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

[Joe Groff]

> 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.

-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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> 
> -- 
> -Dave
> 
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