Improve filter for Kotlin when-expressions with String

[Godin]

For the following Example.kt

fun example1(p: String) {
  when (p) {
    "a"  -> return
    "b"  -> return
    else -> return
  }
}

fun example2(p: String) {
  when (p) { // first case has biggest hashCode value
    "b"  -> return
    "a"  -> return
    else -> return
  }
}

fun main(args: Array<String>) {
  example1("")
  example1("a")
  example1("b")
  
  example2("")
  example2("a")
  example2("b")
}

Execution of

kotlin/bin/kotlinc Example.kt

java \
  -javaagent:jacoco-0.8.6/lib/jacocoagent.jar \
  -cp kotlin/lib/kotlin-stdlib.jar:. \
  ExampleKt

java \
  -jar jacoco-0.8.6/lib/jacococli.jar \
  report jacoco.exec \
  --classfiles ExampleKt.class \
  --sourcefiles . \
  --html report

produces following report

Execution of

javap -v -p ExampleKt.class

(tested for kotlinc versions from 1.3.0 to 1.4.30)

shows that Kotlin compiler generates

  public static final void example1(java.lang.String);
    descriptor: (Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL
    Code:
      stack=2, locals=2, args_size=1
         0: aload_0
         1: ldc           #9                  // String p
         3: invokestatic  #15                 // Method kotlin/jvm/internal/Intrinsics.checkNotNullParameter:(Ljava/lang/Object;Ljava/lang/String;)V
         6: aload_0
         7: astore_1
         8: aload_1
         9: invokevirtual #21                 // Method java/lang/String.hashCode:()I
        12: tableswitch   { // 97 to 98
                      97: 36
                      98: 48
                 default: 62
            }
        36: aload_1
        37: ldc           #23                 // String a
        39: invokevirtual #27                 // Method java/lang/String.equals:(Ljava/lang/Object;)Z
        42: ifeq          62
        45: goto          60
        48: aload_1
        49: ldc           #29                 // String b
        51: invokevirtual #27                 // Method java/lang/String.equals:(Ljava/lang/Object;)Z
        54: ifeq          62
        57: goto          61
        60: return
        61: return
        62: return

  public static final void example2(java.lang.String);
    descriptor: (Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL
    Code:
      stack=2, locals=2, args_size=1
         0: aload_0
         1: ldc           #9                  // String p
         3: invokestatic  #15                 // Method kotlin/jvm/internal/Intrinsics.checkNotNullParameter:(Ljava/lang/Object;Ljava/lang/String;)V
         6: aload_0
         7: astore_1
         8: aload_1
         9: invokevirtual #21                 // Method java/lang/String.hashCode:()I
        12: tableswitch   { // 97 to 98
                      97: 36
                      98: 48
                 default: 59
            }
        36: aload_1
        37: ldc           #23                 // String a
        39: invokevirtual #27                 // Method java/lang/String.equals:(Ljava/lang/Object;)Z
        42: ifeq          59
        45: goto          58
        48: aload_1
        49: ldc           #29                 // String b
        51: invokevirtual #27                 // Method java/lang/String.equals:(Ljava/lang/Object;)Z
        54: ifeq          59
        57: return
        58: return
        59: return

where method example1 has goto at offset 57,
whereas no such goto in example2.

KotlinWhenStringFilter does not filter bytecode of example2 due to absence of this goto

jacoco/org.jacoco.core/src/org/jacoco/core/internal/analysis/filter/KotlinWhenStringFilter.java

Lines 86 to 87 in 492f421

	// jump to case
	nextIs(Opcodes.GOTO);

This was discovered by @margarita-nedzelska-sonarsource ❤️

[Godin]

The same example after this change

[marchof]

👏 Thank‘s @Godin! You‘re the first human decompiler 😅