JVM IR: minor optimizations in IrTypeMapper.classInternalName

Replace cubic complexity with square when computing internal name for
nested classes by using StringBuilder instead of concatenating strings,
and don't do getLocalClassType/classNameOverride checks on each step of
the recursion.

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/codegen/IrTypeMapper.kt

@@ -52,44 +52,52 @@ class IrTypeMapper(private val context: JvmBackendContext) : KotlinTypeMapperBas
                 error("Unknown descriptor: $classifier")
         }
 
-    fun classInternalName(irClass: IrClass): String = getClassInternalName(irClass.symbol)
+    private fun computeClassInternalName(irClass: IrClass): StringBuilder {
+        val shortName = SpecialNames.safeIdentifier(irClass.name).identifier
 
-    override fun getClassInternalName(typeConstructor: TypeConstructorMarker): String {
+        when (val parent = irClass.parent) {
-        val irClass = (typeConstructor as IrClassSymbol).owner
+            is IrPackageFragment ->
-        fun report(): Nothing {
+                return StringBuilder().apply {
-            error(
+                    val fqName = parent.fqName
-                "Local class should have its name computed in InventNamesForLocalClasses: ${irClass.fqNameWhenAvailable}\n" +
+                    if (!fqName.isRoot) {
-                        "Ensure that any lowering that transforms elements with local class info (classes, function references) " +
+                        append(fqName.asString().replace('.', '/')).append("/")
-                        "invokes `copyAttributes` on the transformed element."
+                    }
-            )
+                    append(shortName)
+                }
+            is IrClass ->
+                return computeClassInternalName(parent).append("$").append(shortName)
+            is IrFunction ->
+                if (parent.isSuspend && parent.parentAsClass.origin == JvmLoweredDeclarationOrigin.DEFAULT_IMPLS) {
+                    return computeClassInternalName(parent.parentAsClass.parentAsClass)
+                        .append("$").append(parent.name.asString())
+                }
         }
 
-        context.getLocalClassType(irClass)?.internalName?.let { return it }
+        val localClassType = context.getLocalClassType(irClass)
+        if (localClassType != null) {
+            return StringBuilder(localClassType.internalName)
+        }
 
+        error(
+            "Local class should have its name computed in InventNamesForLocalClasses: ${irClass.fqNameWhenAvailable}\n" +
+                    "Ensure that any lowering that transforms elements with local class info (classes, function references) " +
+                    "invokes `copyAttributes` on the transformed element."
+        )
+    }
+
+    fun classInternalName(irClass: IrClass): String {
+        context.getLocalClassType(irClass)?.internalName?.let { return it }
         context.classNameOverride[irClass]?.let { return it.internalName }
 
-        val className = SpecialNames.safeIdentifier(irClass.name).identifier
+        return JvmCodegenUtil.sanitizeNameIfNeeded(
-
+            computeClassInternalName(irClass).toString(),
-        val internalName = when (val parent = irClass.parent) {
+            context.state.languageVersionSettings
-            is IrPackageFragment -> {
+        )
-                val fqName = parent.fqName
-                val prefix = if (fqName.isRoot) "" else fqName.asString().replace('.', '/') + "/"
-                prefix + className
-            }
-            is IrClass -> {
-                getClassInternalName(parent.symbol) + "$" + className
-            }
-            is IrFunction -> {
-                if (parent.isSuspend && parent.parentAsClass.origin == JvmLoweredDeclarationOrigin.DEFAULT_IMPLS) {
-                    val interfaceClass = parent.parentAsClass.parent as IrClass
-                    getClassInternalName(interfaceClass.symbol) + "$" + parent.name.asString()
-                } else report()
-            }
-            else -> report()
-        }
-        return JvmCodegenUtil.sanitizeNameIfNeeded(internalName, context.state.languageVersionSettings)
     }
 
+    override fun getClassInternalName(typeConstructor: TypeConstructorMarker): String =
+        classInternalName((typeConstructor as IrClassSymbol).owner)
+
     fun writeFormalTypeParameters(irParameters: List<IrTypeParameter>, sw: JvmSignatureWriter) {
         if (sw.skipGenericSignature()) return
         with(KotlinTypeMapper) {