[JS IR BE] Remove conservative unit materialization

Remove UnitMaterializationLowering
Remove Unit insertion in BridgeConstruction
Fix codegen for when expressions without else branch

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/UnitMaterializationLowering.kt

@@ -1,45 +0,0 @@
-/*
- * Copyright 2010-2018 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.common.lower
-
-import org.jetbrains.kotlin.backend.common.BackendContext
-import org.jetbrains.kotlin.backend.common.FileLoweringPass
-import org.jetbrains.kotlin.ir.UNDEFINED_OFFSET
-import org.jetbrains.kotlin.ir.declarations.IrFile
-import org.jetbrains.kotlin.ir.expressions.IrCall
-import org.jetbrains.kotlin.ir.expressions.IrExpression
-import org.jetbrains.kotlin.ir.expressions.impl.IrCompositeImpl
-import org.jetbrains.kotlin.ir.expressions.impl.IrGetObjectValueImpl
-import org.jetbrains.kotlin.ir.symbols.IrClassSymbol
-import org.jetbrains.kotlin.ir.types.classifierOrFail
-import org.jetbrains.kotlin.ir.types.isUnit
-import org.jetbrains.kotlin.ir.visitors.IrElementTransformerVoid
-import org.jetbrains.kotlin.ir.visitors.transformChildrenVoid
-
-class UnitMaterializationLowering(context: BackendContext) : FileLoweringPass {
-
-    private val unitType = context.irBuiltIns.unitType
-    private val unitValue
-        get() = IrGetObjectValueImpl(
-            UNDEFINED_OFFSET,
-            UNDEFINED_OFFSET,
-            unitType,
-            unitType.classifierOrFail as IrClassSymbol
-        )
-
-    override fun lower(irFile: IrFile) {
-        irFile.transformChildrenVoid(object : IrElementTransformerVoid() {
-            override fun visitCall(expression: IrCall): IrExpression {
-                expression.transformChildrenVoid(this)
-                return if (expression.type.isUnit()) expression.let {
-                    IrCompositeImpl(it.startOffset, it.endOffset, it.type, it.origin, listOf(it, unitValue))
-                }
-                else expression
-            }
-        })
-    }
-
-}

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/JsLoweringPhases.kt

@@ -133,13 +133,6 @@ private val tailrecLoweringPhase = makeJsModulePhase(
     description = "Replace `tailrec` callsites with equivalent loop"
 )
 
-private val unitMaterializationLoweringPhase = makeJsModulePhase(
-    ::UnitMaterializationLowering,
-    name = "UnitMaterializationLowering",
-    description = "Insert Unit object where it is supposed to be",
-    prerequisite = setOf(tailrecLoweringPhase)
-)
-
 private val enumClassConstructorLoweringPhase = makeJsModulePhase(
     ::EnumClassConstructorLowering,
     name = "EnumClassConstructorLowering",
@@ -201,8 +194,7 @@ private val innerClassConstructorCallsLoweringPhase = makeJsModulePhase(
 private val suspendFunctionsLoweringPhase = makeJsModulePhase(
     ::JsSuspendFunctionsLowering,
     name = "SuspendFunctionsLowering",
-    description = "Transform suspend functions into CoroutineImpl instance and build state machine",
+    description = "Transform suspend functions into CoroutineImpl instance and build state machine"
-    prerequisite = setOf(unitMaterializationLoweringPhase)
 )
 
 private val privateMembersLoweringPhase = makeJsModulePhase(
@@ -389,7 +381,6 @@ val jsPhases = namedIrModulePhase(
             enumClassLoweringPhase then
             enumUsageLoweringPhase then
             returnableBlockLoweringPhase then
-            unitMaterializationLoweringPhase then
             suspendFunctionsLoweringPhase then
             privateMembersLoweringPhase then
             callableReferenceLoweringPhase then

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/lower/BridgesConstruction.kt

@@ -105,8 +105,6 @@ class BridgesConstruction(val context: JsIrBackendContext) : ClassLoweringPass {
         }
     }
 
-    private val unitValue = JsIrBuilder.buildGetObjectValue(context.irBuiltIns.unitType, context.irBuiltIns.unitClass)
-
     // Ported from from jvm.lower.BridgeLowering
     private fun createBridge(
         function: IrSimpleFunction,
@@ -168,18 +166,7 @@ class BridgesConstruction(val context: JsIrBackendContext) : ClassLoweringPass {
                 call.putValueArgument(i, irCastIfNeeded(irGet(valueParameter), delegateTo.valueParameters[i].type))
             }
 
-            // This is required for Unit materialization
+            +irReturn(call)
-            // TODO: generalize for boxed types and inline classes
-            // TODO: use return type in signature too
-            val returnValue = if (delegateTo.returnType.isUnit() && !function.returnType.isUnit()) {
-                irComposite(resultType = irFunction.returnType) {
-                    +call
-                    +unitValue
-                }
-            } else {
-                call
-            }
-            +irReturn(returnValue)
         }.apply {
             irFunction.body = this
         }

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/transformers/irToJs/IrElementToJsExpressionTransformer.kt

@@ -5,12 +5,14 @@
 
 package org.jetbrains.kotlin.ir.backend.js.transformers.irToJs
 
+import org.jetbrains.kotlin.backend.common.ir.isElseBranch
 import org.jetbrains.kotlin.descriptors.ClassKind
 import org.jetbrains.kotlin.ir.backend.js.utils.*
 import org.jetbrains.kotlin.ir.declarations.IrClass
 import org.jetbrains.kotlin.ir.declarations.IrConstructor
 import org.jetbrains.kotlin.ir.declarations.IrSimpleFunction
 import org.jetbrains.kotlin.ir.expressions.*
+import org.jetbrains.kotlin.ir.types.isUnit
 import org.jetbrains.kotlin.ir.util.*
 import org.jetbrains.kotlin.js.backend.ast.*
 import org.jetbrains.kotlin.util.OperatorNameConventions
@@ -309,8 +311,16 @@ class IrElementToJsExpressionTransformer : BaseIrElementToJsNodeTransformer<JsEx
     }
 
     override fun visitWhen(expression: IrWhen, context: JsGenerationContext): JsExpression {
-        // TODO check when w/o else branch and empty when
+        val lastBranch = expression.branches.lastOrNull()
-        return expression.toJsNode(this, context, ::JsConditional)!!
+        val implicitElse =
+            if (lastBranch == null || !isElseBranch(lastBranch))
+                JsPrefixOperation(JsUnaryOperator.VOID, JsIntLiteral(0))
+            else
+                null
+
+        assert(implicitElse == null || expression.type.isUnit()) { "Non unit when-expression must have else branch" }
+
+        return expression.toJsNode(this, context, ::JsConditional, implicitElse)!!
     }
 
     override fun visitTypeOperator(expression: IrTypeOperatorCall, data: JsGenerationContext): JsExpression {

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/transformers/irToJs/jsAstUtils.kt

@@ -23,9 +23,10 @@ fun jsVar(name: JsName, initializer: IrExpression?, context: JsGenerationContext
 fun <T : JsNode> IrWhen.toJsNode(
     tr: BaseIrElementToJsNodeTransformer<T, JsGenerationContext>,
     data: JsGenerationContext,
-    node: (JsExpression, T, T?) -> T
+    node: (JsExpression, T, T?) -> T,
+    implicitElse: T? = null
 ): T? =
-    branches.foldRight(null) { br, n ->
+    branches.foldRight(implicitElse) { br, n ->
         val body = br.result.accept(tr, data)
         if (isElseBranch(br)) body
         else {