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

[Stephen Canon]

> On Nov 10, 2016, at 1:30 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
> on Thu Nov 10 2016, Joe Groff <jgroff-AT-apple.com> wrote:
> 
>>> On Nov 8, 2016, at 9:29 AM, John McCall <rjmccall at apple.com> wrote:
>>> 
>>>> On Nov 8, 2016, at 7:44 AM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
>>>>> On Nov 7, 2016, at 3:55 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>>>>> 
>> 
>>>>> 
>>>>> 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.
>>>> 
>>>> We've discussed the possibility of types being able to control
>>>> their "borrowed" representation. Even if this isn't something we
>>>> generalize, arrays and contiguous buffers might be important enough
>>>> to the language that your safe BufferPointer could be called
>>>> 'borrowed ArraySlice<T>', with the owner backreference optimized
>>>> out of the borrowed representation. Perhaps Array's own borrowed
>>>> representation would benefit from acting like a slice rather than a
>>>> whole-buffer borrow too.
>>> 
>>> The disadvantage of doing this is that it much more heavily
>>> penalizes the case where we actually do a copy from a borrowed
>>> reference — it becomes an actual array copy, not just a reference
>>> bump.
>> 
>> Fair point, though the ArraySlice/Array dichotomy strikes me as
>> already kind of encouraging this—you might pass ArraySlices down into
>> your algorithm, but we encourage people to use Array at storage and
>> API boundaries, forcing copies.
>> 
>> From a philosophical perspective of making systems Swift feel like
>> "the same language" as Swift today, it feels better to me to try to
>> express this as making our high-level safe abstractions efficient
>> rather than making our low-level unsafe abstractions safe. 
> 
> +1, or maybe 10
> 
> What worries me is that if systems programmers are trying to get static
> guarantees that there's no ARC traffic, they won't be willing to handle
> a copyable thing that carries ownership.

FWIW, we (frequently) only need a static guarantee of no ARC traffic *within a critical section*. If we can guarantee that whatever ARC operations need to be done happen in a precisely-controlled manner at a known interface boundary, that’s often good enough.

– Steve