Nested inline functions should not generate bytecodes

[Glavo]

Because nested functions are only called in external functions, it is meaningless to generate bytecode for nested inline functions.

[nicolasstucki]

I guess it is in functions like bar in

object Foo {
  def foo: Unit = {
    inline def bar(): Unit = ()
    bar()
  }
}

where we should not generate the code for bar.

Note that the code at backend is

package <empty> {
  @scala.annotation.internal.SourceFile("Foo.scala") final module class Foo$
     extends
   Object { 
    def <init>(): Unit = 
      {
        super()
        ()
      }
    def foo(): Unit = 
      {
        println("aa")
      }
    private inline def bar$1(): Unit = println("aa")
  }
  final lazy module val Foo: Foo$ = new Foo$()
}

We should be able to juts drop inline method while lifting.

[nicolasstucki]

Actually, with -optimise this bar$1 is removed.

[smarter]

In scalac, it looks like unused inner methods are eliminated by typer:

object Test {
  def foo: Int = {
    def bar: Int = 3
    2
  }
}

[[syntax trees at end of                     typer]] // dce.scala
package <empty> {
  object Test extends scala.AnyRef {
    def <init>(): Test.type = {
      Test.super.<init>();
      ()
    };
    def foo: Int = 2
  }
}

But I can't find the logic reponsible for this (any idea @retronym ?). In any case, if we did something like that in Dotty it should be done after pickler for the IDE sake.

[retronym]

@smarter It is in constant folding. The behaviour could be seen as a bug IMO.

See scala/bug#7173 (comment) / scala/bug#10340

[smarter]

Oh I see, that's a very weird and inconsistent behavior of scalac then, since it means that nothing is eliminated if the block expression is not "safe to inline":

object Test {
  def foo: Int = {
    def bar: Int = 3
    println("")
  }
}

[[syntax trees at end of                     typer]] // dce.scala
package <empty> {
  object Test extends scala.AnyRef {
    def <init>(): Test.type = {
      Test.super.<init>();
      ()
    };
    def foo: Int = {
      def bar: Int = 3;
      scala.Predef.println("")
    }
  }
}

[abgruszecki]

@nicolasstucki Right now, we emit no bytecode for local inline methods.

See decompilation output

$ cat Test.scala 
class Test {
  def test = {
    inline def five = 5
    five
  }
}
$ ../bin/dotc Test.scala
$ javap -verbose Test.class 
Classfile /home/gruszecki/code/dotty/play/Test.class
  Last modified Apr 4, 2019; size 353 bytes
  MD5 checksum 37b4ed17a682f4aac43de007a8070f84
  Compiled from "Test.scala"
public class Test
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Utf8               Test
   #2 = Class              #1             // Test
   #3 = Utf8               java/lang/Object
   #4 = Class              #3             // java/lang/Object
   #5 = Utf8               Test.scala
   #6 = Utf8               
   #7 = Utf8               ()V
   #8 = NameAndType        #6:#7          // "":()V
   #9 = Methodref          #4.#8          // java/lang/Object."":()V
  #10 = Utf8               this
  #11 = Utf8               LTest;
  #12 = Utf8               test
  #13 = Utf8               ()I
  #14 = Utf8               Code
  #15 = Utf8               LocalVariableTable
  #16 = Utf8               LineNumberTable
  #17 = Utf8               SourceFile
  #18 = Utf8               TASTY
  #19 = Utf8               Scala
{
  public Test();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #9                  // Method java/lang/Object."":()V
         4: return
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   LTest;
      LineNumberTable:
        line 1: 0
public int test();

descriptor: ()I

flags: ACC_PUBLIC

Code:

stack=1, locals=1, args_size=1

0: iconst_5

1: ireturn

LocalVariableTable:

Start  Length  Slot  Name   Signature

0       2     0  this   LTest;

LineNumberTable:

line 2: 0

}

SourceFile: "Test.scala"

Error: unknown attribute

TASTY: length = 0x10

00 BD FD 67 CD 78 4F 00 00 94 8D 87 A0 90 A3 00
Error: unknown attribute

Scala: length = 0x0

Do we want to test for this in any way, or do we want to just close this issue outright?

[smarter]

We should test it if there's no test for it yet, you can just do a run-test that lists all the members of the current class using Java reflection.