[Wasm] Use final Wasm structs for types without subtypes

A version of PR https://github.com/JetBrains/kotlin/pull/5175
 contributed by Yair Morgenstern, modified by Slava Kuzmich

^KT-60505 Fixed

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

@@ -215,7 +215,8 @@ class DeclarationGenerator(
         val struct = WasmStructDeclaration(
             name = "classITable",
             fields = fields,
-            superType = null
+            superType = null,
+            isFinal = true,
         )
 
         iFacesUnion.forEach {
@@ -227,6 +228,7 @@ class DeclarationGenerator(
         methods: List<VirtualMethodMetadata>,
         name: String,
         superType: WasmSymbolReadOnly<WasmTypeDeclaration>? = null,
+        isFinal: Boolean,
     ): WasmStructDeclaration {
         val tableFields = methods.map {
             WasmStructFieldDeclaration(
@@ -240,6 +242,7 @@ class DeclarationGenerator(
             name = name,
             fields = tableFields,
             superType = superType,
+            isFinal = isFinal
         )
     }
 
@@ -250,7 +253,8 @@ class DeclarationGenerator(
         val vtableStruct = createVirtualTableStruct(
             metadata.virtualMethods,
             vtableName,
-            superType = metadata.superClass?.klass?.symbol?.let(context::referenceVTableGcType)
+            superType = metadata.superClass?.klass?.symbol?.let(context::referenceVTableGcType),
+            isFinal = klass.modality == Modality.FINAL
         )
         context.defineVTableGcType(metadata.klass.symbol, vtableStruct)
 
@@ -357,7 +361,8 @@ class DeclarationGenerator(
             if (symbol in hierarchyDisjointUnions) {
                 val vtableStruct = createVirtualTableStruct(
                     methods = context.getInterfaceMetadata(symbol).methods,
-                    name = "$nameStr.itable"
+                    name = "$nameStr.itable",
+                    isFinal = true,
                 )
                 context.defineVTableGcType(symbol, vtableStruct)
             }
@@ -384,7 +389,8 @@ class DeclarationGenerator(
             val structType = WasmStructDeclaration(
                 name = nameStr,
                 fields = fields,
-                superType = superClass?.let { context.referenceGcType(superClass.klass.symbol) }
+                superType = superClass?.let { context.referenceGcType(superClass.klass.symbol) },
+                isFinal = declaration.modality == Modality.FINAL
             )
             context.defineGcType(symbol, structType)
             context.generateTypeInfo(symbol, binaryDataStruct(metadata))

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

@@ -155,7 +155,8 @@ data class WasmFunctionType(
 class WasmStructDeclaration(
     name: String,
     val fields: List<WasmStructFieldDeclaration>,
-    val superType: WasmSymbolReadOnly<WasmTypeDeclaration>?
+    val superType: WasmSymbolReadOnly<WasmTypeDeclaration>?,
+    val isFinal: Boolean
 ) : WasmTypeDeclaration(name)
 
 class WasmArrayDeclaration(

wasm/wasm.ir/src/org/jetbrains/kotlin/wasm/ir/convertors/WasmIrToBinary.kt

@@ -62,8 +62,8 @@ class WasmIrToBinary(
     private var b: ByteWriter = ByteWriter.OutputStream(outputStream)
 
     // "Stack" of offsets waiting initialization. 
-    // Since blocks has as a prefix variable length number encoding its size we can't calculate absolute offsets inside those blocks 
-    // until we generate whole block and generate size. So, we put them into "stack" and initialize as soo as we have all required data.
+    // Since blocks have as a prefix variable length number encoding its size, we can't calculate absolute offsets inside those blocks
+    // until we generate the whole block and generate size. So, we put them into "stack" and initialize as soon as we have all required data.
     private var offsets = persistentListOf<Box>()
 
     fun appendWasmModule() {
@@ -76,7 +76,7 @@ class WasmIrToBinary(
                 val numRecGroups = if (recGroupTypes.isEmpty()) 0 else 1
                 appendVectorSize(functionTypes.size + numRecGroups)
                 functionTypes.forEach { appendFunctionTypeDeclaration(it) }
-                if (!recGroupTypes.isEmpty()) {
+                if (recGroupTypes.isNotEmpty()) {
                     b.writeVarInt7(WasmBinary.REC_GROUP)
                     appendVectorSize(recGroupTypes.size)
                     recGroupTypes.forEach {
@@ -361,14 +361,26 @@ class WasmIrToBinary(
     }
 
     private fun appendStructTypeDeclaration(type: WasmStructDeclaration) {
-        b.writeVarInt7(WasmBinary.SUB_TYPE)
-
         val superType = type.superType
-        if (superType != null) {
-            appendVectorSize(1)
-            appendModuleFieldReference(superType.owner)
+
+        // https://webassembly.github.io/gc/core/binary/types.html#binary-subtype
+        if (superType == null && type.isFinal) {
+            // Short encoding form for final types without subtypes.
         } else {
-            appendVectorSize(0)
+            // General encoding
+            b.writeVarInt7(
+                if (type.isFinal)
+                    WasmBinary.SUB_FINAL_TYPE
+                else
+                    WasmBinary.SUB_TYPE
+            )
+
+            if (superType != null) {
+                appendVectorSize(1)
+                appendModuleFieldReference(superType.owner)
+            } else {
+                appendVectorSize(0)
+            }
         }
 
         b.writeVarInt7(WasmBinary.STRUCT_TYPE)