[swift-dev] Having 64-bit swift_retain/release ignore all negative pointer values

[John McCall]

> On Mar 1, 2016, at 3:05 PM, Greg Parker via swift-dev <swift-dev at swift.org> wrote:
>> On Mar 1, 2016, at 1:33 PM, Joe Groff via swift-dev <swift-dev at 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.
> 
> We can't do that unless we can get a guarantee from the OS folks that a "negative" pointer will never be a valid userspace pointer in any future OS version.

We have that guarantee, actually.  The top eight bits are guaranteed to be clear in the user space on both ARM64 and x86-64.

John.