Merge pull request #9 from cl4es/pr_20273_inject_cache

Cached prototype

Author: wenshao

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

@@ -28,6 +28,7 @@
 import jdk.internal.misc.Unsafe;
 import jdk.internal.util.DecimalDigits;
 import jdk.internal.vm.annotation.ForceInline;
+import jdk.internal.vm.annotation.Stable;
 
 import java.lang.invoke.MethodHandle;
 import java.lang.invoke.MethodHandles;
@@ -40,6 +41,24 @@
  */
 final class StringConcatHelper {
 
+
+    static class StringConcatBase {
+        @Stable String[] constants;
+        @Stable int length;
+        @Stable byte coder;
+        StringConcatBase(String[] constants) {
+            int length = 0;
+            byte coder = String.LATIN1;
+            this.constants = constants;
+            for (String c : constants) {
+                length += c.length();
+                coder |= c.coder();
+            }
+            this.length = length;
+            this.coder = coder;
+        }
+    }
+
     private StringConcatHelper() {
         // no instantiation
     }