8338937: Optimize the string concatenation of ClassDesc

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

@@ -4647,7 +4647,7 @@ public String descriptorString() {
             return Wrapper.forPrimitiveType(this).basicTypeString();
 
         if (isArray()) {
-            return "[" + componentType.descriptorString();
+            return "[".concat(componentType.descriptorString());
         } else if (isHidden()) {
             String name = getName();
             int index = name.indexOf('/');
@@ -4660,11 +4660,7 @@ public String descriptorString() {
                     .toString();
         } else {
             String name = getName().replace('.', '/');
-            return new StringBuilder(name.length() + 2)
-                    .append('L')
-                    .append(name)
-                    .append(';')
-                    .toString();
+            return StringConcatHelper.concat("L", name, ";");
         }
     }
 

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

@@ -783,4 +783,20 @@ static int checkOverflow(int value) {
         }
         throw new OutOfMemoryError("Overflow: String length out of range");
     }
+
+    @ForceInline
+    private static String concat0(String prefix, String str, String suffix) {
+        byte coder = (byte) (prefix.coder() | str.coder() | suffix.coder());
+        int len = prefix.length() + str.length();
+        byte[] buf = newArrayWithSuffix(suffix, len, coder);
+        prepend(len, coder, buf, str, prefix);
+        return new String(buf, coder);
+    }
+
+    @ForceInline
+    static String concat(String prefix, Object value, String suffix) {
+        if (prefix == null) prefix = "null";
+        if (suffix == null) suffix = "null";
+        return concat0(prefix, stringOf(value), suffix);
+    }
 }

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

@@ -2639,6 +2639,10 @@ public String join(String prefix, String suffix, String delimiter, String[] elem
                 return String.join(prefix, suffix, delimiter, elements, size);
             }
 
+            public String concat(String prefix, Object value, String suffix) {
+                return StringConcatHelper.concat(prefix, value, suffix);
+            }
+
             public Object classData(Class<?> c) {
                 return c.getClassData();
             }

src/java.base/share/classes/java/lang/constant/ClassDesc.java

@@ -36,6 +36,7 @@
 import static jdk.internal.constant.ConstantUtils.MAX_ARRAY_TYPE_DESC_DIMENSIONS;
 import static jdk.internal.constant.ConstantUtils.arrayDepth;
 import static jdk.internal.constant.ConstantUtils.binaryToInternal;
+import static jdk.internal.constant.ConstantUtils.concat;
 import static jdk.internal.constant.ConstantUtils.forPrimitiveType;
 import static jdk.internal.constant.ConstantUtils.internalToBinary;
 import static jdk.internal.constant.ConstantUtils.validateBinaryClassName;
@@ -83,7 +84,7 @@ public sealed interface ClassDesc
      */
     static ClassDesc of(String name) {
         validateBinaryClassName(name);
-        return ClassDesc.ofDescriptor("L" + binaryToInternal(name) + ";");
+        return ClassDesc.ofDescriptor(concat("L", binaryToInternal(name), ";"));
     }
 
     /**
@@ -109,7 +110,7 @@ static ClassDesc of(String name) {
      */
     static ClassDesc ofInternalName(String name) {
         validateInternalClassName(name);
-        return ClassDesc.ofDescriptor("L" + name + ";");
+        return ClassDesc.ofDescriptor(concat("L", name, ";"));
     }
 
     /**
@@ -132,8 +133,8 @@ static ClassDesc of(String packageName, String className) {
             return of(className);
         }
         validateMemberName(className, false);
-        return ofDescriptor("L" + binaryToInternal(packageName) +
-                "/" + className + ";");
+        return ofDescriptor('L' + binaryToInternal(packageName) +
+                '/' + className + ';');
     }
 
     /**
@@ -361,7 +362,7 @@ else if (isArray()) {
             ClassDesc c = this;
             for (int i=0; i<depth; i++)
                 c = c.componentType();
-            return c.displayName() + "[]".repeat(depth);
+            return c.displayName().concat("[]".repeat(depth));
         }
         else
             throw new IllegalStateException(descriptorString());

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

@@ -125,7 +125,7 @@ private InvokerBytecodeGenerator(LambdaForm lambdaForm, int localsMapSize,
             name = makeDumpableClassName(name);
         }
         this.name = name;
-        this.className = CLASS_PREFIX + name;
+        this.className = CLASS_PREFIX.concat(name);
         this.classDesc = ClassDesc.ofInternalName(className);
         this.lambdaForm = lambdaForm;
         this.invokerName = invokerName;
@@ -180,11 +180,16 @@ private static String makeDumpableClassName(String className) {
             if (ctr == null)  ctr = 0;
             DUMP_CLASS_FILES_COUNTERS.put(className, ctr+1);
         }
-        String sfx = ctr.toString();
-        while (sfx.length() < 3)
-            sfx = "0" + sfx;
-        className += sfx;
-        return className;
+
+        var buf = new StringBuilder(className.length() + 3).append(className);
+        int ctrVal = ctr;
+        if (ctrVal < 10) {
+            buf.repeat('0', 2);
+        } else if (ctrVal < 100) {
+            buf.append('0');
+        }
+        buf.append(ctrVal);
+        return buf.toString();
     }
 
     static class ClassData {

src/java.base/share/classes/jdk/internal/access/JavaLangAccess.java

@@ -453,6 +453,11 @@ public interface JavaLangAccess {
      */
     String join(String prefix, String suffix, String delimiter, String[] elements, int size);
 
+    /**
+     * Concatenation of prefix and suffix characters to a String for early bootstrap
+     */
+    String concat(String prefix, Object value, String suffix);
+
     /*
      * Get the class data associated with the given class.
      * @param c the class

src/java.base/share/classes/jdk/internal/constant/ConstantUtils.java

@@ -35,12 +35,16 @@
 import java.util.List;
 import java.util.Set;
 
+import jdk.internal.access.JavaLangAccess;
+import jdk.internal.access.SharedSecrets;
 import static jdk.internal.constant.PrimitiveClassDescImpl.*;
 
 /**
  * Helper methods for the implementation of {@code java.lang.constant}.
  */
 public final class ConstantUtils {
+    private static final JavaLangAccess JLA = SharedSecrets.getJavaLangAccess();
+
     /** an empty constant descriptor */
     public static final ConstantDesc[] EMPTY_CONSTANTDESC = new ConstantDesc[0];
     public static final ClassDesc[] EMPTY_CLASSDESC = new ClassDesc[0];
@@ -66,7 +70,7 @@ private ConstantUtils() {}
      * @param binaryName a binary name
      */
     public static ClassDesc binaryNameToDesc(String binaryName) {
-        return ReferenceClassDescImpl.ofValidated("L" + binaryToInternal(binaryName) + ";");
+        return ReferenceClassDescImpl.ofValidated(concat("L", binaryToInternal(binaryName), ";"));
     }
 
     /**
@@ -378,4 +382,8 @@ private static IllegalArgumentException maxArrayTypeDescDimensions() {
                         "Cannot create an array type descriptor with more than %d dimensions",
                         ConstantUtils.MAX_ARRAY_TYPE_DESC_DIMENSIONS));
     }
+
+    public static String concat(String prefix, Object value, String suffix) {
+        return JLA.concat(prefix, value, suffix);
+    }
 }