Fold _canHavePrefixUri into Xobj._bits to shrink AttrXobj by 8 bytes

NamedNodeXobj carries a single boolean, _canHavePrefixUri, to record whether a
node was created through the DOM Level 1 factory methods. Xobj already ends on
an 8-byte boundary, so that one byte costs AttrXobj a whole 8-byte slot: 88
bytes of fields become a 96-byte object with 7 bytes of padding.

Xobj._bits already holds kind, domType and three flags, with the highest bit in
use at 0x400. Moving the boolean there as CAN_HAVE_PREFIX_URI = 0x800 removes
the field, and AttrXobj drops to 88 bytes. Measured with Unsafe.objectFieldOffset
on JDK 17 and 21, with and without compressed oops.

ElementXobj is unchanged at 96 - it adds a 4-byte _attributes reference on top
of 88.

Reported against POI as a 600MB workbook holding 2.35 million AttrXobj, where
this is roughly 19MB.

Refs apache/poi#992

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: pjfanning

src/main/java/org/apache/xmlbeans/impl/store/DomImpl.java

@@ -661,7 +661,7 @@ public static Element document_createElement(Dom d, String name) {
         c.createElement(l.makeQualifiedQName("", name));
         ElementXobj e = (ElementXobj) c.getDom();
         c.release();
-        e._canHavePrefixUri = false;
+        e.clearBit(Xobj.CAN_HAVE_PREFIX_URI);
         return e;
     }
 
@@ -698,7 +698,7 @@ public static Attr document_createAttribute(Dom d, String name) {
         c.createAttr(l.makeQualifiedQName("", name));
         AttrXobj e = (AttrXobj) c.getDom();
         c.release();
-        e._canHavePrefixUri = false;
+        e.clearBit(Xobj.CAN_HAVE_PREFIX_URI);
         return e;
     }
 

src/main/java/org/apache/xmlbeans/impl/store/NamedNodeXobj.java

@@ -18,13 +18,12 @@
 abstract class NamedNodeXobj extends NodeXobj {
     NamedNodeXobj(Locale l, int kind, int domType) {
         super(l, kind, domType);
-        _canHavePrefixUri = true;
+        setBit(CAN_HAVE_PREFIX_URI);
     }
 
     public boolean nodeCanHavePrefixUri() {
-        return _canHavePrefixUri;
+        return bitIsSet(CAN_HAVE_PREFIX_URI);
     }
 
-    boolean _canHavePrefixUri;
 }
 

src/main/java/org/apache/xmlbeans/impl/store/Xobj.java

@@ -567,8 +567,7 @@ final void setName(QName newName) {
 
             _name = newName;
             if (this instanceof NamedNodeXobj) {
-                NamedNodeXobj me = (NamedNodeXobj) this;
-                me._canHavePrefixUri = true;
+                setBit(CAN_HAVE_PREFIX_URI);
             }
 
             if (!isProcinst()) {
@@ -1298,6 +1297,9 @@ final boolean bitIsClear(int mask) {
     static final int VACANT = 0x100;
     static final int STABLE_USER = 0x200;
     static final int INHIBIT_DISCONNECT = 0x400;
+    // only NamedNodeXobj reads this one - it lives here so that NamedNodeXobj
+    // needs no field of its own, which would cost 8 bytes per AttrXobj in padding
+    static final int CAN_HAVE_PREFIX_URI = 0x800;
 
     final boolean isVacant() {
         return bitIsSet(VACANT);

src/test/java/org/apache/xmlbeans/impl/store/NodePrefixUriFlagTest.java

@@ -0,0 +1,104 @@
+/*   Licensed to the Apache Software Foundation (ASF) under one or more
+ *   contributor license agreements.  See the NOTICE file distributed with
+ *   this work for additional information regarding copyright ownership.
+ *   The ASF licenses this file to You under the Apache License, Version 2.0
+ *   (the "License"); you may not use this file except in compliance with
+ *   the License.  You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *   Unless required by applicable law or agreed to in writing, software
+ *   distributed under the License is distributed on an "AS IS" BASIS,
+ *   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *   See the License for the specific language governing permissions and
+ *   limitations under the License.
+ */
+
+package org.apache.xmlbeans.impl.store;
+
+import org.apache.xmlbeans.XmlCursor;
+import org.apache.xmlbeans.XmlException;
+import org.apache.xmlbeans.XmlObject;
+import org.junit.jupiter.api.Test;
+import org.w3c.dom.Attr;
+import org.w3c.dom.Document;
+import org.w3c.dom.Element;
+
+import javax.xml.namespace.QName;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+/**
+ * DOM Level 1 factory methods produce nodes with no namespace information, which
+ * the store tracks per node. These pin that behaviour across the three places the
+ * state is written.
+ */
+public class NodePrefixUriFlagTest {
+
+    private static final String URI = "http://example.org/ns";
+
+    private static XmlObject parse() throws XmlException {
+        return XmlObject.Factory.parse("<root/>");
+    }
+
+    private static Document doc() throws XmlException {
+        return (Document) parse().getDomNode();
+    }
+
+    @Test
+    void level1ElementHasNoNamespaceInfo() throws XmlException {
+        Element e = doc().createElement("foo");
+
+        assertNull(e.getLocalName());
+        assertNull(e.getNamespaceURI());
+        assertNull(e.getPrefix());
+    }
+
+    @Test
+    void level2ElementKeepsNamespaceInfo() throws XmlException {
+        Element e = doc().createElementNS(URI, "p:foo");
+
+        assertEquals("foo", e.getLocalName());
+        assertEquals(URI, e.getNamespaceURI());
+        assertEquals("p", e.getPrefix());
+    }
+
+    @Test
+    void level1AttributeHasNoNamespaceInfo() throws XmlException {
+        Attr a = doc().createAttribute("bar");
+
+        assertNull(a.getLocalName());
+        assertNull(a.getNamespaceURI());
+        assertNull(a.getPrefix());
+    }
+
+    @Test
+    void level2AttributeKeepsNamespaceInfo() throws XmlException {
+        Attr a = doc().createAttributeNS(URI, "p:bar");
+
+        assertEquals("bar", a.getLocalName());
+        assertEquals(URI, a.getNamespaceURI());
+        assertEquals("p", a.getPrefix());
+    }
+
+    @Test
+    void renamingALevel1ElementRestoresNamespaceInfo() throws XmlException {
+        XmlObject xo = parse();
+        Document doc = (Document) xo.getDomNode();
+
+        Element child = doc.createElement("child");
+        doc.getDocumentElement().appendChild(child);
+        assertNull(child.getLocalName());
+
+        try (XmlCursor c = xo.newCursor()) {
+            c.toFirstChild(); // root
+            c.toFirstChild(); // child
+            c.setName(new QName(URI, "child", "p"));
+        }
+
+        assertEquals("child", child.getLocalName());
+        assertEquals(URI, child.getNamespaceURI());
+        assertEquals("p", child.getPrefix());
+    }
+}