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

[Joe Groff]

> On Nov 10, 2016, at 1:02 PM, Dave Abrahams <dabrahams at apple.com> wrote:
> 
> 
> on Thu Nov 10 2016, Stephen Canon <scanon-AT-apple.com> wrote:
> 
>>> 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.
> 
> I don't think you can get those guarantees without static protection
> against escaping borrowed references, though, can you?

You shouldn't be able to do that without copying it, and copying a borrow seems like it ought to at least be explicit.

-Joe