From 0a108f9ef2f9f4c88ac146aef6b62032cfc4b91e Mon Sep 17 00:00:00 2001
From: "Tagir F. Valeev" <tvaleev@openjdk.org>
Date: Tue, 16 Jun 2020 03:16:41 +0000
Subject: [PATCH] 8247605: Avoid array allocation when concatenating with empty
 string

Reviewed-by: redestad, plevart
---
 .../classes/java/lang/StringConcatHelper.java     |  8 ++++++++
 .../org/openjdk/bench/java/lang/StringConcat.java | 15 ++++++++++++---
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/src/java.base/share/classes/java/lang/StringConcatHelper.java b/src/java.base/share/classes/java/lang/StringConcatHelper.java
index 7e676307865..641def5bb93 100644
--- a/src/java.base/share/classes/java/lang/StringConcatHelper.java
+++ b/src/java.base/share/classes/java/lang/StringConcatHelper.java
@@ -406,6 +406,14 @@ final class StringConcatHelper {
     static String simpleConcat(Object first, Object second) {
         String s1 = stringOf(first);
         String s2 = stringOf(second);
+        if (s1.isEmpty()) {
+            // newly created string required, see JLS 15.18.1
+            return new String(s2);
+        }
+        if (s2.isEmpty()) {
+            // newly created string required, see JLS 15.18.1
+            return new String(s1);
+        }
         // start "mixing" in length and coder or arguments, order is not
         // important
         long indexCoder = mix(initialCoder(), s1);
diff --git a/test/micro/org/openjdk/bench/java/lang/StringConcat.java b/test/micro/org/openjdk/bench/java/lang/StringConcat.java
index dffff9b5d8a..b15b1d0a0c0 100644
--- a/test/micro/org/openjdk/bench/java/lang/StringConcat.java
+++ b/test/micro/org/openjdk/bench/java/lang/StringConcat.java
@@ -28,11 +28,8 @@ import org.openjdk.jmh.annotations.Mode;
 import org.openjdk.jmh.annotations.OutputTimeUnit;
 import org.openjdk.jmh.annotations.Param;
 import org.openjdk.jmh.annotations.Scope;
-import org.openjdk.jmh.annotations.Setup;
 import org.openjdk.jmh.annotations.State;
-import org.openjdk.jmh.infra.Blackhole;
 
-import java.util.Random;
 import java.util.concurrent.TimeUnit;
 
 /**
@@ -54,6 +51,8 @@ public class StringConcat {
 
     public byte byteValue = (byte)-128;
 
+    public String emptyString = "";
+
     @Benchmark
     public String concatConstInt() {
         return "string" + intValue;
@@ -64,6 +63,16 @@ public class StringConcat {
         return "string" + stringValue;
     }
 
+    @Benchmark
+    public String concatEmptyRight() {
+        return stringValue + emptyString;
+    }
+
+    @Benchmark
+    public String concatEmptyLeft() {
+        return emptyString + stringValue;
+    }
+
     @Benchmark
     public String concatMethodConstString() {
         return "string".concat(stringValue);