When a method is called with fewer than two parameter lists,
transform it into a fully-applied call by wrapping it in a
closure.
Eg,
Foo.bar => { self in { args... self.bar(args...) } }
foo.bar => { self in { args... self.bar(args...) } }(self)
super.bar => { args... in super.bar(args...) }
With this change, SILGen only ever sees fully-applied calls,
which will allow ripping out some code.
This new way of doing curry thunks fixes a long-standing bug
where unbound references to protocol methods did not work.
This is because such a reference must open the existential
*inside* the closure, after 'self' has been applied, whereas
the old SILGen implementation of curry thunks really wanted
the type of the method reference to match the opened type of
the method.
A follow-up cleanup will remove the SILGen curry thunk
implementation.
Fixes rdar://21289579 and https://bugs.swift.org/browse/SR-75.
Signature optimization is slightly different to (most) other thunks, in that
it's taking an existing function and turning that into a thunk, rather than
creating a thunk that calls an existing function. These symbols can be public,
etc. and so need to be handled a bit different to other types of thunks.
This consists of 3 parts:
1) Extend CallerAnalysis to also provide information if a function is partially applied
2) A new DeadArgSignatureOpt pass, similar to FunctionSignatureOpts, which just specializes for dead arguments of partially applied functions.
3) Let CapturePropagation eliminate such partial_apply instructions and replace them with a thin_to_thick conversion of the specialized functions.
This optimzation improves benchmarks where static struct or class functions are passed as a closure (e.g. -20% for SortStrings).
Such functions have a additional metatype parameter. We used to create a partial_apply in this case, which allocates a context, etc.
But this is not necessary as the metatype parameter is not used in most cases.
rdar://problem/27513085
