> On Oct 13, 2016, at 5:14 PM, Joe Groff <jgr...@apple.com> wrote:
> 
> 
>> On Oct 13, 2016, at 2:04 PM, Alexis <abeingess...@apple.com> wrote:
>> 
>> 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?
> 
> That's correct, but we'd likely already have to have a separate "kernel" ABI 
> due to our assumptions about spare bits in pointers. It also seems unlikely 
> to me that kernel developers would want to use our refcounting scheme as is.

True, but the types being discussed here seem to mostly be language features 
that are implicitly falling back to reference counting when escape analysis 
fails. And specifically the tagging you’re proposing is for the cases where 
some special analysis passes and we can avoid the ref-counting machinery, 
right? Sounds like exactly the things they want. Although perhaps if they want 
to always avoid the ref-counting machinery, then we can actually have more 
aggressive pointer tagging tricks in the kernel ABI. 

Well, as long as we’re aware that this is more complexity we’re adopting, seems 
fine.

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