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

[Bryan Chan]

John McCall via swift-dev <swift-dev at swift.org> wrote on 2016-03-01
06:23:24 PM:

> > 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.

This may be the case currently on Linux (even for non-x86 architectures),
but what I have heard is that the kernel architecture allows using more
levels of page tables, and the full 64-bit address space. The "guarantee"
may not hold true in the future.

Negative pointer values are also used on other operating systems, so I
would suggest not relying on this assumption for the sake of portability.

Bryan

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