[swift-evolution] [Pitch] Make the formal type of 'self' consistent in class methods

[Slava Pestov]

> On Jun 23, 2016, at 2:02 PM, Andrew Trick <atrick at apple.com> wrote:
> 
>> 
>> On Jun 23, 2016, at 1:48 PM, Slava Pestov <spestov at apple.com <mailto:spestov at apple.com>> wrote:
>> 
>> 
>>> On Jun 23, 2016, at 1:46 PM, Andrew Trick <atrick at apple.com <mailto:atrick at apple.com>> wrote:
>>> 
>>> 
>>>> On Jun 23, 2016, at 12:53 PM, Slava Pestov via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>> The proposal is to change the type of self to always be Self, which can be thought of as a special generic type parameter bound to the dynamic type of the instance.
>>> 
>>> We’re currently specializing functions that take `self` as an argument. I don’t think that will be possible after your proposed change.
>>> 
>>> - Andy
>> 
>> I’m not sure what that means. Do you currently punt on certain optimizations if a method returns ‘Self’?
>> 
>> It should be possible to keep the reified type information around, by passing in a metatype or something for example. Can you give a concrete code snippet demonstrating the optimization and how this change would inhibit it?
> 
> We bail out of generic specialization, inlining, and function signature specialization when a type substitution contains dynamic self. (hasDynamicSelfTypes). So, yes we currently almost entirely punt on optimization for methods that return Self.

I see. That makes sense.

I think the problem is that if we specialize a top-level function with a substitution involving Self, we have no way to recover what the ‘Self’ type actually is in IRGen. However I think it could be made to work by passing in a metatype for Self, and somehow ensuring we don’t mix up Self from two different contexts...

This is certainly a trickier change than I first imagined, but it would be nice to figure out how to solve this in a principled way so that we can get these optimizations to be more generally applicable. It seems even more surprising, now, if changing the return type of a method inhibits optimizations in a non-obvious way, especially ones that can have a drastic effect on performance.

I think next week I’ll try implementing this proposal behind a staging flag, and play around with the optimizer to see how hard it would be plumb through the relevant type information.

Slava

> 
> I don’t have an interesting case to point out. You can look into any trivial example:
> 
> func foo<T>(_: T) {}
> 
> func method() {
>   foo(self)
> }
> 
> -Andy

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