--- /dev/null	2020-06-23 09:58:47.120242046 +0200
+++ new/src/hotspot/share/gc/shenandoah/shenandoahEvacLockingBitmap.hpp	2020-06-23 17:04:59.879878660 +0200
@@ -0,0 +1,74 @@
+/*
+ * Copyright (c) 2020, Red Hat, Inc. 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_GC_SHENANDOAH_SHENANDOAHEVACLOCKINGBITMAP_HPP
+#define SHARE_GC_SHENANDOAH_SHENANDOAHEVACLOCKINGBITMAP_HPP
+
+#include "memory/allocation.hpp"
+#include "memory/memRegion.hpp"
+#include "oops/oopsHierarchy.hpp"
+#include "runtime/globals.hpp"
+#include "utilities/bitMap.hpp"
+
+/*
+ * Protects the evacuation slow-path from concurrent access/races:
+ * - Only one thread must be allowed to (successfully) evacuate an object
+ * - In case of evacuation failure by one thread, no other thread must be allowed to evacuate
+ *   an object
+ *
+ * This is achieved by a locking bitmap. Each bit corresponds to a card of
+ * 2^ShenandoahEvacGranularity heap words. Before evacuating, a thread must acquire
+ * the corresponding bit, and release it after evacuation is done. It is preferable
+ * to use the scoped ShenandoahEvacLocker for that.
+ */
+class ShenandoahEvacLockingBitmap : public CHeapObj<mtGC> {
+private:
+  MemRegion _covered;    // The heap area covered by this bitmap.
+  const int _shifter;    // Shift amount from heap index to bit index in the bitmap.
+  CHeapBitMap _bm;        // The actual bitmap.
+
+  // Convert from address to bit offset.
+  inline size_t addr_to_offset(const HeapWord* addr) const;
+
+public:
+  ShenandoahEvacLockingBitmap(MemRegion heap) :
+  _covered(heap),
+  _shifter((1 + ShenandoahEvacLockGranularity) * LogHeapWordSize),
+  _bm(_covered.word_size() >> _shifter, mtGC) {
+}
+
+  inline void acquire(oop obj);
+  inline void release(oop obj);
+};
+
+class ShenandoahEvacLocker : public StackObj {
+private:
+  ShenandoahEvacLockingBitmap* _bitmap;
+  oop const _obj;
+public:
+  inline ShenandoahEvacLocker(ShenandoahEvacLockingBitmap* bitmap, oop obj);
+  inline ~ShenandoahEvacLocker();
+};
+
+#endif // SHARE_GC_SHENANDOAH_SHENANDOAHEVACLOCKINGBITMAP_HPP