8325730: StringBuilder.toString allocation for the empty String

Backport-of: d2590c69b4efe5aa2b48b08070e0dbafb04ef202

src/java.base/share/classes/java/lang/StringBuffer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1994, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1994, 2024, 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
@@ -735,6 +735,9 @@ public synchronized StringBuffer repeat(CharSequence cs, int count) {
     @Override
     @IntrinsicCandidate
     public synchronized String toString() {
+        if (length() == 0) {
+            return "";
+        }
         if (toStringCache == null) {
             return toStringCache = new String(this, null);
         }

src/java.base/share/classes/java/lang/StringBuilder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2003, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2024, 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
@@ -471,8 +471,11 @@ public StringBuilder repeat(CharSequence cs, int count) {
     @Override
     @IntrinsicCandidate
     public String toString() {
+        if (length() == 0) {
+            return "";
+        }
         // Create a copy, don't share the array
-        return new String(this);
+        return new String(this, null);
     }
 
     /**

test/micro/org/openjdk/bench/java/lang/StringBuffers.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2014, 2024, 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
@@ -80,4 +80,10 @@ public String substring() {
         return blaha.substring(30, 35);
     }
 
+    StringBuffer sb = new StringBuffer();
+
+    @Benchmark
+    public String emptyToString() {
+        return sb.toString();
+    }
 }

test/micro/org/openjdk/bench/java/lang/StringBuilders.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014, 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2014, 2024, 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
@@ -364,6 +364,11 @@ public char charAtUtf16() {
         return sbUtf16.charAt(charAt_index);
     }
 
+    @Benchmark
+    public String emptyToString(Data data) {
+        return data.sbEmpty.toString();
+    }
+
     @State(Scope.Thread)
     public static class Data {
         int i = 0;
@@ -380,6 +385,7 @@ public CharSequence next() {
             }
         }
 
+        StringBuilder sbEmpty;
         String str;
         String utf16Str;
         CharSequence cs;
@@ -398,6 +404,8 @@ public void setup() {
         }
 
         private void generateData() {
+            sbEmpty = new StringBuilder(length);
+
             char[] chars = "abcdefghijklmnopqrstuvwxyz0123456789".toCharArray();
 
             StringBuilder sb = new StringBuilder(length);