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.
On Mar 1, 2016, at 1:33 PM, Joe Groff via swift-dev <firstname.lastname@example.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.