Can we avoid the allocation when using Option with Extractor

Language Design
1 
import java.util.concurrent.atomic.AtomicInteger

object Nat:
  inline def unapply(x: AtomicInteger): Option[Int] =  x.get() match {
    case n if n > 0 => Some(n)
    case _ => None
  }

object Main:
  def test(n: AtomicInteger): String =
    n match
      case Nat(m) => s"$m is a natural number"
      case _ => s"$n is not a natural number"

  def main(args: Array[String]): Unit =
    println(test(new AtomicInteger(5))) // Output: 5 is a natural number

// class version 52.0 (52)
// access flags 0x31
public final class Main$ implements java/io/Serializable {

  // compiled from: main.scala

  ATTRIBUTE Scala : unknown

  // access flags 0x19
  public final static LMain$; MODULE$

  // access flags 0x2
  private <init>()V
   L0
    LINENUMBER 9 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object.<init> ()V
    RETURN
   L1
    LOCALVARIABLE this LMain$; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1

  // access flags 0x9
  public static <clinit>()V
   L0
    LINENUMBER 10 L0
    NEW Main$
    DUP
    INVOKESPECIAL Main$.<init> ()V
    PUTSTATIC Main$.MODULE$ : LMain$;
    RETURN
    MAXSTACK = 2
    MAXLOCALS = 0

  // access flags 0x2
  private writeReplace()Ljava/lang/Object;
   L0
    LINENUMBER 9 L0
    NEW scala/runtime/ModuleSerializationProxy
    DUP
    LDC LMain$;.class
    INVOKESPECIAL scala/runtime/ModuleSerializationProxy.<init> (Ljava/lang/Class;)V
    ARETURN
   L1
    LOCALVARIABLE this LMain$; L0 L1 0
    MAXSTACK = 3
    MAXLOCALS = 1

  // access flags 0x1
  public test(Ljava/util/concurrent/atomic/AtomicInteger;)Ljava/lang/String;
    // parameter final  n
   L0
    LINENUMBER 11 L0
    ALOAD 1
    ASTORE 2
   L1
    LINENUMBER 12 L1
    ALOAD 2
    IFNULL L2
   L3
    LINENUMBER 12 L3
    ALOAD 2
    INVOKEVIRTUAL java/util/concurrent/atomic/AtomicInteger.get ()I
    ISTORE 4
   L4
    LINENUMBER 5 L4
    ILOAD 4
    ISTORE 5
   L5
    ILOAD 5
    ICONST_0
    IF_ICMPLE L6
    GETSTATIC scala/Some$.MODULE$ : Lscala/Some$;
    ILOAD 5
    INVOKESTATIC scala/runtime/BoxesRunTime.boxToInteger (I)Ljava/lang/Integer;
    INVOKEVIRTUAL scala/Some$.apply (Ljava/lang/Object;)Lscala/Some;
    GOTO L7
   L6
    LINENUMBER 6 L6
    GETSTATIC scala/None$.MODULE$ : Lscala/None$;
    GOTO L7
   L7
    ASTORE 3
   L8
    LINENUMBER 12 L8
    ALOAD 3
    INVOKEVIRTUAL scala/Option.isEmpty ()Z
    IFNE L2
    ALOAD 3
    INVOKEVIRTUAL scala/Option.get ()Ljava/lang/Object;
    INVOKESTATIC scala/runtime/BoxesRunTime.unboxToInt (Ljava/lang/Object;)I
    ISTORE 6
    ILOAD 6
    ISTORE 7
   L9
    NEW java/lang/StringBuilder
    DUP
    LDC 20
    INVOKESPECIAL java/lang/StringBuilder.<init> (I)V
    ILOAD 7
    INVOKEVIRTUAL java/lang/StringBuilder.append (I)Ljava/lang/StringBuilder;
    LDC " is a natural number"
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    ARETURN
   L2
    LINENUMBER 13 L2
    NEW java/lang/StringBuilder
    DUP
    LDC 24
    INVOKESPECIAL java/lang/StringBuilder.<init> (I)V
    ALOAD 1
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/Object;)Ljava/lang/StringBuilder;
    LDC " is not a natural number"
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    ARETURN
   L10
    LOCALVARIABLE n I L5 L6 5
    LOCALVARIABLE m I L9 L2 7
    LOCALVARIABLE this LMain$; L0 L10 0
    LOCALVARIABLE n Ljava/util/concurrent/atomic/AtomicInteger; L0 L10 1
    MAXSTACK = 3
    MAXLOCALS = 8

  // access flags 0x1
  // signature ([Ljava/lang/String;)V
  // declaration: void main(java.lang.String[])
  public main([Ljava/lang/String;)V
    // parameter final  args
   L0
    LINENUMBER 16 L0
    GETSTATIC scala/Predef$.MODULE$ : Lscala/Predef$;
    ALOAD 0
    NEW java/util/concurrent/atomic/AtomicInteger
    DUP
    ICONST_5
    INVOKESPECIAL java/util/concurrent/atomic/AtomicInteger.<init> (I)V
    INVOKEVIRTUAL Main$.test (Ljava/util/concurrent/atomic/AtomicInteger;)Ljava/lang/String;
    INVOKEVIRTUAL scala/Predef$.println (Ljava/lang/Object;)V
    RETURN
   L1
    LOCALVARIABLE this LMain$; L0 L1 0
    LOCALVARIABLE args [Ljava/lang/String; L0 L1 1
    MAXSTACK = 5
    MAXLOCALS = 2
}

I was thinking , the unapply is inline method, and the compiler will treat Some(n) and None special and rewrite the code to something like :

import java.util.concurrent.atomic.AtomicInteger

object Main:

  def main(args: Array[String]): Unit =
    println({
      (new AtomicInteger(5)).get() match
        case n if n > 0 => s"$n is a natural number"
        case x => s"$x is not a natural number"

    }) // Output: 5 is a natural number
1 Like
2

IIRC, if this is in the hot path, JVM will elide away the Option allocation, because the Option instance does not escape the method test

1 Like
3

you can eliminate the allocation with by-name extractors: Option-less pattern matching

4
  1. Do we need the AnyVal or is there no new Nat.
  2. Will the ref.get() be called twice if both the isEmpty and get both need to access the ref’s value.
1 Like