> On Nov 10, 2016, at 1:30 PM, Dave Abrahams via swift-evolution > <[email protected]> wrote: > > > on Thu Nov 10 2016, Joe Groff <jgroff-AT-apple.com> wrote: > >>> On Nov 8, 2016, at 9:29 AM, John McCall <[email protected]> wrote: >>> >>>> On Nov 8, 2016, at 7:44 AM, Joe Groff via swift-evolution >>>> <[email protected]> wrote: >>>>> On Nov 7, 2016, at 3:55 PM, Dave Abrahams via swift-evolution >>>>> <[email protected]> wrote: >>>>> >> >>>>> >>>>> on Mon Nov 07 2016, John McCall <[email protected]> wrote: >>>>> >>>>>>> On Nov 6, 2016, at 1:20 PM, Dave Abrahams via swift-evolution >>>>>>> <[email protected]> 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 _______________________________________________ swift-evolution mailing list [email protected] https://lists.swift.org/mailman/listinfo/swift-evolution
