Eliminate box-unbox pairs, tuples, *Refs

[retronym]

Miguel's code: UnBoxElider, fixpointGivenConsumer

NOTE: benchmark before doing anything. It seems the JVM optimizes *Refs modified in a while loop.

Related:

• eliminate tuples in multi-returns (val (a, b) = foo())
• eliminate *Ref objects for caputred variables after inlining a closure

Also eliminate boxing of values that are discarded: see for exmple SI-9160

[lrytz]

IntRef example:

def f(a: Array[Int]): Int = {
  var x = 0
  a.foreach(y => x += y)
  x
}

[lrytz]

Box-Unbox example below. Specialization fixes it for the common Function case, but using a higher-arity, non-specialized function also incurs boxing. Also, mixing primitve and reference types prevents specialization ((s: String) => s.length).

scala> abstract class BoxingFun[T] { def f(x: T): T }
scala> class KKK {
  def foo = {
    val fun = new BoxingFun[Int] { def f(x: Int) = x + 10 }
    fun.f(1)
  }
}
scala> :javap KKK
  public int foo();
        10: iconst_1
        11: invokestatic  #36   // boxToInteger:(I)Ljava/lang/Integer;
        14: invokevirtual #40   // $read$$iw$$iw$BoxingFun.f:(Ljava/lang/Object;)Ljava/lang/Object;
        17: invokestatic  #44   // unboxToInt:(Ljava/lang/Object;)I

[lrytz]

Similar to box-unbox paris, also eliminate value class instantiation and value extraction pairs.

class A(val x: Int) extends AnyVal
class T {
  def f(n: A) = {
    val f = (a: A) => a
    f(n)
  }
}

produces (cfr-decompiler)

    public int f(int n) {
        A a = new A(n);
        return new A(a.x()).x();
    }

[lrytz]

scala/scala#4858 / scala/scala@29d772d