[swift-evolution] Contiguous Memory and the Effect of Borrowing on Safety

Dave Abrahams dabrahams at apple.com
Mon Nov 7 17:55:35 CST 2016

on Mon Nov 07 2016, John McCall <swift-evolution at swift.org> wrote:

>> On Nov 6, 2016, at 1:20 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>> Given that we're headed for ABI (and thus stdlib API) stability, I've
>> been giving lots of thought to the bottom layer of our collection
>> abstraction and how it may limit our potential for efficiency.  In
>> particular, I want to keep the door open for optimizations that work on
>> contiguous memory regions.  Every cache-friendly data structure, even if
>> it is not an array, contains contiguous memory regions over which
>> operations can often be vectorized, that should define boundaries for
>> parallelism, etc.  Throughout Cocoa you can find patterns designed to
>> exploit this fact when possible (NSFastEnumeration).  Posix I/O bottoms
>> out in readv/writev, and MPI datatypes essentially boil down to
>> identifying the contiguous parts of data structures.  My point is that
>> this is an important class of optimization, with numerous real-world
>> examples.
>> If you think about what it means to build APIs for contiguous memory
>> into abstractions like Sequence or Collection, at least without
>> penalizing the lowest-level code, it means exposing UnsafeBufferPointers
>> as a first-class part of the protocols, which is really
>> unappealing... unless you consider that *borrowed* UnsafeBufferPointers
>> can be made safe.  
>> [Well, it's slightly more complicated than that because
>> UnsafeBufferPointer is designed to bypass bounds checking in release
>> builds, and to ensure safety you'd need a BoundsCheckedBuffer—or
>> something—that checks bounds unconditionally... but] the point remains
>> that
>>  A thing that is unsafe when it's arbitrarily copied can become safe if
>>  you ensure that it's only borrowed (in accordance with well-understood
>>  lifetime rules).
> UnsafeBufferPointer today is a copyable type.  Having a borrowed value
> doesn't prevent you from making your own copy, which could then escape
> the scope that was guaranteeing safety.
> This is fixable, of course, but it's a more significant change to the
> type and how it would be used.

It sounds like you're saying that, to get static safety benefits from
ownership, we'll need a whole parallel universe of safe move-only
types. Seems a cryin' shame.


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