JVM_IR: generate WhenMappings for enum ordinal subjects

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/DeclarationOrigins.kt

@@ -35,6 +35,7 @@ interface JvmLoweredDeclarationOrigin : IrDeclarationOrigin {
         IrDeclarationOriginImpl("SYNTHETIC_METHOD_FOR_PROPERTY_ANNOTATIONS", isSynthetic = true)
     object GENERATED_PROPERTY_REFERENCE : IrDeclarationOriginImpl("GENERATED_PROPERTY_REFERENCE", isSynthetic = true)
     object GENERATED_SAM_IMPLEMENTATION : IrDeclarationOriginImpl("GENERATED_SAM_IMPLEMENTATION", isSynthetic = true)
+    object ENUM_MAPPINGS_FOR_WHEN : IrDeclarationOriginImpl("ENUM_MAPPINGS_FOR_WHEN", isSynthetic = true)
 }
 
 interface JvmLoweredStatementOrigin : IrStatementOrigin {

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/JvmLower.kt

@@ -61,12 +61,6 @@ private val propertiesPhase = makeIrFilePhase<CommonBackendContext>(
     stickyPostconditions = setOf((PropertiesLowering)::checkNoProperties)
 )
 
-private val enumWhenPhase = makeIrFilePhase(
-    ::EnumWhenLowering,
-    name = "EnumWhenLowering",
-    description = "Replace `when` subjects of enum types with their ordinals"
-)
-
 val jvmPhases = namedIrFilePhase<JvmBackendContext>(
     name = "IrLowering",
     description = "IR lowering",

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/MappedEnumWhenLowering.kt

@@ -0,0 +1,136 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.backend.jvm.lower
+
+import org.jetbrains.kotlin.backend.common.CommonBackendContext
+import org.jetbrains.kotlin.backend.common.ir.createImplicitParameterDeclarationWithWrappedDescriptor
+import org.jetbrains.kotlin.backend.common.lower.EnumWhenLowering
+import org.jetbrains.kotlin.backend.common.lower.createIrBuilder
+import org.jetbrains.kotlin.backend.common.phaser.makeIrFilePhase
+import org.jetbrains.kotlin.backend.jvm.JvmLoweredDeclarationOrigin
+import org.jetbrains.kotlin.ir.IrStatement
+import org.jetbrains.kotlin.ir.UNDEFINED_OFFSET
+import org.jetbrains.kotlin.ir.builders.*
+import org.jetbrains.kotlin.ir.builders.declarations.addField
+import org.jetbrains.kotlin.ir.builders.declarations.buildClass
+import org.jetbrains.kotlin.ir.declarations.IrClass
+import org.jetbrains.kotlin.ir.declarations.IrEnumEntry
+import org.jetbrains.kotlin.ir.declarations.IrField
+import org.jetbrains.kotlin.ir.expressions.IrExpression
+import org.jetbrains.kotlin.ir.expressions.impl.IrGetEnumValueImpl
+import org.jetbrains.kotlin.ir.types.getClass
+import org.jetbrains.kotlin.ir.util.*
+import org.jetbrains.kotlin.ir.visitors.transformChildrenVoid
+import org.jetbrains.kotlin.name.Name
+import org.jetbrains.kotlin.util.OperatorNameConventions
+
+internal val enumWhenPhase = makeIrFilePhase(
+    ::MappedEnumWhenLowering,
+    name = "EnumWhenLowering",
+    description = "Replace `when` subjects of enum types with their ordinals"
+)
+
+// A version of EnumWhenLowering that is more friendly to incremental compilation. For example,
+// suppose the code initially looks like this:
+//
+//     // 1.kt
+//     enum E { X }
+//
+//     // 2.kt
+//     fun f(e: E) = when (e) { E.X -> 1 }
+//
+// EnumWhenLowering would transform 2.kt into this:
+//
+//     fun f(e: E) = when (e.ordinal()) { 0 -> 1 }
+//
+// While this lowering would generate (approximately) this instead:
+//
+//     fun f(e: E) = when (WhenMappings.$EnumSwitchMapping$0[e.ordinal()]) { 1 -> 1 }
+//
+//     object WhenMappings {
+//         // Note the runtime call to ordinal(): 0 is not hardcoded.
+//         val $EnumSwitchMapping$0 = IntArray(E.values().size).also { it[E.X.ordinal()] = 1 }
+//     }
+//
+// The latter would not need to be recompiled if new entries were added before `X`
+// at the negligible cost of an additional initializer per run + one array read per call.
+//
+private class MappedEnumWhenLowering(context: CommonBackendContext) : EnumWhenLowering(context) {
+    private val intArray = context.irBuiltIns.primitiveArrayForType.getValue(context.irBuiltIns.intType)
+    private val intArrayConstructor = intArray.constructors.single { it.owner.valueParameters.size == 1 }
+    private val intArrayGet = intArray.functions.single { it.owner.name == OperatorNameConventions.GET }
+    private val intArraySet = intArray.functions.single { it.owner.name == OperatorNameConventions.SET }
+    private val refArraySize = context.irBuiltIns.arrayClass.owner.properties.single { it.name.toString() == "size" }.getter!!
+
+    // To avoid visibility-related issues, classes containing the mappings are direct children
+    // of the classes in which they are used. This field tracks which container is the innermost one.
+    private var state: EnumMappingState? = null
+
+    private class EnumMappingState {
+        val mappings = mutableMapOf<IrClass /* enum */, Pair<MutableMap<IrEnumEntry, Int>, IrField>>()
+        val mappingsClass by lazy {
+            buildClass {
+                name = Name.identifier("WhenMappings")
+                origin = JvmLoweredDeclarationOrigin.ENUM_MAPPINGS_FOR_WHEN
+            }.apply {
+                createImplicitParameterDeclarationWithWrappedDescriptor()
+            }
+        }
+    }
+
+    override fun mapConstEnumEntry(entry: IrEnumEntry): Int {
+        val (mapping, _) = state!!.mappings.getOrPut(entry.parentAsClass) {
+            mutableMapOf<IrEnumEntry, Int>() to state!!.mappingsClass.addField {
+                name = Name.identifier("\$EnumSwitchMapping\$${state!!.mappings.size}")
+                type = intArray.owner.defaultType
+                origin = JvmLoweredDeclarationOrigin.ENUM_MAPPINGS_FOR_WHEN
+                isFinal = true
+                isStatic = true
+            }
+        }
+        // Index 0 (default value for integers) is reserved for unknown ordinals.
+        return mapping.getOrPut(entry) { mapping.size + 1 }
+    }
+
+    override fun mapRuntimeEnumEntry(builder: IrBuilderWithScope, subject: IrExpression): IrExpression =
+        builder.irCall(intArrayGet).apply {
+            val (_, field) = state!!.mappings[subject.type.getClass()!!]
+                ?: throw AssertionError("no values mapped for enum class ${subject.type}")
+            dispatchReceiver = builder.irGetField(null, field)
+            putValueArgument(0, super.mapRuntimeEnumEntry(builder, subject))
+        }
+
+    override fun visitClassNew(declaration: IrClass): IrStatement {
+        val oldState = state
+        state = EnumMappingState()
+        super.visitClassNew(declaration)
+
+        for ((enum, mappingAndField) in state!!.mappings) {
+            val (mapping, field) = mappingAndField
+            val builder = context.createIrBuilder(state!!.mappingsClass.symbol)
+            val enumValues = enum.functions.single { it.name.toString() == "values" }
+            field.initializer = builder.irExprBody(builder.irBlock {
+                val enumSize = irCall(refArraySize).apply { dispatchReceiver = irCall(enumValues) }
+                val result = irTemporary(irCall(intArrayConstructor).apply { putValueArgument(0, enumSize) })
+                for ((entry, index) in mapping) {
+                    val runtimeEntry = IrGetEnumValueImpl(UNDEFINED_OFFSET, UNDEFINED_OFFSET, enum.defaultType, entry.symbol)
+                    +irCall(intArraySet).apply {
+                        dispatchReceiver = irGet(result)
+                        putValueArgument(0, super.mapRuntimeEnumEntry(builder, runtimeEntry)) // <entry>.ordinal()
+                        putValueArgument(1, irInt(index))
+                    }
+                }
+                +irGet(result)
+            })
+        }
+
+        if (state!!.mappings.isNotEmpty()) {
+            declaration.declarations += state!!.mappingsClass.apply { parent = declaration }
+        }
+        state = oldState
+        return declaration
+    }
+}