Implement null / non-null analysis

[retronym]

Can be used to avoid unnecessary null-checks when inlining an instance method.

Miguel's code: NullnessPropagator

Below there are two null checks in t, none is necessary: t is an instance method, we know in it's body that ALOAD 0 is non-null.

class C {
  @inline final def f = 1
  def t = f + f
}

[lrytz]

Nullness analysis is implemented in scala/scala#4519, performance of aliasing analysis improved in scala/scala#4711.

still to do: Peephole opts around null

• see miguel's NullnessPropagator.transform

also to do: take null tests into account (if (x == null) { [x known null here] })

Miguel's treats module loads as not null. Jason provided a counterexample:

class C(a: Any)
object X { def o = O}
object O extends C(println(X.o)) { println(O) }
object Test extends App { O }

scalac sandbox/test.scala && qscala Test
null
O$@4c873330

A bit simpler:

class C { val b = O }
object O extends C

object Test extends App {
  println(O.b) // null
}

Another example:

object A1 extends {
  val e = B1
} with Object {
  val b1 = B1
}

object B1 {
  val a1 = A1
}

object Test extends App {
  println(A1.b1) // B1
  println(B1.a1) // null
  println(A1.e)  // B1
}

See also #16

[lrytz]

PatMat generates null tests that can potentially be eliminated:

  public f(I)I
   L0
    LINENUMBER 3 L0
    NEW C
    DUP
    ILOAD 1
    INVOKESPECIAL C.<init> (I)V
    ASTORE 3
    ALOAD 3
    IFNULL L1
    [...]

[lrytz]

Implemented in scala/scala#4858 / scala/scala@80b0660