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Subject: Re: New colony proposal - feedback requested
From: Matthew Bentley (mattreecebentley_at_[hidden])
Date: 2020-05-02 00:05:53


First of all, thanks Jens for the thorough overview - please consider any
points I don't comment on below as accepted criticism and as things I will
action upon:

On Fri, 1 May 2020 at 22:21, Jens Maurer <Jens.Maurer_at_[hidden]> wrote:

> - Please suggest stable labels and a location in the standard
> where the additional container should go.
>

When you say "stable labels", do you mean numbered headings as in "26.3.8.5
deque specialized algorithms".

>
> - The "Skipfield" template parameter is not specified.
> Which restrictions apply? (For example, can it be void,
> an array of unknown bound, an abstract base class, a function
> type, or a non-copyable class type?)
>

I don't think it's necessary to leave it anything other than
implementation-defined? The reference implementation must use unsigned
integral types, but I don't want to obviate future potential improvements
on implementations, that could, for example, use a floating-point type
(somehow).

  - Would it be useful to give the default template argument
> for Skipfield a user-visible name? We also use the public
> name std::allocator for the default argument of Allocator etc.
>

Since the type is meant to be implementation-defined, I'm unsure how to do
this.

> - "const size_type min/max_block_capacity"
> drop "const"; it's ignored in a non-defining function declaration anyway.
>
>
There's plenty of use of const in the existing standard, for containers,
not sure what is different about what I'm doing. For example:
"iterator insert(const_iterator position, size_type n, const T& x);"
under 26.3.8.1 Class template deque overview.

  - The synopsis should show "assign" member functions near "operator=".
>

There are no assign member functions. I was following the path of other
unordered containers by not including them. Is there an obvious reason for
including them? Implementation would be clear();reserve();insert().

  - " // Since this could be derived from --end() depending on
> implementation, and that could result in undefined behaviour in a colony
> with no insertions and unbounded iterators, whether this is noexcept is
> implementation-defined"
>
> "Since" is rationale that doesn't belong in normative text. Do you want
> to say
> noexcept( see below ) , or what?
>
> Why is "rbegin() const" not so marked? Same question for crbegin().
>

I assumed the reader would be able to extrapolate from one to the others.
Plus it's a large block of text. Maybe should go in a footnote and use a
reference marker.

> Why do we need this funny restriction anyway? Can the implementation
> specialize reverse_iterator<colony::iterator> to do the right thing and
> avoid the undefined behavior for rbegin() with no runtime cost and no
> insane code cost? If so, that should be required, not
> implementation-defined.
>

The only way to avoid that behaviour is to store a permanent reference to
the current back element in-code, which would need to be updated in many
places, and that's costly. I don't like reverse iterators for performance
reasons and I personally wouldn't ever sacrifice overall performance to
obtain an undefined-behaviour free function for rbegin. I want to give
implementors the same ability. Frankly, as an unordered-insert container,
the relevance of reverse_iterators is sparse at-best.

> - These block capacity functions use a "Skipfield" type (plus redundant
> const),
> but the default arguments in the constructor use size_type. That seems
> inconsistent.
>

Good point. This is relevant to the current implementation but possibly not
to others. Will fix.

>
> - In general, the standard doesn't do "get_" and "set_" prefixes for
> getting/setting values.
> Instead, we have
> name_for_value(x) to set a value
> name_for_value() to retrieve the value
>

I don't think that's clear, but okay.

> If you need to return several values, use a struct with nice member
> names.
>
>
... Why. How is creating an arbitrary struct type, having the user
instantiate one, and then have to get rid of it, any better than
passing-by-reference. It's a waste of time for both the implementor and the
user.

> - similar for get_index_from_iterator; Why can't we use
> std::distance(begin(), it)
> instead?
> (We have "Complexity: Same as distance specialized algorithm, see below.",
> so there
> seems to be no benefit.)
>

If you read the implementation you'll find there's a lot of code you can
get rid of when you have the assumption of counting from begin(), which
makes for a faster function. Whether an implementor decides to act on that
is up to them.

ctor p6:
>
> "If n is larger than min_block_capacity, the capacity of the first block
> created will be the smaller of n or max_block_capacity."
>
> If you want the min/max block capacity to have any normative impact,
> you need to establish an actual abstract model you can tie into.
> Otherwise, why does the capacity of some block matter at all?
>

Because the length of the block affects potential cache locality. The
abstract model is described in Design Decisions.

  - Why do we need MoveInsertable for "reserve" anyway? Aren't we just
> allocating
> a new memory block of the desired size, without any moves?
>

That's an open question that I want addressed, so thanks for bringing it up
- see 'questions for the committee' section.

"Complexity: Implementation-defined. One strong strategy involves using the
> standard library's sort function to sort an array of pointers to colony
> elements by the value of the elements they point to, and would have
> std::sort()'s complexity."
>
> This is, of course, idle chatter. If there is a reasonable implementation
> strategy that is O(n log n), we should simply require that complexity and
> be
> done with it.
>

That also is an open question. See Questions for Committee section.

> - splice p17:
> "Better performance may be gained in some cases by allowing the source
> blocks to go to the front rather than the back, depending on how full the
> final block in x's iterative sequence is. This is because unused elements
> that are not at the back of colony's iterative sequence will need to be
> marked as skipped, and skipping over large numbers of elements will incur a
> small performance disadvantage during iteration compared to skipping over a
> small number of elements, due to memory locality."
>
> You must be kidding me, right? Burn that with fire and move the ashes
> to the prose section of the paper.
>

Okay calm down.

> - approximate_memory_use p19
> "Complexity: Implementation-defined. Should typically be constant."
>
> If there is a reasonable implementation strategy to make this constant,
> then make it so
> in the spec.
>

Constant relies on the same code (maintaining sequential memory block
numbers or alternatively having vector of memory blocks rather than
linked-list) that allows operators >/</>=/<= to exist. If the committee
decides that they don't want those, you would have to maintain those just
to allow for constant implementation of this function, which I'm not in
favour of. However I see benefit to retaining >/</>=/<= , so adopting a
wait-and-see attitude at this point.

>
> - Why is reinitialize needed? Can't you just do my_colony =
> colony(min_block_size, max_block_size) ?
> Oh, we don't have that constructor, but it seems we should definitely have
> it.
>

That constructor is not possible as it creates overload conflict with the
fill constructor when using integers.

> - get_iterator_from_pointer p1: "pointer is not invalid" : The name of
> the parameter is element_pointer
> (which is a bit verbose, though, just call it "p"). I think we need a
> precondition that the pointer
> points to an element of *this.
>
> - In general, I think we need to be more clear which operations
> invalidate iterators,
> and we should do so in an overall introductory paragraph of the normative
> wording.
>

This is in the first appendix, but I'm happy to move it upwards if you can
name the section you think it should be in.

> and of course the lengthy explanation of the complexity doesn't belong
> here. Just say O(n)
>

Where? You mean in the technical spec? Or in the paper in general?
Thanks
Matt



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