> On Oct 17, 2016, at 10:19 AM, Joe Groff <jgr...@apple.com> 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.
Sure. I thought that switch_enum was always a consuming operation semantically
(that the optimizer just chose to ignore). I agree with you here that this fits
the enum model better (definitely for optionals which are used more like tuples
than like classes).
>
> -Joe
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