[swift-evolution] Add wrappers for allocWithTailElems_{n} routines

David Sweeris davesweeris at mac.com
Tue Dec 13 15:59:05 CST 2016

> On Dec 13, 2016, at 1:37 PM, Karl via swift-evolution <swift-evolution at swift.org> wrote:
>> On 13 Dec 2016, at 20:33, Dave Abrahams via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> on Tue Dec 13 2016, Alexey Komnin <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> Hi everyone!
>>> I really don't know whether you have discussed it before. So, here is the deal.
>>> Currently, Builtin routines like allocWithTailElems_{n} not available
>>> outside of stdlib. On the other hand, those routines are widely used
>>> in implementation of containers: Array, Set, Dictionary and others.
>>> On the other hand, some Builtin routines are wrapped by publicly
>>> available operators and functions like unsafeBitCast, (==).
>>> ManagedBuffer.create(minimumCapacity:) class method is just a wrapper
>>> around call to allocWithTailElems_1 builtin function. So, it is
>>> possible to implement some containers, like Deque or linked-list using
>>> ManagedBuffer, but it's not enough if developer is going to implement
>>> advanced data structures like OrderedSet, LRU-cache or something more
>>> complex.
>>> Instead, the developers are forced to use complex pointer arithmetic
>>> with UnsafeRawPtr. My point is: I think the language must provide all
>>> the available tools to build algorithms and data structures. First of
>>> all, it should provide the tools used for stdlib implementation.
>>> Could we make some ManagedBuffer-alike wrappers around
>>> allocWithTailElems_{n} routines?
>> Feel free to design and propose these interfaces (in phase 2 of course).
> Would the optimiser hints also possibly be up for making public (_onFastPath() and so)?
> You can use them anyway; I don’t know why they’re underscored…

I believe the underscored stuff is consider an implementation detail that should be private, but can’t be for some technical reason, and is subject to change without notice. I think.

Now that I’ve said that, someone’s going to go do an analysis of all the “_*” symbols and point out that 95% of them haven’t changed since the first Mac OS X Public Beta back in 2000. Then someone else will say that “subject to” doesn’t mean “will”.

- Dave Sweeris
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