> 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.
Advertising
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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