> On Oct 17, 2016, at 12:01 PM, Michael Gottesman <mgottes...@apple.com> wrote: > > >> On Oct 17, 2016, at 11:53 AM, Michael Gottesman via swift-dev >> <swift-dev@swift.org <mailto:swift-dev@swift.org>> wrote: >> >> >>> On Oct 17, 2016, at 10:00 AM, Joe Groff <jgr...@apple.com >>> <mailto:jgr...@apple.com>> wrote: >>> >>> >>>> On Oct 17, 2016, at 9:57 AM, Michael Gottesman <mgottes...@apple.com >>>> <mailto:mgottes...@apple.com>> wrote: >>>> >>>> >>>>> On Oct 17, 2016, at 9:42 AM, Joe Groff via swift-dev <swift-dev@swift.org >>>>> <mailto:swift-dev@swift.org>> wrote: >>>>> >>>>> >>>>>> On Oct 16, 2016, at 1:10 PM, Dave Abrahams via swift-dev >>>>>> <swift-dev@swift.org <mailto:swift-dev@swift.org>> wrote: >>>>>> >>>>>> >>>>>> on Thu Oct 13 2016, Joe Groff <swift-dev-AT-swift.org >>>>>> <http://swift-dev-at-swift.org/>> wrote: >>>>>> >>>>>>>> On Oct 13, 2016, at 1:18 PM, Greg Parker <gpar...@apple.com >>>>>>>> <mailto:gpar...@apple.com>> wrote: >>>>>>>> >>>>>>>> >>>>>>>>> On Oct 13, 2016, at 10:46 AM, John McCall via swift-dev >>>>>>>>> <swift-dev@swift.org <mailto:swift-dev@swift.org>> wrote: >>>>>>>>> >>>>>>> >>>>>>>>>> On Oct 13, 2016, at 9:04 AM, Joe Groff via swift-dev >>>>>>>>>> <swift-dev@swift.org <mailto:swift-dev@swift.org>> wrote: >>>>>>>>>> >>>>>>>>>>> On Mar 1, 2016, at 1:33 PM, Joe Groff via swift-dev >>>>>>>>>>> <swift-dev@swift.org <mailto: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. >> >> I am saying something stronger. What I am saying is that, you could have 0 >> retain/release operations on the SmallString path. > > Let me elaborate a little bit, and then I am going to drop my point here > since as Joe pointed out to me offlist, this is orthogonal to the ABI > discussion. > > What I am trying to say is that the optimizer will eliminate all retain, > release operations on trivial values. Any code path which uses the top level > enum can not take advantage of this property since the top level enum /could/ > have the BigString contained in it. So what you want to do to get rid of the > most retain/release operations is to move the enum switch to the entrances of > the API so that one has the largest region of code where the optimizer can > clearly see that it has a small string. > > Now we /could/ specialize on enum cases. I will file a radar for this.
rdar://28805035 > > Michael > >> >>> >>> -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 <mailto:swift-dev@swift.org> >>>>>>> https://lists.swift.org/mailman/listinfo/swift-dev >>>>>>> <https://lists.swift.org/mailman/listinfo/swift-dev> >>>>>> >>>>>> -- >>>>>> -Dave >>>>>> >>>>>> _______________________________________________ >>>>>> swift-dev mailing list >>>>>> swift-dev@swift.org <mailto:swift-dev@swift.org> >>>>>> https://lists.swift.org/mailman/listinfo/swift-dev >>>>>> <https://lists.swift.org/mailman/listinfo/swift-dev> >>>>> >>>>> _______________________________________________ >>>>> swift-dev mailing list >>>>> swift-dev@swift.org <mailto:swift-dev@swift.org> >>>>> https://lists.swift.org/mailman/listinfo/swift-dev >>>>> <https://lists.swift.org/mailman/listinfo/swift-dev> >>>> >>> >> >> _______________________________________________ >> swift-dev mailing list >> swift-dev@swift.org <mailto:swift-dev@swift.org> >> https://lists.swift.org/mailman/listinfo/swift-dev >
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