[Wasm] Revert a7ed496a and few fixes to make Kotlin/Wasm compatible with Firefox

Revert changes as soon as the bug is fixed https://bugzilla.mozilla.org/show_bug.cgi?id=1811932

a7ed496a "[WASM] Use wasm bottom types for Nothing?"

#KT-56166 In-Progress

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

@@ -427,6 +427,12 @@ 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",
@@ -655,5 +661,6 @@ val wasmPhases = SameTypeNamedCompilerPhase(
 
             virtualDispatchReceiverExtractionPhase then
             staticMembersLoweringPhase then
+            wasmNullSpecializationLowering then
             validateIrAfterLowering
 )

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

@@ -303,7 +303,7 @@ class BodyGenerator(
             return
         }
 
-        body.buildRefNull(WasmHeapType.Simple.NullNone, location) // this = null
+        body.buildRefNull(WasmHeapType.Type(wasmGcType), location) // this = null
         generateCall(expression)
     }
 
@@ -725,14 +725,8 @@ class BodyGenerator(
         // NOTHING -> TYPE -> TRUE
         if (actualType.isNothing()) return
 
-        // NOTHING? -> TYPE? -> (NOTHING?)NULL
-        if (actualType.isNullableNothing() && expectedType.isNullable()) {
-            if (expectedType.getClass()?.isExternal == true) {
-                body.buildDrop(location)
-                body.buildRefNull(WasmHeapType.Simple.NullNoExtern, location)
-            }
-            return
-        }
+        // NOTHING? -> TYPE? -> TRUE
+        if (actualType.isNullableNothing() && expectedType.isNullable()) return
 
         val expectedClassErased = expectedType.getRuntimeClass(irBuiltIns)
 

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

@@ -22,7 +22,6 @@ 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
@@ -486,8 +485,6 @@ fun generateDefaultInitializerForType(type: WasmType, g: WasmExpressionBuilder)
             WasmF32 -> g.buildConstF32(0f, location)
             WasmF64 -> g.buildConstF64(0.0, location)
             is WasmRefNullType -> g.buildRefNull(type.heapType, location)
-            is WasmRefNullNoneType -> g.buildRefNull(WasmHeapType.Simple.NullNone, location)
-            is WasmRefNullExternrefType -> g.buildRefNull(WasmHeapType.Simple.NullNoExtern, location)
             is WasmAnyRef -> g.buildRefNull(WasmHeapType.Simple.Any, location)
             is WasmExternRef -> g.buildRefNull(WasmHeapType.Simple.Extern, location)
             WasmUnreachableType -> error("Unreachable type can't be initialized")
@@ -511,10 +508,7 @@ fun generateConstExpression(
     location: SourceLocation
 ) =
     when (val kind = expression.kind) {
-        is IrConstKind.Null -> {
-            val bottomType = if (expression.type.getClass()?.isExternal == true) WasmRefNullExternrefType else WasmRefNullNoneType
-            body.buildInstr(WasmOp.REF_NULL, location, WasmImmediate.HeapType(bottomType))
-        }
+        is IrConstKind.Null -> generateDefaultInitializerForType(context.transformType(expression.type), body)
         is IrConstKind.Boolean -> body.buildConstI32(if (kind.valueOf(expression)) 1 else 0, location)
         is IrConstKind.Byte -> body.buildConstI32(kind.valueOf(expression).toInt(), location)
         is IrConstKind.Short -> body.buildConstI32(kind.valueOf(expression).toInt(), location)

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

@@ -82,7 +82,7 @@ class WasmTypeTransformer(
                 WasmF64
 
             builtIns.nothingNType ->
-                WasmRefNullNoneType
+                WasmAnyRef
 
             // 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

@@ -0,0 +1,36 @@
+/*
+ * 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
+
+// TODO revert the commit where this was reintroduced as soon the bug in the Firefox is fixed https://bugzilla.mozilla.org/show_bug.cgi?id=1811932
+/**
+ * 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

@@ -25,8 +25,6 @@ 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)
@@ -49,9 +47,8 @@ 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)})"
@@ -69,8 +66,6 @@ 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