Read class from object header

src/hotspot/share/oops/markWord.inline.hpp

@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 2021, 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
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#ifndef SHARE_OOPS_MARKWORD_INLINE_HPP
+#define SHARE_OOPS_MARKWORD_INLINE_HPP
+
+#include "oops/compressedOops.inline.hpp"
+#include "oops/markWord.hpp"
+#include "runtime/safepoint.hpp"
+
+#ifdef _LP64
+narrowKlass markWord::narrow_klass() const {
+  return narrowKlass(value() >> klass_shift);
+}
+
+Klass* markWord::klass() const {
+  return CompressedKlassPointers::decode_not_null(narrow_klass());
+}
+
+Klass* markWord::klass_or_null() const {
+  return CompressedKlassPointers::decode(narrow_klass());
+}
+
+markWord markWord::set_narrow_klass(const narrowKlass nklass) const {
+  return markWord((value() & ~klass_mask_in_place) | ((uintptr_t) nklass << klass_shift));
+}
+
+Klass* markWord::safe_klass() const {
+  assert(SafepointSynchronize::is_at_safepoint(), "only call at safepoint");
+  markWord mrk = *this;
+  if (mrk.has_displaced_mark_helper()) {
+    mrk = mrk.displaced_mark_helper();
+  }
+  return CompressedKlassPointers::decode_not_null(mrk.narrow_klass());
+}
+
+markWord markWord::set_klass(const Klass* klass) const {
+  assert(UseCompressedClassPointers, "expect compressed klass pointers");
+  // TODO: Don't cast to non-const, change CKP::encode() to accept const Klass* instead.
+  narrowKlass nklass = CompressedKlassPointers::encode(const_cast<Klass*>(klass));
+  return set_narrow_klass(nklass);
+}
+#endif
+
+#endif // SHARE_OOPS_MARKWORD_INLINE_HPP