> On Oct 17, 2016, at 12:40 PM, John McCall <rjmcc...@apple.com> wrote:
>> On Oct 17, 2016, at 10:19 AM, Joe Groff via swift-dev <swift-dev@swift.org> 
>> wrote:
>>> On Oct 17, 2016, at 9:57 AM, Michael Gottesman <mgottes...@apple.com> wrote:
>>>> On Oct 17, 2016, at 9:42 AM, Joe Groff via swift-dev <swift-dev@swift.org> 
>>>> wrote:
>>>>> On Oct 16, 2016, at 1:10 PM, Dave Abrahams via swift-dev 
>>>>> <swift-dev@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 <gpar...@apple.com> wrote:
>>>>>>>> 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> wrote:
>>>>>>>>>> On Mar 1, 2016, at 1:33 PM, Joe Groff via swift-dev 
>>>>>>>>>> <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.
>>>>>> 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.
>>> I need to catch up on this thread, but there is an important thing to 
>>> remember. If you use an enum like this there are a few potential issues:
>>> 1. In the implementation, you will /not/ want to use the enum internally. 
>>> This would prevent the optimizer from eliminating all of the Small Case 
>>> reference counting operations. This means you would rewrap the internal 
>>> value when you return one and when you enter into an internal 
>>> implementation code path try to immediately switch to a specialized small 
>>> case path if you can.
>> This poses an interesting question for the semantic ARC model with enums. It 
>> seems to me that, if switching or projecting the payload of an enum was a 
>> consuming operation, that we could avoid this optimization pitfall. 
>> Switching the enum { case Big(Class), Small(Trivial) } or similar case would 
>> semantically eliminate the nontrivial enum value and leave only the trivial 
>> payload behind.
> We can't make it *exclusively* a consuming operation; it has to be possible 
> to switch on a borrowed value.

Yes. Perhaps the right way to think about it is in the context of considering 
putting conventions on SILArguments/Terminators. Then you have a natural way to 
express this and could optimize (potentially) in between such forms.

> John.

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