to say it is good the way it is, but that its of
> less practical importance).
>> I could still submit a patch to fix that and make all auto_vec
>> specializations safely copyable but why should I bother?  You
>> have made it abundantly clear that you don't support it and
>> several others have taken your position despite all the problems
>> I have repeatedly pointed out.
> Well, personally if you wanted to define something like
> template<typename T> T gcc::copy (const T &) = delete;  WIth
> specializations for things that could be coppied including anything
> that's POD, I think that might be reasonable, and much clearer than "="
> that calls malloc, if your issue is that .copy () on vec isn't the same
> as for other objects.  Personally I think its mostly YAGNI and there's
> few enough places coppying things for it to be a real problem, but at
> the same time I think there's no particular need .copy () is a member,
> just that its use is more obvious to the author and reader than
> "=".

auto_vec::copy() returns a vec so it's not the same thing as a copy
ctor/assignment.  vec happens to be convertible to auto_vec so it's
possible to copy auto_vec A to auto_vec B by B = A.copy() but that's
a convoluted way of copying or assigning one object to another.

Without looking up the types of the operands, the meaning of
B = A.copy() is certainly less obvious than that of B = A.  Same
for copy (B, A).  To those of us accustomed to basic C++ idioms,
it's unusual to have to call a named function to do what's
canonically accomplished by a native operator.  It raises
the question of what else the call might do that the expected
alternative wouldn't.  To Richi's concern about the inefficiency
of deep copies, forcing users to call A.copy() to make one runs
the risk that they will do it even when it's not necessary and
when the move ctor or assignment would be suitable.  It's much
better (and in line with the intended use of the feature) to
use the familiar initialization or assignment syntax and trust
the compiler make the right call whether to do a deep copy or
to move.

More generally, obviating the distinction between native types
and user-defined types is one of the most powerful features of
C++.  It makes it possible to write generic code: not just
templates but any piece of code that's agnostic of the types
it works with.  There are many benefits: code that uses common
idioms and operators is easier to understand, swapping one data
type for another becomes trivial, and similarities between
different pieces of code are more apparent, opening up
opportunities for reuse and refactoring.  Designing classes
to be gratuitously different means missing out on some of
the most valuable benefits of using the language.  But I'm
just paraphrasing arguments that have been made much more
convincingly by authors of the many excellent C++ books out
there (some of my favorites are Herb Sutter's Exceptional C++


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