Expects: `getfield` but `INVOKEINTERFACE` when using trait parameters

[He-Pin]

• Scala version 3.3.5
• feature: trait parameter
• bug: I expect the bytecode to be a getfield but got an INVOKEINTERFACE

background:

I am doing performance optimization, where I need to access a _tag field.
see:

But I got:

  // access flags 0x1
  public visitExpr(Lsjsonnet/Expr;[Lsjsonnet/Lazy;)Lsjsonnet/Val;
    // parameter final  e
    // parameter final  scope
    TRYCATCHBLOCK L0 L1 L2 java/lang/Throwable
    TRYCATCHBLOCK L3 L4 L2 java/lang/Throwable
   L0
    LINENUMBER 33 L0
    ALOAD 1
    INVOKEINTERFACE sjsonnet/Expr._tag ()I (itf) //I would like this to be a getfield.

My local change is :

trait Expr (private[sjsonnet] val _tag :Int = ExprTags.UNTAGGED){
  def pos: Position

  /** The name of this expression type to be shown in error messages */
  def exprErrorString: String = {
    val n = getClass.getName
    if(n.startsWith("sjsonnet.Expr$")) n.substring(14) else n
  }

  override def toString: String = s"$exprErrorString@$pos"
}

and then something like:

case class ImportStr(pos: Position, value: String) extends Expr(ExprTags.ImportStr)

But when I use it as code below, it still INVOKEINTERFACE sjsonnet/Expr._tag ()I (itf)

  override def visitExpr(e: Expr)(implicit scope: ValScope): Val = try {
    (e._tag: @switch) match {
      case ExprTags.ValidId => visitValidId(e.asInstanceOf[ValidId])
      case ExprTags.BinaryOp => visitBinaryOp(e.asInstanceOf[BinaryOp])

refs: databricks/sjsonnet#313

btw, as Scala is now using Java 17, there is a https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/runtime/SwitchBootstraps.html

Will Scala leverage that?

[sjrd]

Java interfaces cannot have fields. So the fields are introduced in the classes that extend that trait. When accessing the "field" through the interface, we have to go through a dispatch getter, implemented in every class, that reads the particular field of said class.

If you want a direct field access, use an abstract class instead.

[He-Pin]

Thank you。

I was expecting Scala can copy this field to all its subclasses when all subclasses are .scala, but yes, we can't limit the code to not being used by Java.

Making the Expr an abstract class will need to change more code in sjsonnet, but that may not matter in real-world cases.

[He-Pin]

@sjrd, what if the trait is sealed? Will it still not be compiled into a field?

[sjrd]

No, still not. Sealed or not, a trait of a Java interfaces, and Java interfaces cannot have fields.