CODEWITHSUNDEEP
scalabytecodeimplicitvalue-class
Gabor Juhasz
Gabor Juhasz

Reputation: 317

Why do implicit value classes have an extra method invocation?

I was inspecting the byte-code generated by implicit classes and wanted to compare to what is generated when they extend AnyVal.

Without implicit:

object Example1 {
  class Wrapper(val self: Int) extends AnyVal {
    def add(n: Int): Int = self + n
  }
  def foo(w: Wrapper): Wrapper = new Wrapper(w.add(42))
}

The (relevant part of) bytecode:

scala>:javap Example1

[...]

public int foo(int);
  descriptor: (I)I
  flags: ACC_PUBLIC
  Code:
    stack=3, locals=2, args_size=2
       0: getstatic     #19                 // Field Example1$Wrapper$.MODULE$:LExample1$Wrapper$;
       3: iload_1
       4: bipush        42
       6: invokevirtual #23                 // Method Example1$Wrapper$.add$extension:(II)I
       9: ireturn
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0      10     0  this   LExample1$;
          0      10     1     w   I
    LineNumberTable:
      line 11: 3

[...]

With implicit:

object Example2 {
  implicit class Wrapper(val self: Int) extends AnyVal {
    def add(n: Int): Int = self + n
  }
  def foo(w: Wrapper): Wrapper = w.add(42)
}

The (relevant part of) byte-code:

scala>:javap Example2

[...]

public int Wrapper(int);
  descriptor: (I)I
  flags: ACC_PUBLIC
  Code:
    stack=1, locals=2, args_size=2
       0: iload_1
       1: ireturn
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0       2     0  this   LExample2$;
          0       2     1  self   I
    LineNumberTable:
      line 9: 0

public int foo(int);
  descriptor: (I)I
  flags: ACC_PUBLIC
  Code:
    stack=4, locals=2, args_size=2
       0: aload_0
       1: getstatic     #23                 // Field Example2$Wrapper$.MODULE$:LExample2$Wrapper$;
       4: iload_1
       5: bipush        42
       7: invokevirtual #27                 // Method Example2$Wrapper$.add$extension:(II)I
      10: invokevirtual #29                 // Method Wrapper:(I)I
      13: ireturn
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0      14     0  this   LExample2$;
          0      14     1     w   I
    LineNumberTable:
      line 12: 0

[...]

As a result of extending AnyVal the call to add is invoked on the companion object, and the type Wrapper does not show in the type signature of foo (public int foo(int);) in both versions.

Hovewer, in the second version, there is a call just before return: 10: invokevirtual #29. It calls public int Wrapper(int); which seemingly doesn't do anything. (Although I might be wrong, as I don't have much experience with reading the byte-code)

So the question is, what's the significance of that call? Can't it be omitted?

Upvotes: 3

Views: 148

Answers (1)

Jasper-M
Jasper-M

Reputation: 15086

The problem is that your code snippets are not equivalent. An implicit class Foo is compiled/desugared into a class Foo and an implicit conversion method Foo. That's also the reason that implicit classes (currently) can't be top level.

So your first snippet should be:

object Example1 {
  class Wrapper(val self: Int) extends AnyVal {
    def add(n: Int): Int = self + n
  }
  def Wrapper(self: Int): Wrapper = new Wrapper(self)
  def foo(w: Wrapper): Wrapper = Wrapper(w.add(42))
}

The compiler erases calls to value class constructors if possible. But it doesn't erase the call to the Wrapper method, implicit or not.

I guess that the JIT compiler in the JVM will eventually erase that method call anyway.

Upvotes: 3

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