> On Oct 17, 2016, at 12:40 PM, John McCall <rjmcc...@apple.com> wrote: > >> On Oct 17, 2016, at 10:19 AM, Joe Groff via swift-dev <swift-dev@swift.org> >> wrote: >>> On Oct 17, 2016, at 9:57 AM, Michael Gottesman <mgottes...@apple.com> wrote: >>> >>>> >>>> On Oct 17, 2016, at 9:42 AM, Joe Groff via swift-dev <swift-dev@swift.org> >>>> wrote: >>>> >>>> >>>>> On Oct 16, 2016, at 1:10 PM, Dave Abrahams via swift-dev >>>>> <swift-dev@swift.org> wrote: >>>>> >>>>> >>>>> on Thu Oct 13 2016, Joe Groff <swift-dev-AT-swift.org> wrote: >>>>> >>>>>>> On Oct 13, 2016, at 1:18 PM, Greg Parker <gpar...@apple.com> wrote: >>>>>>> >>>>>>> >>>>>>>> On Oct 13, 2016, at 10:46 AM, John McCall via swift-dev >>>>>>>> <swift-dev@swift.org> wrote: >>>>>>>> >>>>>> >>>>>>>>> On Oct 13, 2016, at 9:04 AM, Joe Groff via swift-dev >>>>>>>>> <swift-dev@swift.org> wrote: >>>>>>>>> >>>>>>>>>> On Mar 1, 2016, at 1:33 PM, Joe Groff via swift-dev >>>>>>>>>> <swift-dev@swift.org> wrote: >>>>>>>>>> >>>>>>>>>> In swift_retain/release, we have an early-exit check to pass >>>>>>>>>> through a nil pointer. Since we're already burning branch, I'm >>>>>>>>>> thinking we could pass through not only zero but negative pointer >>>>>>>>>> values too on 64-bit systems, since negative pointers are never >>>>>>>>>> valid userspace pointers on our 64-bit targets. This would give >>>>>>>>>> us room for tagged-pointer-like optimizations, for instance to >>>>>>>>>> avoid allocations for tiny closure contexts. >>>>>>>>> >>>>>>>>> I'd like to resurrect this thread as we look to locking down the >>>>>>>>> ABI. There were portability concerns about doing this unilaterally >>>>>>>>> for all 64-bit targets, but AFAICT it should be safe for x86-64 >>>>>>>>> and Apple AArch64 targets. The x86-64 ABI limits the userland >>>>>>>>> address space, per section 3.3.2: >>>>>>>>> >>>>>>>>> Although the AMD64 architecture uses 64-bit pointers, >>>>>>>>> implementations are only required to handle 48-bit >>>>>>>>> addresses. Therefore, conforming processes may only use addresses >>>>>>>>> from 0x00000000 00000000 to 0x00007fff ffffffff. >>>>>>>>> >>>>>>>>> Apple's ARM64 platforms always enable the top-byte-ignore >>>>>>>>> architectural feature, restricting the available address space to >>>>>>>>> the low 56 bits of the full 64-bit address space in >>>>>>>>> practice. Therefore, "negative" values should never be valid >>>>>>>>> user-space references to Swift-refcountable objects. Taking >>>>>>>>> advantage of this fact would enable us to optimize small closure >>>>>>>>> contexts, Error objects, and, if we move to a reference-counted >>>>>>>>> COW model for existentials, small `Any` values, which need to be >>>>>>>>> refcountable for ABI reasons but don't semantically promise a >>>>>>>>> unique identity like class instances do. >>>>>>>> >>>>>>>> This makes sense to me. if (x <= 0) return; should be just as cheap >>>>>>>> as is (x == 0) return; >>>>>>> >>>>>>> Conversely, I wanted to try to remove such nil checks. Currently >>>>>>> they look haphazard: some functions have them and some do not. >>>>>>> >>>>>>> Allowing ABI space for tagged pointer objects is a much bigger >>>>>>> problem than the check in swift_retain/release. For example, all >>>>>>> vtable and witness table dispatch sites to AnyObject or any other >>>>>>> type that might someday have a tagged pointer subclass would need to >>>>>>> compile in a fallback path now. You can't dereference a tagged >>>>>>> pointer to get its class pointer. >>>>>> >>>>>> True. I don't think we'd want to use this optimization for class >>>>>> types; I was specifically thinking of other things for which we use >>>>>> nullable refcounted representations, particularly closure >>>>>> contexts. The ABI for function types requires the context to be >>>>>> refcountable by swift_retain/release, but it doesn't necessarily have >>>>>> to be a valid pointer, if the closure formation site and invocation >>>>>> function agree on a tagged-pointer representation. >>>>> >>>>> Well, but we'd like to take advantage of the same kind of optimization >>>>> for the small string optimization. It doesn't seem like this should be >>>>> handled differently just because the string buffer is a class instance >>>>> and not a closure context. >>>> >>>> String is a struct, and small strings don't have to be modeled as class >>>> instances. An enum { case Big(StringStorage), Small(Int63) } or similar >>>> layout should be able to take advantage of swift_retain/release ignoring >>>> negative values too. >>> >>> I need to catch up on this thread, but there is an important thing to >>> remember. If you use an enum like this there are a few potential issues: >>> >>> 1. In the implementation, you will /not/ want to use the enum internally. >>> This would prevent the optimizer from eliminating all of the Small Case >>> reference counting operations. This means you would rewrap the internal >>> value when you return one and when you enter into an internal >>> implementation code path try to immediately switch to a specialized small >>> case path if you can. >> >> This poses an interesting question for the semantic ARC model with enums. It >> seems to me that, if switching or projecting the payload of an enum was a >> consuming operation, that we could avoid this optimization pitfall. >> Switching the enum { case Big(Class), Small(Trivial) } or similar case would >> semantically eliminate the nontrivial enum value and leave only the trivial >> payload behind. > > We can't make it *exclusively* a consuming operation; it has to be possible > to switch on a borrowed value.
Yes. Perhaps the right way to think about it is in the context of considering putting conventions on SILArguments/Terminators. Then you have a natural way to express this and could optimize (potentially) in between such forms. > > John.
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