> 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. -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