swift234

Introduce @_specialize(<type list>) internal attribute.

This attribute can be attached to generic functions. The attribute's
arguments must be a list of concrete types to be substituted in the
function's generic signature. Any number of specializations may be
associated with a generic function.

This attribute provides a hint to the compiler. At -O, the compiler
will generate the specified specializations and emit calls to the
specialized code in the original generic function guarded by type
checks.

The current attribute is designed to be an internal tool for
performance experimentation. It does not affect the language or
API. This work may be extended in the future to add user-visible
attributes that do provide API guarantees and/or direct dispatch to
specialized code.

This attribute works on any generic function: a freestanding function
with generic type parameters, a nongeneric method declared in a
generic class, a generic method in a nongeneric class or a generic
method in a generic class. A function's generic signature is a
concatenation of the generic context and the function's own generic
type parameters.

e.g.

  struct S<T> {
    var x: T
    @_specialize(Int, Float)
    mutating func exchangeSecond<U>(u: U, _ t: T) -> (U, T) {
      x = t
      return (u, x)
    }
  }
  // Substitutes: <T, U> with <Int, Float> producing:
  // S<Int>::exchangeSecond<Float>(u: Float, t: Int) -> (Float, Int)