> 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.
> 2. {Retain,Release}Values will be created outside. We are talking about some 
> ways of fixing this from a code-size perspective by using a value witness, 
> but in the present this may cause additional code-size increase.

This is exactly the case that would be improved, since retain/release_value on 
such an enum would boil down to a single swift_retain/release call if the 
runtime functions ignored the tagged small case values.


>> -Joe
>>>> We could also do interesting things with enums; if one payload type is
>>>> a class reference and the rest are trivial, we could lay the enum out
>>>> in such a way that we can use swift_retain/release on it by setting
>>>> the high bit when tagging the trivial representations, saving us the
>>>> need to emit a switch. We wouldn't actually dereference the pointer
>>>> representation without checking it first.
>>>> I know we've discussed taking the nil check out of
>>>> swift_retain/release, and possibly having separate variants that do
>>>> include the null check for when we know we're working with
>>>> Optionals. How much of difference would that really make, though? I'd
>>>> expect it to be a fairly easily predictable branch, since most objects
>>>> are likely to be nonnull in practice.
>>>> -Joe
>>>> _______________________________________________
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>>>> https://lists.swift.org/mailman/listinfo/swift-dev
>>> -- 
>>> -Dave
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