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 <> 
> wrote:
>> On Mar 1, 2016, at 1:33 PM, Joe Groff via swift-dev < 
>> <>> 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.
> -Joe
> _______________________________________________
> swift-dev mailing list

swift-dev mailing list

Reply via email to