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 >> <[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.
I don't think you can get those guarantees without static protection against escaping borrowed references, though, can you? -- -Dave _______________________________________________ swift-evolution mailing list [email protected] https://lists.swift.org/mailman/listinfo/swift-evolution
