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

[Dave Abrahams]

on Fri Nov 11 2016, Alexis <abeingessner-AT-apple.com> wrote:

>> On Nov 10, 2016, at 8:17 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> 
>> on Thu Nov 10 2016, Joe Groff <jgroff-AT-apple.com <http://jgroff-at-apple.com/>> wrote:
>> 
>
>>>> 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.
>> 
>> I don't think that's true for all types (e.g. Int).
>> 
>> But regardless, we've come full circle, because I'm talking
>> about needing to do something explicit to copy a a borrowed type when
>> copying it will be unsafe.
>
> I’m a bit confused by this. Presumably 99.999% of string slices would
> be an immutable view. Almost no unicode correct code has any business
> mutating the contents of a fixed-sized string buffer. (data point: the
> `&mut str` type exists in Rust, but literally every interface I’ve
> ever seen handles `&str`). If the views are immutable, they’re safe to
> copy as long as every copy “remembers” that it’s noescape or whatever
> you want to call it.

If copies can do that, they are the moral equivalent of borrows.  The
unsafe copies are the ones that can escape.

-- 
-Dave