Correct me if I’m wrong, but aren’t all kernel addresses negative on x64 and
AArch64? Would this then mean any attempt to use Swift in kernel-space requires
a distinct ABI?
> On Oct 13, 2016, at 12:04 PM, Joe Groff via swift-dev <email@example.com>
>> On Mar 1, 2016, at 1:33 PM, Joe Groff via swift-dev <firstname.lastname@example.org
>> <mailto:email@example.com>> 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
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