Unoptimized code generated for Enum dispatch

[nmichael44]

When generating code for a simple Enum, the compiler generates a huge chain of if-else-if statements instead of a tableswitch. The same problem exists for more general ADTs of course (and it's more complicated to fix this for those since we would have to generate a hidden tag etc), but for just plain enums I would expect this to work. Everything is there for you - just detect this case (of a simple Enum), call ordinal() on it and index into the table. Even Kotlin does this.

Compiler version

"3.6.4"

Minimized code

  enum Z:
    case A, B, C, D, E, F, G

  def f(n: Z): Int =
    import Z.*
    n match {
      case A => 23
      case B => 12
      case C => -1
      case D => 4
      case E => 12
      case F => -11
      case G => 21
    }

Output

  public f(Lapp/Main$Z;)I
    // parameter final  n
   L0
    LINENUMBER 47 L0
    ALOAD 1
    ASTORE 2
   L1
    LINENUMBER 48 L1
    GETSTATIC app/Main$Z$.A : Lapp/Main$Z;
    ALOAD 2
    ASTORE 3
    DUP
    IFNONNULL L2
    POP
    ALOAD 3
    IFNULL L3
    GOTO L4
   L2
    ALOAD 3
    INVOKEVIRTUAL java/lang/Object.equals (Ljava/lang/Object;)Z
    IFEQ L4
   L3
    BIPUSH 23
    IRETURN
   L4
    LINENUMBER 49 L4
    GETSTATIC app/Main$Z$.B : Lapp/Main$Z;
    ALOAD 2
    ASTORE 4
    DUP
    IFNONNULL L5
    POP
    ALOAD 4
    IFNULL L6
    GOTO L7
   L5
    ALOAD 4
    INVOKEVIRTUAL java/lang/Object.equals (Ljava/lang/Object;)Z
    IFEQ L7
   L6
    BIPUSH 12
    IRETURN
   L7
    LINENUMBER 50 L7
    GETSTATIC app/Main$Z$.C : Lapp/Main$Z;
    ALOAD 2
    ASTORE 5
    DUP
    IFNONNULL L8
    POP
    ALOAD 5
    IFNULL L9
    GOTO L10
   L8
    ALOAD 5
    INVOKEVIRTUAL java/lang/Object.equals (Ljava/lang/Object;)Z
    IFEQ L10
   L9
    ICONST_M1
    IRETURN
   L10
    LINENUMBER 51 L10
    GETSTATIC app/Main$Z$.D : Lapp/Main$Z;
    ALOAD 2
    ASTORE 6
    DUP
    IFNONNULL L11
    POP
    ALOAD 6
    IFNULL L12
    GOTO L13
   L11
    ALOAD 6
    INVOKEVIRTUAL java/lang/Object.equals (Ljava/lang/Object;)Z
    IFEQ L13
   L12
    ICONST_4
    IRETURN
   L13
    LINENUMBER 52 L13
    GETSTATIC app/Main$Z$.E : Lapp/Main$Z;
    ALOAD 2
    ASTORE 7
    DUP
    IFNONNULL L14
    POP
    ALOAD 7
    IFNULL L15
    GOTO L16
   L14
    ALOAD 7
    INVOKEVIRTUAL java/lang/Object.equals (Ljava/lang/Object;)Z
    IFEQ L16
   L15
    BIPUSH 12
    IRETURN
   L16
    LINENUMBER 53 L16
    GETSTATIC app/Main$Z$.F : Lapp/Main$Z;
    ALOAD 2
    ASTORE 8
    DUP
    IFNONNULL L17
    POP
    ALOAD 8
    IFNULL L18
    GOTO L19
   L17
    ALOAD 8
    INVOKEVIRTUAL java/lang/Object.equals (Ljava/lang/Object;)Z
    IFEQ L19
   L18
    BIPUSH -11
    IRETURN
   L19
    LINENUMBER 54 L19
    GETSTATIC app/Main$Z$.G : Lapp/Main$Z;
    ALOAD 2
    ASTORE 9
    DUP
    IFNONNULL L20
    POP
    ALOAD 9
    IFNULL L21
    GOTO L22
   L20
    ALOAD 9
    INVOKEVIRTUAL java/lang/Object.equals (Ljava/lang/Object;)Z
    IFEQ L22
   L21
    BIPUSH 21
    IRETURN
   L22
    NEW scala/MatchError
    DUP
    ALOAD 2
    INVOKESPECIAL scala/MatchError.<init> (Ljava/lang/Object;)V
    ATHROW
   L23
    LOCALVARIABLE this Lapp/Main$; L0 L23 0
    LOCALVARIABLE n Lapp/Main$Z; L0 L23 1
    MAXSTACK = 3
    MAXLOCALS = 10

Expectation

[som-snytt]

A long-standing request, which is to say, an old request for a contribution. Is GSOC set for this year?

[odersky]

The Java version is described here: #5537 (comment). It uses a table switch, but is a lot more complex than a simple switch on ordinal. What exactly does Kotlin do? As I understand it, there's a trade-off between code simplicity and speed vs separate compilation guarantees.

[nmichael44]

@odersky @som-snytt Here is the same function in Kotlin and the code generated.

enum class Z {
    A, B, C, D, E, F, G
}

fun f(n: Z): Int =
  when (n) {
      Z.A -> 23
      Z.B -> 12
      Z.C -> -1
      Z.D -> 4
      Z.E -> 12
      Z.F -> -11
      Z.G -> 21
  }

Generated code for f:

  public final static f(Lneo/org/app/Z;)I
    // annotable parameter count: 1 (invisible)
    @Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 0
   L0
    ALOAD 0
    LDC "n"
    INVOKESTATIC kotlin/jvm/internal/Intrinsics.checkNotNullParameter (Ljava/lang/Object;Ljava/lang/String;)V
   L1
    LINENUMBER 116 L1
    ALOAD 0
    GETSTATIC neo/org/app/AppKt$WhenMappings.$EnumSwitchMapping$1 : [I
    SWAP
    INVOKEVIRTUAL neo/org/app/Z.ordinal ()I
    IALOAD
    TABLESWITCH
      1: L2
      2: L3
      3: L4
      4: L5
      5: L6
      6: L7
      7: L8
      default: L9
   L2
    LINENUMBER 117 L2
    BIPUSH 23
    GOTO L10
   L3
    LINENUMBER 118 L3
    BIPUSH 12
    GOTO L10
   L4
    LINENUMBER 119 L4
    ICONST_M1
    GOTO L10
   L5
    LINENUMBER 120 L5
    ICONST_4
    GOTO L10
   L6
    LINENUMBER 121 L6
    BIPUSH 12
    GOTO L10
   L7
    LINENUMBER 122 L7
    BIPUSH -11
    GOTO L10
   L8
    LINENUMBER 123 L8
    BIPUSH 21
    GOTO L10
   L9
    LINENUMBER 116 L9
    NEW kotlin/NoWhenBranchMatchedException
    DUP
    INVOKESPECIAL kotlin/NoWhenBranchMatchedException.<init> ()V
    ATHROW
   L10
    LINENUMBER 124 L10
    IRETURN
   L11
    LOCALVARIABLE n Lneo/org/app/Z; L0 L11 0
    MAXSTACK = 2
    MAXLOCALS = 1

[odersky]

Thanks for reporting this! So Kotlin does use a simple table switch based on ordinal. Whereas Java jumps through a lot of hoops, involving one inner class for each enum switch, to ensure that reorderings of enums don't break binary compatibility.

Personally, I am not convinced binary compatibility under re-orderings is necessary here. Reordering an enum changes the ordinal values of enum constants. That's a semantic change, which should be done only if all clients can be inspected to not rely on precise ordinal values. So it seems strange to require a manual verification of client code but not require a recompile. Seen in another way, no public library should re-order enums, because clients could break. So why insist on that change being binary compatible?

[nmichael44]

@odersky I agree with your reasoning. Doing this optimization is a good change.

[He-Pin]

@nmichael44 @odersky maybe #23264 can help sometime?
And in Java 17 there is a https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/runtime/SwitchBootstraps.html#enumSwitch(java.lang.invoke.MethodHandles.Lookup,java.lang.String,java.lang.invoke.MethodType,java.lang.Object...)