[WASM] Use wasm bottom types for Nothing?

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/WasmLoweringPhases.kt

@@ -427,12 +427,6 @@ private val autoboxingTransformerPhase = makeWasmModulePhase(
     description = "Insert box/unbox intrinsics"
 )
 
-private val wasmNullSpecializationLowering = makeWasmModulePhase(
-    { context -> WasmNullCoercingLowering(context) },
-    name = "WasmNullCoercingLowering",
-    description = "Specialize assigning Nothing? values to other types."
-)
-
 private val staticMembersLoweringPhase = makeWasmModulePhase(
     ::StaticMembersLowering,
     name = "StaticMembersLowering",
@@ -661,6 +655,5 @@ val wasmPhases = NamedCompilerPhase(
 
             virtualDispatchReceiverExtractionPhase then
             staticMembersLoweringPhase then
-            wasmNullSpecializationLowering then
             validateIrAfterLowering
 )

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/ir2wasm/BodyGenerator.kt

@@ -589,8 +589,14 @@ class BodyGenerator(
         // NOTHING -> TYPE -> TRUE
         if (actualType.isNothing()) return
 
-        // NOTHING? -> TYPE? -> TRUE
-        if (actualType.isNullableNothing() && expectedType.isNullable()) return
+        // NOTHING? -> TYPE? -> (NOTHING?)NULL
+        if (actualType.isNullableNothing() && expectedType.isNullable()) {
+            if (expectedType.getClass()?.isExternal == true) {
+                body.buildDrop()
+                body.buildRefNull(WasmHeapType.Simple.NullNoExtern)
+            }
+            return
+        }
 
         val expectedClassErased = expectedType.getRuntimeClass(irBuiltIns)
 

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/ir2wasm/DeclarationGenerator.kt

@@ -22,6 +22,7 @@ import org.jetbrains.kotlin.ir.expressions.IrConst
 import org.jetbrains.kotlin.ir.expressions.IrConstKind
 import org.jetbrains.kotlin.ir.expressions.IrExpression
 import org.jetbrains.kotlin.ir.symbols.IrClassSymbol
+import org.jetbrains.kotlin.ir.types.getClass
 import org.jetbrains.kotlin.ir.util.*
 import org.jetbrains.kotlin.ir.visitors.IrElementVisitorVoid
 import org.jetbrains.kotlin.ir.visitors.acceptVoid
@@ -442,13 +443,14 @@ class DeclarationGenerator(
     }
 }
 
-
 fun generateDefaultInitializerForType(type: WasmType, g: WasmExpressionBuilder) = when (type) {
     WasmI32 -> g.buildConstI32(0)
     WasmI64 -> g.buildConstI64(0)
     WasmF32 -> g.buildConstF32(0f)
     WasmF64 -> g.buildConstF64(0.0)
     is WasmRefNullType -> g.buildRefNull(type.heapType)
+    is WasmRefNullNoneType -> g.buildRefNull(WasmHeapType.Simple.NullNone)
+    is WasmRefNullExternrefType -> g.buildRefNull(WasmHeapType.Simple.NullNoExtern)
     is WasmAnyRef -> g.buildRefNull(WasmHeapType.Simple.Any)
     is WasmExternRef -> g.buildRefNull(WasmHeapType.Simple.Extern)
     WasmUnreachableType -> error("Unreachable type can't be initialized")
@@ -466,7 +468,10 @@ fun IrFunction.isExported(): Boolean =
 
 fun generateConstExpression(expression: IrConst<*>, body: WasmExpressionBuilder, context: WasmBaseCodegenContext) {
     when (val kind = expression.kind) {
-        is IrConstKind.Null -> generateDefaultInitializerForType(context.transformType(expression.type), body)
+        is IrConstKind.Null -> {
+            val bottomType = if (expression.type.getClass()?.isExternal == true) WasmRefNullExternrefType else WasmRefNullNoneType
+            body.buildInstr(WasmOp.REF_NULL, WasmImmediate.HeapType(bottomType))
+        }
         is IrConstKind.Boolean -> body.buildConstI32(if (kind.valueOf(expression)) 1 else 0)
         is IrConstKind.Byte -> body.buildConstI32(kind.valueOf(expression).toInt())
         is IrConstKind.Short -> body.buildConstI32(kind.valueOf(expression).toInt())

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/ir2wasm/TypeTransformer.kt

@@ -82,7 +82,7 @@ class WasmTypeTransformer(
                 WasmF64
 
             builtIns.nothingNType ->
-                WasmAnyRef
+                WasmRefNullNoneType
 
             // Value will not be created. Just using a random Wasm type.
             builtIns.nothingType ->

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/lower/WasmNullCoercionLowering.kt

@@ -1,35 +0,0 @@
-/*
- * Copyright 2010-2020 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.wasm.lower
-
-import org.jetbrains.kotlin.backend.wasm.WasmBackendContext
-import org.jetbrains.kotlin.ir.UNDEFINED_OFFSET
-import org.jetbrains.kotlin.ir.backend.js.JsCommonBackendContext
-import org.jetbrains.kotlin.ir.backend.js.ir.JsIrBuilder
-import org.jetbrains.kotlin.ir.backend.js.lower.AbstractValueUsageLowering
-import org.jetbrains.kotlin.ir.expressions.IrConstKind
-import org.jetbrains.kotlin.ir.expressions.IrExpression
-import org.jetbrains.kotlin.ir.expressions.impl.IrConstImpl
-import org.jetbrains.kotlin.ir.types.IrType
-import org.jetbrains.kotlin.ir.types.isNothing
-import org.jetbrains.kotlin.ir.types.isNullable
-import org.jetbrains.kotlin.ir.types.makeNotNull
-
-/**
- * Replace null constants of type Nothing? with null constants of a concrete class types.
- *
- * Wasm GC doesn't have a nullref type anymore.
- */
-class WasmNullCoercingLowering(private val contextx: WasmBackendContext) : AbstractValueUsageLowering(contextx) {
-    override fun IrExpression.useExpressionAsType(actualType: IrType, expectedType: IrType): IrExpression =
-        if (actualType.makeNotNull().isNothing() && actualType.isNullable() && !expectedType.makeNotNull().isNothing() && expectedType != contextx.wasmSymbols.voidType)
-            JsIrBuilder.buildComposite(
-                expectedType,
-                listOf(this, IrConstImpl(UNDEFINED_OFFSET, UNDEFINED_OFFSET, expectedType, IrConstKind.Null, null))
-            )
-        else
-            this
-}

wasm/wasm.ir/src/org/jetbrains/kotlin/wasm/ir/Types.kt

@@ -25,6 +25,8 @@ object WasmFuncRef : WasmType("funcref", -0x10)
 object WasmExternRef : WasmType("externref", -0x11)
 object WasmAnyRef : WasmType("anyref", -0x12)
 object WasmEqRef : WasmType("eqref", -0x13)
+object WasmRefNullNoneType : WasmType("nullnone", -0x1b)
+object WasmRefNullExternrefType : WasmType("nullexternref", -0x17)
 
 data class WasmRefNullType(val heapType: WasmHeapType) : WasmType("ref null", -0x14)
 data class WasmRefType(val heapType: WasmHeapType) : WasmType("ref", -0x15)
@@ -47,8 +49,9 @@ sealed class WasmHeapType {
         object Extern : Simple("extern", -0x11)
         object Any : Simple("any", -0x12)
         object Eq : Simple("eq", -0x13)
-
         object Data : Simple("data", -0x19)
+        object NullNone : Simple("nullref", -0x1b)
+        object NullNoExtern : Simple("nullexternref", -0x17)
 
         override fun toString(): String {
             return "Simple:$name(${code.toString(16)})"
@@ -66,6 +69,8 @@ fun WasmType.getHeapType(): WasmHeapType =
     when (this) {
         is WasmRefType -> heapType
         is WasmRefNullType -> heapType
+        is WasmRefNullNoneType -> WasmHeapType.Simple.NullNone
+        is WasmRefNullExternrefType -> WasmHeapType.Simple.NullNoExtern
         is WasmEqRef -> WasmHeapType.Simple.Eq
         is WasmAnyRef -> WasmHeapType.Simple.Any
         is WasmFuncRef -> WasmHeapType.Simple.Func