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

[Joe Groff]

> On Oct 17, 2016, at 10:54 AM, Dave Abrahams <dabrahams at apple.com> wrote:
> 
> 
> on Mon Oct 17 2016, Joe Groff <jgroff-AT-apple.com> wrote:
> 
>>> On Oct 16, 2016, at 1:10 PM, Dave Abrahams via swift-dev <swift-dev at swift.org> wrote:
>>> 
>>> 
>>> on Thu Oct 13 2016, Joe Groff <swift-dev-AT-swift.org> wrote:
>>> 
>> 
>>>>> On Oct 13, 2016, at 1:18 PM, Greg Parker <gparker at apple.com> wrote:
>>>>> 
>>>>> 
>>>>>> On Oct 13, 2016, at 10:46 AM, John McCall via swift-dev <swift-dev at swift.org> wrote:
>>>>>> 
>>>> 
>>>>>>> On Oct 13, 2016, at 9:04 AM, Joe Groff 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.
>>>>>>> 
>>>>>>> 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.
>>>> 
>>>> True. I don't think we'd want to use this optimization for class
>>>> types; I was specifically thinking of other things for which we use
>>>> nullable refcounted representations, particularly closure
>>>> contexts. The ABI for function types requires the context to be
>>>> refcountable by swift_retain/release, but it doesn't necessarily have
>>>> to be a valid pointer, if the closure formation site and invocation
>>>> function agree on a tagged-pointer representation. 
>>> 
>>> Well, but we'd like to take advantage of the same kind of optimization
>>> for the small string optimization.  It doesn't seem like this should be
>>> handled differently just because the string buffer is a class instance
>>> and not a closure context.
>> 
>> String is a struct, and small strings don't have to be modeled as
>> class instances. An enum { case Big(StringStorage), Small(Int63) } or
>> similar layout should be able to take advantage of
>> swift_retain/release ignoring negative values too.
> 
> That would be nice.  Historically I've had to bypass enums either
> because the codegen or the optimizer wasn't smart enough.
> 
> We *had* wanted to use some of the same tagged pointer representations
> as Cocoa does for NSString, and if we give up layout to the enum codegen
> machinery we won't get that.  That would be a very minor
> performance win, though, so it probably doesn't matter.

Yeah, I figured we would still need some variant of Builtin.BridgeObject around to give the standard library the control it needs. My only point was that we don't necessarily need to support a concept of tagged-pointer class instances, with all the runtime complexity that entails, to be able to take advantage of tagged pointer optimizations for non-class refcounted types in Swift.

-Joe