> 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 >> <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; John.
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