> On Oct 13, 2016, at 10:46 AM, John McCall via swift-dev <swift-dev@swift.org> 
> wrote:
>> On Oct 13, 2016, at 9:04 AM, Joe Groff via swift-dev <swift-dev@swift.org 
>> <mailto:swift-dev@swift.org>> wrote:
>>> On Mar 1, 2016, at 1:33 PM, Joe Groff via swift-dev <swift-dev@swift.org 
>>> <mailto:swift-dev@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.
>> 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.
> This makes sense to me.  if (x <= 0) return; should be just as cheap as is (x 
> == 0) return;

Conversely, I wanted to try to remove such nil checks. Currently they look 
haphazard: some functions have them and some do not.

Allowing ABI space for tagged pointer objects is a much bigger problem than the 
check in swift_retain/release. For example, all vtable and witness table 
dispatch sites to AnyObject or any other type that might someday have a tagged 
pointer subclass would need to compile in a fallback path now. You can't 
dereference a tagged pointer to get its class pointer. 

Greg Parker     gpar...@apple.com <mailto:gpar...@apple.com>     Runtime 

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