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

[Dave Abrahams]

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?

-- 
-Dave