[swift-evolution] [swift-evolution-announce] [Review] SE-0184: Unsafe[Mutable][Raw][Buffer]Pointer: add missing methods, adjust existing labels for clarity, and remove deallocation size

[Taylor Swift]

On Thu, Sep 28, 2017 at 7:59 PM, Andrew Trick <atrick at apple.com> wrote:

>
> On Sep 6, 2017, at 10:15 PM, Taylor Swift <kelvin13ma at gmail.com> wrote:
>
> okay so I think so far what’s been agreed on is
>
> *1*. rename “Bytes” to “Memory” in the raw pointer API. Note: this brings
> the `copyBytes<C>(from:)` collection method into the scope of this
> proposal
>
> *2*. change raw offsets to be in terms of bytes, not typed strides. This
> argument will be called `atByteOffset:` and will only appear in
> UnsafeMutableRawBufferPointer. “at:” arguments are no longer needed in
> UnsafeMutableRawPointer, since we can just use pointer arithmetic now.
>
>
> *3*. move UnsafeMutableBufferPointer’s `at:` arguments to the front of
> the parameter list. mostly cause any pointer arithmetic happens in the
> front so structurally we want to mirror that.
>
> *4*. add dual (to:) single element initializers and assigners to
> UnsafeMutablePointer, but not UnsafeMutableRawPointer because it’s
> apparently not useful there. `UnsafeMutableRawPointer.initi
> alizeMemory<T>(as:repeating:count:)` still loses its default count to
> prevent confusion with its buffer variant.
>
> *5*. memory deallocation on buffer pointers is clearly documented to only
> be defined behavior when the buffer matches a whole heap block.
>
>
> Kelvin,
>
> Attempting to limit the scope of this proposal backfired. I was hoping to
> avoid discussing changes to the slice API, instead providing basic
> functionality within the buffer API itself. However, Dave Abrahams has
> argued that once the slice API is extended, the positional arguments are
> extraneous and less clear.
>
> Instead of
>
>   buf.intialize(at: i, from: source)
>
> We want to force a more obvious idiom:
>
>   buf[i..<n].intialize(from: source)
>
> I think this is a reasonable argument and convinced myself that it's
> possible to extend the slice API. I'm also convinced now that we don't need
> overloads to handle an UnsafeBufferPointer source, instead we can provide a
> single generic entry point on both UnsafeMutableBufferPointer and its
> slice, optimized within the implementation:
>
>  `initialize<S : Sequence>(from: S) -> (S.Iterator, Index)
>
> We can always drop down to the UnsafePointer API to get back to a direct
> unsafe implementation as a temporary workaround for performance issues.
>
> Let's set aside for now whether we support full or partial
> initialization/assignment, how to handle moveInitialize, and whether we
> need to return the Iterator/Index. This is going to require another
> iteration on swift-evolution, which *we should discuss in a separate
> thread*.
>
> At this point, I suggest removing the controversial aspects of SE-0184 so
> that we can put the other changes behind us and focus future discussion
> around a smaller follow-up proposal.
>
> Here I've summarized the changes that I think could be accepted as-is:
> https://gist.github.com/atrick/c1ed7afb598e5cc943bdac7683914e3e
>
> If you amend SE-0184 accordingly and file a new PR, I think it can be
> quickly approved.
>
> -Andy
>
>
Part one of SE-0184 is here as SE-0184 A
<https://github.com/kelvin13/swift-evolution/blob/improved-pointers/proposals/0184a-unsafe-pointers-part-1.md>
I’ll implement it tomorrow and then make the PR
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