swift VTables

VTables

https://github.com/apple/swift/blob/master/docs/SIL.rst#vtables

decl ::= sil-vtable

sil-vtable ::= 'sil_vtable' identifier '{' sil-vtable-entry* '}'

 

sil-vtable-entry ::= sil-decl-ref ':' sil-linkage? sil-function-name

SIL represents dynamic dispatch for class methods using the class_method, super_method, objc_method, and objc_super_method instructions.

The potential destinations for class_method and super_method are tracked in sil_vtable declarations for every class type. The declaration contains a mapping from every method of the class (including those inherited from its base class) to the SIL function that implements the method for that class:

class A {

  func foo()

  func bar()

  func bas()

}

 

sil @A_foo : $@convention(thin) (@owned A) -> ()

sil @A_bar : $@convention(thin) (@owned A) -> ()

sil @A_bas : $@convention(thin) (@owned A) -> ()

 

sil_vtable A {

  #A.foo!1: @A_foo

  #A.bar!1: @A_bar

  #A.bas!1: @A_bas

}

 

class B : A {

  func bar()

}

 

sil @B_bar : $@convention(thin) (@owned B) -> ()

 

sil_vtable B {

  #A.foo!1: @A_foo

  #A.bar!1: @B_bar

  #A.bas!1: @A_bas

}

 

class C : B {

  func bas()

}

 

sil @C_bas : $@convention(thin) (@owned C) -> ()

 

sil_vtable C {

  #A.foo!1: @A_foo

  #A.bar!1: @B_bar

  #A.bas!1: @C_bas

}

Note that the declaration reference in the vtable is to the least-derived method visible through that class (in the example above, B's vtable references A.bar and not B.bar, and C's vtable references A.bas and not C.bas). The Swift AST maintains override relationships between declarations that can be used to look up overridden methods in the SIL vtable for a derived class (such as C.bas in C's vtable).

In case the SIL function is a thunk, the function name is preceded with the linkage of the original implementing function.