reflect: NamedOf

[crawshaw]

Consider adding a function to the reflect package for creating a new named type with a method set:

func NamedOf(t Type, name string, methods []Method) Type

(Broken out from #4146.)

[crawshaw]

This turns out to require more invasive changes than I originally realized.

To be useful, it needs to be possible to add methods to types created at run time. That means taking some equivalent to func(receiver reflect.Value, args []reflect.Value) (results []reflect.Value) as the user's implementation of a method.

But methods are called directly from pointers to the text segment (stored in the itab). The unboxed receiver value is on the stack for the method body to use, but no type is available. So there's no way for the reflect package to take the receiver value and box it up into a reflect.Value when the method is called.

Assuming we want to avoid code generation, some change to dynamic method calling would have to be made. The smallest is probably dropping the underlying type and method number onto the stack. A more significant change would be making the function pointers in the itab indirect like func() pointers (https://golang.org/s/go11func). A third option would be a note on the itab saying these are func()-style pointers. Any of these actions incurs some cost on dynamic method calls.

Still pondering.

[glycerine]

The unboxed receiver value is on the stack for the method body to use, but no type is available.

I thought the precise GC work back in Go1.4 meant all types on the stack are known to the runtime. Is that no longer true?

[randall77]

We only have pointer/nonpointer bits for each word on the stack. We do not have full type information.