8295302: Do not use ArrayList when LambdaForm has a single ClassData

Reviewed-by: vlivanov, redestad, mchung

src/java.base/share/classes/java/lang/invoke/InvokerBytecodeGenerator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012, 2021, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2012, 2022, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -285,24 +285,6 @@ String classData(Object arg) {
         return name;
     }
 
-    List<Object> classDataValues() {
-        final List<ClassData> cd = classData;
-        return switch(cd.size()) {
-            case 0 -> List.of();
-            case 1 -> List.of(cd.get(0).value);
-            case 2 -> List.of(cd.get(0).value, cd.get(1).value);
-            case 3 -> List.of(cd.get(0).value, cd.get(1).value, cd.get(2).value);
-            case 4 -> List.of(cd.get(0).value, cd.get(1).value, cd.get(2).value, cd.get(3).value);
-            default -> {
-                Object[] data = new Object[classData.size()];
-                for (int i = 0; i < classData.size(); i++) {
-                    data[i] = classData.get(i).value;
-                }
-                yield List.of(data);
-            }
-        };
-    }
-
     private static String debugString(Object arg) {
         if (arg instanceof MethodHandle mh) {
             MemberName member = mh.internalMemberName();
@@ -360,6 +342,32 @@ private void methodEpilogue() {
         mv.visitEnd();
     }
 
+    /**
+     * Returns the class data object that will be passed to `Lookup.defineHiddenClassWithClassData`.
+     * The classData is loaded in the <clinit> method of the generated class.
+     * If the class data contains only one single object, this method returns  that single object.
+     * If the class data contains more than one objects, this method returns a List.
+     *
+     * This method returns null if no class data.
+     */
+    private Object classDataValues() {
+        final List<ClassData> cd = classData;
+        return switch (cd.size()) {
+            case 0 -> null;             // special case (classData is not used by <clinit>)
+            case 1 -> cd.get(0).value;  // special case (single object)
+            case 2 -> List.of(cd.get(0).value, cd.get(1).value);
+            case 3 -> List.of(cd.get(0).value, cd.get(1).value, cd.get(2).value);
+            case 4 -> List.of(cd.get(0).value, cd.get(1).value, cd.get(2).value, cd.get(3).value);
+            default -> {
+                Object[] data = new Object[classData.size()];
+                for (int i = 0; i < classData.size(); i++) {
+                    data[i] = classData.get(i).value;
+                }
+                yield List.of(data);
+            }
+        };
+    }
+
     /*
      * <clinit> to initialize the static final fields with the live class data
      * LambdaForms can't use condy due to bootstrapping issue.
@@ -379,18 +387,23 @@ static void clinit(ClassWriter cw, String className, List<ClassData> classData)
         mv.visitLdcInsn(Type.getType("L" + className + ";"));
         mv.visitMethodInsn(Opcodes.INVOKESTATIC, "java/lang/invoke/MethodHandles",
                            "classData", "(Ljava/lang/Class;)Ljava/lang/Object;", false);
-        // we should optimize one single element case that does not need to create a List
-        mv.visitTypeInsn(Opcodes.CHECKCAST, "java/util/List");
-        mv.visitVarInsn(Opcodes.ASTORE, 0);
-        int index = 0;
-        for (ClassData p : classData) {
-            // initialize the static field
-            mv.visitVarInsn(Opcodes.ALOAD, 0);
-            emitIconstInsn(mv, index++);
-            mv.visitMethodInsn(Opcodes.INVOKEINTERFACE, "java/util/List",
-                               "get", "(I)Ljava/lang/Object;", true);
+        if (classData.size() == 1) {
+            ClassData p = classData.get(0);
             mv.visitTypeInsn(Opcodes.CHECKCAST, p.desc.substring(1, p.desc.length()-1));
             mv.visitFieldInsn(Opcodes.PUTSTATIC, className, p.name, p.desc);
+        } else {
+            mv.visitTypeInsn(Opcodes.CHECKCAST, "java/util/List");
+            mv.visitVarInsn(Opcodes.ASTORE, 0);
+            int index = 0;
+            for (ClassData p : classData) {
+                // initialize the static field
+                mv.visitVarInsn(Opcodes.ALOAD, 0);
+                emitIconstInsn(mv, index++);
+                mv.visitMethodInsn(Opcodes.INVOKEINTERFACE, "java/util/List",
+                                   "get", "(I)Ljava/lang/Object;", true);
+                mv.visitTypeInsn(Opcodes.CHECKCAST, p.desc.substring(1, p.desc.length()-1));
+                mv.visitFieldInsn(Opcodes.PUTSTATIC, className, p.name, p.desc);
+            }
         }
         mv.visitInsn(Opcodes.RETURN);
         mv.visitMaxs(2, 1);