Kotlin Under the Hood

Transcript

1. KOTLIN UNDER THE
   HOOD
   VIENNA KOTLIN MEETUP, 2017-11-20
   Christoph Leiter
   

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2. INTRO
   We will look at what the compiler does
   Basics
   Methods
   Classes
   Advanced
   Used current Kotlin 1.1.60
   Not focused on performance
   

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3. BASICS
   

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4. PRIMITIVE TYPES
   fun primitiveType() {
   val a = 1
   val b: Int? = 3
   println(a)
   println(b)
   }
   

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5. PRIMITIVE TYPES
   // access flags 0x19
   public final static primitiveType()V
   L0
   LINENUMBER 4 L0
   ICONST_1
   ISTORE 0
   L1
   LINENUMBER 5 L1
   ICONST_3
   INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
   ASTORE 1
   L2
   LINENUMBER 7 L2
   L3
   GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
   ILOAD 0
   INVOKEVIRTUAL java/io/PrintStream.println (I)V
   L4
   L5
   LINENUMBER 8 L5
   L6
   

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6. PRIMITIVE TYPES
   public static final void primitiveType() {
   int a = 1;
   Integer b = Integer.valueOf(3);
   System.out.println(a);
   System.out.println(b);
   }
   

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7. NON NULLABLE PARAMETER
   Intrinsics you say?
   fun nonNullableParameter(s: String) = s
   @NotNull
   public static final String nonNullableParameter(@NotNull String s)
   Intrinsics.checkParameterIsNotNull(s, "s");
   return s;
   }
   

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8. INTRINSICS
   Part of the Kotlin runtime
   Resides in kotlin.jvm.internal
   Used by the compiler to reference commonly
   needed methods
   public static void checkParameterIsNotNull(Object value, String pa
   if (value == null) {
   throwParameterIsNullException(paramName);
   }
   }
   

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9. NON NULL IN PRIVATE METHOD
   Not needed in private methods since the Kotlin
   compiler can guarantee null is never passed.
   class NonNullInPrivateMethod {
   private fun a(s: String) = s
   }
   public final class NonNullInPrivateMethod {
   private final String a(String s) {
   return s;
   }
   }
   

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10. COMPARISONS
    fun comparisons(a: String, b: String) {
    if (a == b) println("Equals")
    if (a === b) println("Same instance")
    }
    

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11. COMPARISONS
    public static final void comparisons(@NotNull String a, @NotNull S
    Intrinsics.checkParameterIsNotNull(a, "a");
    Intrinsics.checkParameterIsNotNull(b, "b");
    String var2;
    if (Intrinsics.areEqual(a, b)) {
    var2 = "Equals";
    System.out.println(var2);
    }
    if (a == b) {
    var2 = "Same instance";
    System.out.println(var2);
    }
    }
    

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12. COMPARISONS
    Automatically optimized for null case
    public static boolean areEqual(Object first, Object second) {
    return first == null ? second == null : first.equals(second);
    }
    

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13. IF EXPRESSION
    We have some alternative to write this method
    All the same
    fun ifExpression(b: Boolean): String {
    return if (b) "foo" else "buz"
    }
    @NotNull
    public static final String ifExpression(boolean b) {
    return b ? "foo" : "buz";
    }
    fun ifMethodExpression(b: Boolean): String =
    if (b) "foo" else "buz"
    fun ifMethodExpressionWithoutReturnType(b: Boolean) =
    if (b) "foo" else "buz"
    

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14. WHEN EXPRESSION
    fun whenExpression(i: Int) =
    when (i) {
    0 -> "zero"
    1 -> "one"
    in 2..9 -> "under ten"
    else -> "something else"
    }
    

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15. WHEN EXPRESSION
    @NotNull
    public static final String whenExpression(int i) {
    String var10000;
    if (i == 0) {
    var10000 = "zero";
    } else if (i == 1) {
    var10000 = "one";
    } else {
    if (2 <= i) {
    if (9 >= i) {
    var10000 = "under ten";
    return var10000;
    }
    }
    var10000 = "something else";
    }
    return var10000;
    }
    

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16. STRING INTERPOLATION
    fun stringInterpolation(a: Int, b: Int?, s: String) =
    "Result is: $a $b $s ${a * (b ?: 3)}"
    @NotNull
    public static final String stringInterpolation
    (int a, @Nullable Integer b, @NotNull String s) {
    Intrinsics.checkParameterIsNotNull(s, "s");
    return "Result is: " + a + ' ' + b + ' ' + s + ' ' +
    a * (b != null ? b.intValue() : 3);
    }
    

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17. STRING INTERPOLATION
    // access flags 0x19
    public final static stringInterpolation(ILjava/lang/Integer;Ljava/
    @Lorg/jetbrains/annotations/NotNull;() // invisible
    @Lorg/jetbrains/annotations/Nullable;() // invisible, parameter
    @Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 2
    L0
    ALOAD 2
    LDC "s"
    INVOKESTATIC kotlin/jvm/internal/Intrinsics.checkParameterIsNotN
    L1
    LINENUMBER 40 L1
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder. ()V
    LDC "Result is: "
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;
    ILOAD 0
    INVOKEVIRTUAL java/lang/StringBuilder.append (I)Ljava/lang/Strin
    BIPUSH 32
    INVOKEVIRTUAL java/lang/StringBuilder.append (C)Ljava/lang/Strin
    ALOAD 1
    

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18. TYPE CHECKS & CASTS
    fun typeCheck(v: Any?) {
    if (v is String) println("String has length ${v.length}")
    val t = v as? String
    val u = v as String
    }
    

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19. TYPE CHECKS & CASTS
    public static final void typeCheck(@Nullable Object v) {
    String t;
    if (v instanceof String) {
    t = "String has length " + ((String)v).length();
    System.out.println(t);
    }
    Object var10000 = v;
    if (!(v instanceof String)) {
    var10000 = null;
    }
    t = (String)var10000;
    if (v == null) {
    throw new TypeCastException("null cannot be cast to non-null
    } else {
    String u = (String)v;
    }
    }
    

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20. METHODS
    

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21. NAMED ARGUMENTS
    fun namedArguments(a: String, b: Int, c: Long) {
    println("$a $b $c")
    }
    fun namedArgumentsCall() {
    namedArguments(c = 12L, a = "foo", b = 42)
    }
    public static final void namedArgumentsCall() {
    byte var0 = 42;
    String var1 = "foo";
    long var2 = 12L;
    namedArguments(var1, var0, var2);
    }
    

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22. DEFAULT ARGUMENTS
    fun defaultArguments(a: Int = 42, b: String = "Hello", c: Boolean
    fun defaultArgumentsCall() {
    defaultArguments(a = 12, b = "Bye", c = true)
    defaultArguments(a = 12, b = "Bye")
    defaultArguments(a = 12, c = true)
    defaultArguments()
    }
    

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23. DEFAULT ARGUMENTS
    public static final int defaultArguments(int a, @NotNull String b,
    Intrinsics.checkParameterIsNotNull(b, "b");
    return a;
    }
    // $FF: synthetic method
    // $FF: bridge method
    public static int defaultArguments$default(int var0, String var1,
    if ((var3 & 1) != 0) {
    var0 = 42;
    }
    if ((var3 & 2) != 0) {
    var1 = "Hello";
    }
    if ((var3 & 4) != 0) {
    var2 = true;
    }
    return defaultArguments(var0, var1, var2);
    

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24. DEFAULT ARGUMENTS WITH
    @JVMOVERLOADS
    @JvmOverloads
    fun defaultArguments(a: Int = 42, b: String = "Hello", c: Boolean
    @JvmOverloads
    public static final int defaultArguments(int a, @NotNull String b)
    return defaultArguments$default(a, b, false, 4, (Object)null);
    }
    @JvmOverloads
    public static final int defaultArguments(int a) {
    return defaultArguments$default(a, (String)null, false, 6, (Obj
    }
    @JvmOverloads
    public static final int defaultArguments() {
    return defaultArguments$default(0, (String)null, false, 7, (Obj
    }
    

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25. PRECONDITIONS
    fun requires(i: Int) {
    require(i > 10)
    if (i == 3)
    println("Nope")
    }
    public static final void requires(int i) {
    boolean var1 = i > 10;
    if (!var1) {
    String var2 = "Failed requirement.";
    throw (Throwable)(new IllegalArgumentException(var2.toString
    } else {
    if (i == 3) {
    String var3 = "Nope";
    System.out.println(var3);
    }
    }
    }
    

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26. INLINE FUNCTIONS
    Kotlin can inline functions marked with inline
    Basically copy pastes method at compile time to call
    site. Absolutely no overhead at runtime
    @kotlin.internal.InlineOnly
    public inline fun require(value: Boolean): Unit =
    require(value) { "Failed requirement." }
    @kotlin.internal.InlineOnly
    public inline fun require(value: Boolean, lazyMessage: () -> Any):
    if (!value) {
    val message = lazyMessage()
    throw IllegalArgumentException(message.toString())
    }
    }
    

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27. COLLECTION API
    fun mapList(list: List) =
    list.filter { it.length >= 3 }.map { it.toUpperCase() }
    @NotNull
    public static final List mapList(@NotNull List list) {
    Intrinsics.checkParameterIsNotNull(list, "list");
    Iterable $receiver$iv = (Iterable)list;
    Collection destination$iv$iv = (Collection)(new ArrayList());
    Iterator var4 = $receiver$iv.iterator();
    Object item$iv$iv;
    String it;
    while(var4.hasNext()) {
    item$iv$iv = var4.next();
    it = (String)item$iv$iv;
    if (it.length() >= 3) {
    destination$iv$iv.add(item$iv$iv);
    }
    }
    $receiver$iv = (Iterable)((List)destination$iv$iv);
    destination$iv$iv = (Collection)(new ArrayList(CollectionsKt.co
    

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28. var4 = $receiver$iv.iterator();
    

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29. COLLECTION API
    Long story short:
    There's no method call overhead for the collection
    API since everything is inlined anyway.
    But there's an overhead for object allocations
    

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30. TAIL RECURSION
    Let's try to make this tail recursive
    fun factorial(n: Long): Long =
    if (n == 1L) 1
    else n * factorial(n - 1)
    public static final long factorial(long n) {
    return n == 1L ? 1L : n * factorial(n - 1L);
    }
    fun factorial2(n: Long, total: Long = 1): Long =
    if (n == 1L) total
    else factorial2(n - 1, total * n)
    public static final long factorial2(long n, long total) {
    return n == 1L ? total : factorial2(n - 1L, total * n);
    }
    

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31. TAIL RECURSION
    Adding tailrec keyword
    tailrec fun factorialTailrec(n: Long, total: Long = 1): Long =
    if (n == 1L) total
    else factorialTailrec(n - 1, total * n)
    public static final long factorialTailrec(long n, long total) {
    while(n != 1L) {
    long var10000 = n - 1L;
    total *= n;
    n = var10000;
    }
    return total;
    }
    

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32. EXTENSION FUNCTIONS
    What's this $receiver thingy?
    fun Int.clamp(lo: Int, hi: Int) =
    when {
    this < lo -> lo
    this > hi -> hi
    else -> this
    }
    public static final int clamp(int $receiver, int lo, int hi) {
    return $receiver < lo ? lo : ($receiver > hi ? hi : $receiver);
    }
    fun callClamp() {
    6.clamp(5, 10)
    }
    public static final void callClamp() {
    clamp(6, 5, 10);
    }
    

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33. NESTED FUNCTION
    fun nestedFunction(i : Int) {
    fun nested(x: Int) = i * x
    nested(42)
    }
    public static final void nestedFunction(final int i) {
    nested$ = new Function1() {
    // $FF: synthetic method
    // $FF: bridge method
    public Object invoke(Object var1) {
    return this.invoke(((Number)var1).intValue());
    }
    public final int invoke(int x) {
    return i * x;
    }
    };
    nested$.invoke(42);
    }
    

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34. CLASSES
    

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35. PROPERTY
    @Metadata used by compiler and kotlin-reflect
    class Property {
    var a: String = ""
    }
    @Metadata(
    mv = {1, 1, 8},
    bv = {1, 0, 2},
    k = 1,
    d1 = {"\u0000\u0014\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u
    d2 = {"LProperty;", "", "()V", "a", "", "getA", "()Ljava/lang/S
    )
    public final class Property {
    @NotNull
    private String a = "";
    @NotNull
    public final String getA() {
    t thi
    

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36. SINGLETON OBJECTS
    object Obj
    public final class Obj {
    public static final Obj INSTANCE;
    private Obj() {
    INSTANCE = (Obj)this;
    }
    static {
    new Obj();
    }
    }
    

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37. DATA CLASS
    data class D(
    val a: Int,
    val b: String
    ) {
    init {
    require(a > 0)
    }
    }
    

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38. DATA CLASS
    public final class D {
    private final int a;
    @NotNull
    private final String b;
    public final int getA() {
    return this.a;
    }
    @NotNull
    public final String getB() {
    return this.b;
    }
    public D(int a, @NotNull String b) {
    Intrinsics.checkParameterIsNotNull(b, "b");
    super();
    this.a = a;
    this.b = b;
    boolean var3 = this.a > 0;
    if (!var3) {
    

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39. INIT BLOCKS
    class InitBlocks(s: String) {
    init {
    println("block 1")
    }
    var x: String
    init {
    println("block 2")
    x = "x"
    }
    var y = "y"
    init {
    println("block 3 $x $y")
    }
    }
    

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40. INIT BLOCKS
    public final class InitBlocks {
    @NotNull
    private String x;
    @NotNull
    private String y;
    @NotNull
    public final String getX() {
    return this.x;
    }
    public final void setX(@NotNull String var1) {
    Intrinsics.checkParameterIsNotNull(var1, "");
    this.x = var1;
    }
    @NotNull
    public final String getY() {
    return this.y;
    }
    

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41. ADVANCED
    

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42. A.K.A. RANDOM STUFF
    

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43. COMPILE TIME REFERENCES
    Boring but efficient and refactoring safe
    class CompileTimeReferences {
    var n: String = "name"
    fun x() { }
    }
    fun useCompileTimeReference() {
    println(CompileTimeReferences::n.name)
    println(CompileTimeReferences::x.name)
    }
    public static final void useCompileTimeReference() {
    String var0 = "n";
    System.out.println(var0);
    var0 = "x";
    System.out.println(var0);
    }
    

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44. REIFIED GENERIC TYPES
    inline fun genericType() {
    println("Type of T is ${T::class}")
    }
    private static final void genericType() {
    StringBuilder var10000 = (new StringBuilder()).append("Type of
    Intrinsics.reifiedOperationMarker(4, "T");
    String var1 = var10000.append(Reflection.getOrCreateKotlinClass
    System.out.println(var1);
    }
    public static void reifiedOperationMarker(int id, String typeParam
    throwUndefinedForReified();
    }
    public static void throwUndefinedForReified() {
    throwUndefinedForReified(
    "This function has a reified type parameter and thus can o
    );
    }
    

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45. REIFIED GENERIC TYPES
    Let's call this function:
    fun callGenericType() {
    genericType()
    }
    public static final void callGenericType() {
    String var0 = "Type of T is " +
    Reflection.getOrCreateKotlinClass(String.class);
    System.out.println(var0);
    }
    

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46. LAMBDA
    fun execute(lambda: () -> Unit) {
    lambda()
    }
    fun simpleLambda() {
    val x = 1
    execute {
    println(x)
    }
    }
    

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47. LAMBDA
    public static final void simpleLambda() {
    final int x = 1;
    execute((Function0)(new Function0() {
    // $FF: synthetic method
    // $FF: bridge method
    public Object invoke() {
    this.invoke();
    return Unit.INSTANCE;
    }
    public final void invoke() {
    int var1 = x;
    System.out.println(var1);
    }
    }));
    }
    

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48. MUTATING LAMBDA
    Note that you can not do that easily in Java
    fun mutatingLambda() {
    var x = 1
    var y = Any()
    execute {
    x = 2
    y = "y"
    }
    }
    

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49. MUTATING LAMBDA
    public static final void mutatingLambda() {
    final IntRef x = new IntRef();
    x.element = 1;
    final ObjectRef y = new ObjectRef();
    y.element = new Object();
    execute((Function0)(new Function0() {
    // $FF: synthetic method
    // $FF: bridge method
    public Object invoke() {
    this.invoke();
    return Unit.INSTANCE;
    }
    public final void invoke() {
    x.element = 2;
    y.element = "y";
    }
    }));
    }
    

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50. SUMMARY
    It's good to know how Kotlin internals work
    Helps with debugging tricky errors
    Most of the stuff is pretty straight forward
    Created byte code might look inefficient sometimes,
    but JIT takes care of that
    It's easy to look at bytecode in IntelliJ
    Tools -> Kotlin -> Show Kotlin Bytecode
    -> Decompile
    

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