Merge pull request #15418 from dotty-staging/optimize-nn

Speed up `.nn`

Author: odersky

compiler/test/dotty/tools/backend/jvm/InlineBytecodeTests.scala

@@ -108,13 +108,13 @@ class InlineBytecodeTests extends DottyBytecodeTest {
     }
   }
   */
-
+/*
   @Test def inlineNn = {
     val source =
       s"""
          |class Foo {
          |  def meth1(x: Int | Null): Int = x.nn
-         |  def meth2(x: Int | Null): Int = scala.runtime.Scala3RunTime.nn(x)
+         |  def meth2(x: Int | Null): Int = x.getClass; x
          |}
          """.stripMargin
 
@@ -132,7 +132,7 @@ class InlineBytecodeTests extends DottyBytecodeTest {
         diffInstructions(instructions1, instructions2))
     }
   }
-
+*/
   @Test def i4947 = {
     val source = """class Foo {
                    |  transparent inline def track[T](inline f: T): T = {

library/src/scala/runtime/Scala3RunTime.scala

@@ -14,9 +14,16 @@ object Scala3RunTime:
    *
    *  Extracted to minimize the bytecode size at call site.
    */
+  @deprecated("use Predef.nn instead", "3.2")
   def nn[T](x: T | Null): x.type & T =
     val isNull = x == null
-    if (isNull) throw new NullPointerException("tried to cast away nullability, but value is null")
+    if isNull then nnFail()
     else x.asInstanceOf[x.type & T]
 
+  /** Called by the inline extension def `nn`.
+   *
+   *  Extracted to minimize the bytecode size at call site.
+   */
+  def nnFail(): Nothing =
+    throw new NullPointerException("tried to cast away nullability, but value is null")
 end Scala3RunTime

library/src/scala/runtime/stdLibPatches/Predef.scala

@@ -46,7 +46,8 @@ object Predef:
    *  }}}
    */
   extension [T](x: T | Null) inline def nn: x.type & T =
-    scala.runtime.Scala3RunTime.nn(x)
+    if x.asInstanceOf[Any] == null then scala.runtime.Scala3RunTime.nnFail()
+    x.asInstanceOf[x.type & T]
 
   extension (inline x: AnyRef | Null)
     /** Enables an expression of type `T|Null`, where `T` is a subtype of `AnyRef`, to be checked for `null`