> 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. > 2. {Retain,Release}Values will be created outside. We are talking about some > ways of fixing this from a code-size perspective by using a value witness, > but in the present this may cause additional code-size increase.
This is exactly the case that would be improved, since retain/release_value on such an enum would boil down to a single swift_retain/release call if the runtime functions ignored the tagged small case values. -Joe > > >> >> -Joe >> >>>> We could also do interesting things with enums; if one payload type is >>>> a class reference and the rest are trivial, we could lay the enum out >>>> in such a way that we can use swift_retain/release on it by setting >>>> the high bit when tagging the trivial representations, saving us the >>>> need to emit a switch. We wouldn't actually dereference the pointer >>>> representation without checking it first. >>>> >>>> I know we've discussed taking the nil check out of >>>> swift_retain/release, and possibly having separate variants that do >>>> include the null check for when we know we're working with >>>> Optionals. How much of difference would that really make, though? I'd >>>> expect it to be a fairly easily predictable branch, since most objects >>>> are likely to be nonnull in practice. >>>> >>>> -Joe >>>> _______________________________________________ >>>> swift-dev mailing list >>>> swift-dev@swift.org >>>> https://lists.swift.org/mailman/listinfo/swift-dev >>> >>> -- >>> -Dave >>> >>> _______________________________________________ >>> swift-dev mailing list >>> swift-dev@swift.org >>> https://lists.swift.org/mailman/listinfo/swift-dev >> >> _______________________________________________ >> swift-dev mailing list >> swift-dev@swift.org >> https://lists.swift.org/mailman/listinfo/swift-dev > _______________________________________________ swift-dev mailing list swift-dev@swift.org https://lists.swift.org/mailman/listinfo/swift-dev