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  * 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.  Oracle designates this
  * particular file as subject to the "Classpath" exception as provided
  * by Oracle in the LICENSE file that accompanied this code.
  *
  * 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
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 package java.lang;
 
-import java.lang.ClassValue.ClassValueMap;
 import java.util.WeakHashMap;
 import java.lang.ref.WeakReference;
 import java.util.concurrent.atomic.AtomicInteger;
 
 import static java.lang.ClassValue.ClassValueMap.probeHomeLocation;
 import static java.lang.ClassValue.ClassValueMap.probeBackupLocations;
 
 /**
  * Lazily associate a computed value with (potentially) every type.
  * For example, if a dynamic language needs to construct a message dispatch
  * table for each class encountered at a message send call site,
  * it can use a {@code ClassValue} to cache information needed to
  * perform the message send quickly, for each class encountered.
  * @author John Rose, JSR 292 EG
  * @since 1.7
  */
 public abstract class ClassValue<T> {
     /**
      * Sole constructor.  (For invocation by subclass constructors, typically
      * implicit.)
      */
     protected ClassValue() {
     }
 
     /**
      * Computes the given class's derived value for this {@code ClassValue}.
      * <p>
      * This method will be invoked within the first thread that accesses
      * the value with the {@link #get get} method.
      * <p>
      * Normally, this method is invoked at most once per class,
      * but it may be invoked again if there has been a call to
      * {@link #remove remove}.
      * <p>
      * If this method throws an exception, the corresponding call to {@code get}
      * will terminate abnormally with that exception, and no class value will be recorded.
      *
      * @param type the type whose class value must be computed
      * @return the newly computed value associated with this {@code ClassValue}, for the given class or interface
      * @see #get
      * @see #remove
      */
     protected abstract T computeValue(Class<?> type);
 
     /**
      * Returns the value for the given class.
      * If no value has yet been computed, it is obtained by
      * an invocation of the {@link #computeValue computeValue} method.
      * <p>
      * The actual installation of the value on the class
      * is performed atomically.
      * At that point, if several racing threads have
      * computed values, one is chosen, and returned to
      * all the racing threads.
      * <p>
      * The {@code type} parameter is typically a class, but it may be any type,
      * such as an interface, a primitive type (like {@code int.class}), or {@code void.class}.
      * <p>
      * In the absence of {@code remove} calls, a class value has a simple
      * state diagram:  uninitialized and initialized.
      * When {@code remove} calls are made,
      * the rules for value observation are more complex.
      * See the documentation for {@link #remove remove} for more information.
      *
      * @param type the type whose class value must be computed or retrieved
      * @return the current value associated with this {@code ClassValue}, for the given class or interface
      * @throws NullPointerException if the argument is null
      * @see #remove
      * @see #computeValue
      */
     public T get(Class<?> type) {
         // non-racing this.hashCodeForCache : final int
         Entry<?>[] cache;
         Entry<T> e = probeHomeLocation(cache = getCacheCarefully(type), this);
         // racing e : current value <=> stale value from current cache or from stale cache
         // invariant:  e is null or an Entry with readable Entry.version and Entry.value
         if (match(e))
             // invariant:  No false positive matches.  False negatives are OK if rare.
             // The key fact that makes this work: if this.version == e.version,
             // then this thread has a right to observe (final) e.value.
             return e.value();
         // The fast path can fail for any of these reasons:
         // 1. no entry has been computed yet
         // 2. hash code collision (before or after reduction mod cache.length)
         // 3. an entry has been removed (either on this type or another)
         // 4. the GC has somehow managed to delete e.version and clear the reference
         return getFromBackup(cache, type);
     }
 
     /**
      * Removes the associated value for the given class.
      * If this value is subsequently {@linkplain #get read} for the same class,
      * its value will be reinitialized by invoking its {@link #computeValue computeValue} method.
      * This may result in an additional invocation of the
      * {@code computeValue} method for the given class.
      * <p>
      * In order to explain the interaction between {@code get} and {@code remove} calls,
      * we must model the state transitions of a class value to take into account
      * the alternation between uninitialized and initialized states.
      * To do this, number these states sequentially from zero, and note that
      * uninitialized (or removed) states are numbered with even numbers,
      * while initialized (or re-initialized) states have odd numbers.
      * <p>
      * When a thread {@code T} removes a class value in state {@code 2N},
      * nothing happens, since the class value is already uninitialized.
      * Otherwise, the state is advanced atomically to {@code 2N+1}.
      * <p>
      * When a thread {@code T} queries a class value in state {@code 2N},
      * the thread first attempts to initialize the class value to state {@code 2N+1}
      * by invoking {@code computeValue} and installing the resulting value.
      * <p>
      * When {@code T} attempts to install the newly computed value,
      * if the state is still at {@code 2N}, the class value will be initialized
      * with the computed value, advancing it to state {@code 2N+1}.
      * <p>
      * Otherwise, whether the new state is even or odd,
      * {@code T} will discard the newly computed value
      * and retry the {@code get} operation.
      * <p>
      * Discarding and retrying is an important proviso,
      * since otherwise {@code T} could potentially install
      * a disastrously stale value.  For example:
      * <ul>
      * <li>{@code T} calls {@code CV.get(C)} and sees state {@code 2N}
      * <li>{@code T} quickly computes a time-dependent value {@code V0} and gets ready to install it
      * <li>{@code T} is hit by an unlucky paging or scheduling event, and goes to sleep for a long time
      * <li>...meanwhile, {@code T2} also calls {@code CV.get(C)} and sees state {@code 2N}
      * <li>{@code T2} quickly computes a similar time-dependent value {@code V1} and installs it on {@code CV.get(C)}
      * <li>{@code T2} (or a third thread) then calls {@code CV.remove(C)}, undoing {@code T2}'s work
      * <li> the previous actions of {@code T2} are repeated several times
      * <li> also, the relevant computed values change over time: {@code V1}, {@code V2}, ...
      * <li>...meanwhile, {@code T} wakes up and attempts to install {@code V0}; <em>this must fail</em>
      * </ul>
      * We can assume in the above scenario that {@code CV.computeValue} uses locks to properly
      * observe the time-dependent states as it computes {@code V1}, etc.
      * This does not remove the threat of a stale value, since there is a window of time
      * between the return of {@code computeValue} in {@code T} and the installation
-     * of the the new value.  No user synchronization is possible during this time.
+     * of the new value.  No user synchronization is possible during this time.
      *
      * @param type the type whose class value must be removed
      * @throws NullPointerException if the argument is null
      */
     public void remove(Class<?> type) {
         ClassValueMap map = getMap(type);
         map.removeEntry(this);
     }
 
     // Possible functionality for JSR 292 MR 1
     /*public*/ void put(Class<?> type, T value) {
         ClassValueMap map = getMap(type);
         map.changeEntry(this, value);
     }
 
     /// --------
     /// Implementation...
     /// --------
 
     /** Return the cache, if it exists, else a dummy empty cache. */
     private static Entry<?>[] getCacheCarefully(Class<?> type) {
         // racing type.classValueMap{.cacheArray} : null => new Entry[X] <=> new Entry[Y]
         ClassValueMap map = type.classValueMap;
         if (map == null)  return EMPTY_CACHE;
         Entry<?>[] cache = map.getCache();
         return cache;
         // invariant:  returned value is safe to dereference and check for an Entry
     }
 
     /** Initial, one-element, empty cache used by all Class instances.  Must never be filled. */
     private static final Entry<?>[] EMPTY_CACHE = { null };
 
     /**
      * Slow tail of ClassValue.get to retry at nearby locations in the cache,
      * or take a slow lock and check the hash table.
      * Called only if the first probe was empty or a collision.
      * This is a separate method, so compilers can process it independently.
      */
     private T getFromBackup(Entry<?>[] cache, Class<?> type) {
         Entry<T> e = probeBackupLocations(cache, this);
         if (e != null)
             return e.value();
         return getFromHashMap(type);
     }
 
     // Hack to suppress warnings on the (T) cast, which is a no-op.
     @SuppressWarnings("unchecked")
     Entry<T> castEntry(Entry<?> e) { return (Entry<T>) e; }
 
     /** Called when the fast path of get fails, and cache reprobe also fails.
      */
     private T getFromHashMap(Class<?> type) {
         // The fail-safe recovery is to fall back to the underlying classValueMap.
         ClassValueMap map = getMap(type);
         for (;;) {
             Entry<T> e = map.startEntry(this);
             if (!e.isPromise())
                 return e.value();
             try {
                 // Try to make a real entry for the promised version.
                 e = makeEntry(e.version(), computeValue(type));
             } finally {
                 // Whether computeValue throws or returns normally,
                 // be sure to remove the empty entry.
                 e = map.finishEntry(this, e);
             }
             if (e != null)
                 return e.value();
             // else try again, in case a racing thread called remove (so e == null)
         }
     }
 
     /** Check that e is non-null, matches this ClassValue, and is live. */
     boolean match(Entry<?> e) {
         // racing e.version : null (blank) => unique Version token => null (GC-ed version)
         // non-racing this.version : v1 => v2 => ... (updates are read faithfully from volatile)
         return (e != null && e.get() == this.version);
         // invariant:  No false positives on version match.  Null is OK for false negative.
         // invariant:  If version matches, then e.value is readable (final set in Entry.<init>)
     }
 
     /** Internal hash code for accessing Class.classValueMap.cacheArray. */
     final int hashCodeForCache = nextHashCode.getAndAdd(HASH_INCREMENT) & HASH_MASK;
 
     /** Value stream for hashCodeForCache.  See similar structure in ThreadLocal. */
     private static final AtomicInteger nextHashCode = new AtomicInteger();
 
     /** Good for power-of-two tables.  See similar structure in ThreadLocal. */
     private static final int HASH_INCREMENT = 0x61c88647;
 
     /** Mask a hash code to be positive but not too large, to prevent wraparound. */
     static final int HASH_MASK = (-1 >>> 2);
 
     /**
      * Private key for retrieval of this object from ClassValueMap.
      */
     static class Identity {
     }
     /**
      * This ClassValue's identity, expressed as an opaque object.
      * The main object {@code ClassValue.this} is incorrect since
      * subclasses may override {@code ClassValue.equals}, which
      * could confuse keys in the ClassValueMap.
      */
     final Identity identity = new Identity();
 
     /**
      * Current version for retrieving this class value from the cache.
      * Any number of computeValue calls can be cached in association with one version.
      * But the version changes when a remove (on any type) is executed.
      * A version change invalidates all cache entries for the affected ClassValue,
      * by marking them as stale.  Stale cache entries do not force another call
      * to computeValue, but they do require a synchronized visit to a backing map.
      * <p>
      * All user-visible state changes on the ClassValue take place under
      * a lock inside the synchronized methods of ClassValueMap.
      * Readers (of ClassValue.get) are notified of such state changes
      * when this.version is bumped to a new token.
      * This variable must be volatile so that an unsynchronized reader
      * will receive the notification without delay.
      * <p>
      * If version were not volatile, one thread T1 could persistently hold onto
-     * a stale value this.value == V1, while while another thread T2 advances
+     * a stale value this.value == V1, while another thread T2 advances
      * (under a lock) to this.value == V2.  This will typically be harmless,
      * but if T1 and T2 interact causally via some other channel, such that
      * T1's further actions are constrained (in the JMM) to happen after
      * the V2 event, then T1's observation of V1 will be an error.
      * <p>
      * The practical effect of making this.version be volatile is that it cannot
      * be hoisted out of a loop (by an optimizing JIT) or otherwise cached.
      * Some machines may also require a barrier instruction to execute
      * before this.version.
      */
     private volatile Version<T> version = new Version<>(this);
     Version<T> version() { return version; }
     void bumpVersion() { version = new Version<>(this); }
     static class Version<T> {
         private final ClassValue<T> classValue;
         private final Entry<T> promise = new Entry<>(this);
         Version(ClassValue<T> classValue) { this.classValue = classValue; }
         ClassValue<T> classValue() { return classValue; }
         Entry<T> promise() { return promise; }
         boolean isLive() { return classValue.version() == this; }
     }
 
     /** One binding of a value to a class via a ClassValue.
      *  States are:<ul>
      *  <li> promise if value == Entry.this
      *  <li> else dead if version == null
      *  <li> else stale if version != classValue.version
      *  <li> else live </ul>
      *  Promises are never put into the cache; they only live in the
      *  backing map while a computeValue call is in flight.
      *  Once an entry goes stale, it can be reset at any time
      *  into the dead state.
      */
     static class Entry<T> extends WeakReference<Version<T>> {
         final Object value;  // usually of type T, but sometimes (Entry)this
         Entry(Version<T> version, T value) {
             super(version);
             this.value = value;  // for a regular entry, value is of type T
         }
         private void assertNotPromise() { assert(!isPromise()); }
         /** For creating a promise. */
         Entry(Version<T> version) {
             super(version);
             this.value = this;  // for a promise, value is not of type T, but Entry!
         }
         /** Fetch the value.  This entry must not be a promise. */
         @SuppressWarnings("unchecked")  // if !isPromise, type is T
         T value() { assertNotPromise(); return (T) value; }
         boolean isPromise() { return value == this; }
         Version<T> version() { return get(); }
         ClassValue<T> classValueOrNull() {
             Version<T> v = version();
             return (v == null) ? null : v.classValue();
         }
         boolean isLive() {
             Version<T> v = version();
             if (v == null)  return false;
             if (v.isLive())  return true;
             clear();
             return false;
         }
         Entry<T> refreshVersion(Version<T> v2) {
             assertNotPromise();
             @SuppressWarnings("unchecked")  // if !isPromise, type is T
             Entry<T> e2 = new Entry<>(v2, (T) value);
             clear();
             // value = null -- caller must drop
             return e2;
         }
         static final Entry<?> DEAD_ENTRY = new Entry<>(null, null);
     }
 
     /** Return the backing map associated with this type. */
     private static ClassValueMap getMap(Class<?> type) {
         // racing type.classValueMap : null (blank) => unique ClassValueMap
         // if a null is observed, a map is created (lazily, synchronously, uniquely)
         // all further access to that map is synchronized
         ClassValueMap map = type.classValueMap;
         if (map != null)  return map;
         return initializeMap(type);
     }
 
     private static final Object CRITICAL_SECTION = new Object();
     private static ClassValueMap initializeMap(Class<?> type) {
         ClassValueMap map;
         synchronized (CRITICAL_SECTION) {  // private object to avoid deadlocks
             // happens about once per type
             if ((map = type.classValueMap) == null)
-                type.classValueMap = map = new ClassValueMap(type);
+                type.classValueMap = map = new ClassValueMap();
         }
-            return map;
-        }
+        return map;
+    }
 
     static <T> Entry<T> makeEntry(Version<T> explicitVersion, T value) {
         // Note that explicitVersion might be different from this.version.
         return new Entry<>(explicitVersion, value);
 
         // As soon as the Entry is put into the cache, the value will be
         // reachable via a data race (as defined by the Java Memory Model).
         // This race is benign, assuming the value object itself can be
         // read safely by multiple threads.  This is up to the user.
         //
         // The entry and version fields themselves can be safely read via
         // a race because they are either final or have controlled states.
         // If the pointer from the entry to the version is still null,
         // or if the version goes immediately dead and is nulled out,
         // the reader will take the slow path and retry under a lock.
     }
 
     // The following class could also be top level and non-public:
 
-    /** A backing map for all ClassValues, relative a single given type.
+    /** A backing map for all ClassValues.
      *  Gives a fully serialized "true state" for each pair (ClassValue cv, Class type).
      *  Also manages an unserialized fast-path cache.
      */
     static class ClassValueMap extends WeakHashMap<ClassValue.Identity, Entry<?>> {
-        private final Class<?> type;
         private Entry<?>[] cacheArray;
         private int cacheLoad, cacheLoadLimit;
 
         /** Number of entries initially allocated to each type when first used with any ClassValue.
          *  It would be pointless to make this much smaller than the Class and ClassValueMap objects themselves.
          *  Must be a power of 2.
          */
         private static final int INITIAL_ENTRIES = 32;
 
-        /** Build a backing map for ClassValues, relative the given type.
+        /** Build a backing map for ClassValues.
          *  Also, create an empty cache array and install it on the class.
          */
-        ClassValueMap(Class<?> type) {
-            this.type = type;
+        ClassValueMap() {
             sizeCache(INITIAL_ENTRIES);
         }
 
         Entry<?>[] getCache() { return cacheArray; }
 
         /** Initiate a query.  Store a promise (placeholder) if there is no value yet. */
         synchronized
         <T> Entry<T> startEntry(ClassValue<T> classValue) {
             @SuppressWarnings("unchecked")  // one map has entries for all value types <T>
             Entry<T> e = (Entry<T>) get(classValue.identity);
             Version<T> v = classValue.version();
             if (e == null) {
                 e = v.promise();
                 // The presence of a promise means that a value is pending for v.
                 // Eventually, finishEntry will overwrite the promise.
                 put(classValue.identity, e);
                 // Note that the promise is never entered into the cache!
                 return e;
             } else if (e.isPromise()) {
                 // Somebody else has asked the same question.
                 // Let the races begin!
                 if (e.version() != v) {
                     e = v.promise();
                     put(classValue.identity, e);
                 }
                 return e;
             } else {
                 // there is already a completed entry here; report it
                 if (e.version() != v) {
                     // There is a stale but valid entry here; make it fresh again.
                     // Once an entry is in the hash table, we don't care what its version is.
                     e = e.refreshVersion(v);
                     put(classValue.identity, e);
                 }
                 // Add to the cache, to enable the fast path, next time.
                 checkCacheLoad();
                 addToCache(classValue, e);
                 return e;
             }
         }
 
         /** Finish a query.  Overwrite a matching placeholder.  Drop stale incoming values. */
         synchronized
         <T> Entry<T> finishEntry(ClassValue<T> classValue, Entry<T> e) {
             @SuppressWarnings("unchecked")  // one map has entries for all value types <T>
             Entry<T> e0 = (Entry<T>) get(classValue.identity);
             if (e == e0) {
                 // We can get here during exception processing, unwinding from computeValue.
                 assert(e.isPromise());
                 remove(classValue.identity);
                 return null;
             } else if (e0 != null && e0.isPromise() && e0.version() == e.version()) {
                 // If e0 matches the intended entry, there has not been a remove call
                 // between the previous startEntry and now.  So now overwrite e0.
                 Version<T> v = classValue.version();
                 if (e.version() != v)
                     e = e.refreshVersion(v);
                 put(classValue.identity, e);
                 // Add to the cache, to enable the fast path, next time.
                 checkCacheLoad();
                 addToCache(classValue, e);
                 return e;
             } else {
                 // Some sort of mismatch; caller must try again.
                 return null;
             }
         }
 
         /** Remove an entry. */
         synchronized
         void removeEntry(ClassValue<?> classValue) {
             Entry<?> e = remove(classValue.identity);
             if (e == null) {
                 // Uninitialized, and no pending calls to computeValue.  No change.
             } else if (e.isPromise()) {
                 // State is uninitialized, with a pending call to finishEntry.
                 // Since remove is a no-op in such a state, keep the promise
                 // by putting it back into the map.
                 put(classValue.identity, e);
             } else {
                 // In an initialized state.  Bump forward, and de-initialize.
                 classValue.bumpVersion();
                 // Make all cache elements for this guy go stale.
                 removeStaleEntries(classValue);
             }
         }
 
         /** Change the value for an entry. */
         synchronized
         <T> void changeEntry(ClassValue<T> classValue, T value) {
             @SuppressWarnings("unchecked")  // one map has entries for all value types <T>
             Entry<T> e0 = (Entry<T>) get(classValue.identity);
             Version<T> version = classValue.version();
             if (e0 != null) {
                 if (e0.version() == version && e0.value() == value)
                     // no value change => no version change needed
                     return;
                 classValue.bumpVersion();
                 removeStaleEntries(classValue);
             }
             Entry<T> e = makeEntry(version, value);
             put(classValue.identity, e);
             // Add to the cache, to enable the fast path, next time.
             checkCacheLoad();
             addToCache(classValue, e);
         }
 
         /// --------
         /// Cache management.
         /// --------
 
         // Statics do not need synchronization.
 
         /** Load the cache entry at the given (hashed) location. */
         static Entry<?> loadFromCache(Entry<?>[] cache, int i) {
             // non-racing cache.length : constant
             // racing cache[i & (mask)] : null <=> Entry
             return cache[i & (cache.length-1)];
             // invariant:  returned value is null or well-constructed (ready to match)
         }
 
         /** Look in the cache, at the home location for the given ClassValue. */
         static <T> Entry<T> probeHomeLocation(Entry<?>[] cache, ClassValue<T> classValue) {
             return classValue.castEntry(loadFromCache(cache, classValue.hashCodeForCache));
         }
 
         /** Given that first probe was a collision, retry at nearby locations. */
         static <T> Entry<T> probeBackupLocations(Entry<?>[] cache, ClassValue<T> classValue) {
             if (PROBE_LIMIT <= 0)  return null;
             // Probe the cache carefully, in a range of slots.
             int mask = (cache.length-1);
             int home = (classValue.hashCodeForCache & mask);
             Entry<?> e2 = cache[home];  // victim, if we find the real guy
             if (e2 == null) {
                 return null;   // if nobody is at home, no need to search nearby
             }
             // assume !classValue.match(e2), but do not assert, because of races
             int pos2 = -1;
             for (int i = home + 1; i < home + PROBE_LIMIT; i++) {
                 Entry<?> e = cache[i & mask];
                 if (e == null) {
                     break;   // only search within non-null runs
                 }
                 if (classValue.match(e)) {
                     // relocate colliding entry e2 (from cache[home]) to first empty slot
                     cache[home] = e;
                     if (pos2 >= 0) {
                         cache[i & mask] = Entry.DEAD_ENTRY;
                     } else {
                         pos2 = i;
                     }
                     cache[pos2 & mask] = ((entryDislocation(cache, pos2, e2) < PROBE_LIMIT)
                                           ? e2                  // put e2 here if it fits
                                           : Entry.DEAD_ENTRY);
                     return classValue.castEntry(e);
                 }
                 // Remember first empty slot, if any:
                 if (!e.isLive() && pos2 < 0)  pos2 = i;
             }
             return null;
         }
 
         /** How far out of place is e? */
         private static int entryDislocation(Entry<?>[] cache, int pos, Entry<?> e) {
             ClassValue<?> cv = e.classValueOrNull();
             if (cv == null)  return 0;  // entry is not live!
             int mask = (cache.length-1);
             return (pos - cv.hashCodeForCache) & mask;
         }
 
         /// --------
         /// Below this line all functions are private, and assume synchronized access.
         /// --------
 
         private void sizeCache(int length) {
             assert((length & (length-1)) == 0);  // must be power of 2
             cacheLoad = 0;
             cacheLoadLimit = (int) ((double) length * CACHE_LOAD_LIMIT / 100);
             cacheArray = new Entry<?>[length];
         }
 
         /** Make sure the cache load stays below its limit, if possible. */
         private void checkCacheLoad() {
             if (cacheLoad >= cacheLoadLimit) {
                 reduceCacheLoad();
             }
         }
         private void reduceCacheLoad() {
             removeStaleEntries();
             if (cacheLoad < cacheLoadLimit)
                 return;  // win
             Entry<?>[] oldCache = getCache();
             if (oldCache.length > HASH_MASK)
                 return;  // lose
             sizeCache(oldCache.length * 2);
             for (Entry<?> e : oldCache) {
                 if (e != null && e.isLive()) {
                     addToCache(e);
                 }
             }
         }
 
         /** Remove stale entries in the given range.
          *  Should be executed under a Map lock.
          */
         private void removeStaleEntries(Entry<?>[] cache, int begin, int count) {
             if (PROBE_LIMIT <= 0)  return;
             int mask = (cache.length-1);
             int removed = 0;
             for (int i = begin; i < begin + count; i++) {
                 Entry<?> e = cache[i & mask];
                 if (e == null || e.isLive())
                     continue;  // skip null and live entries
                 Entry<?> replacement = null;
                 if (PROBE_LIMIT > 1) {
                     // avoid breaking up a non-null run
                     replacement = findReplacement(cache, i);
                 }
                 cache[i & mask] = replacement;
                 if (replacement == null)  removed += 1;
             }
             cacheLoad = Math.max(0, cacheLoad - removed);
         }
 
         /** Clearing a cache slot risks disconnecting following entries
          *  from the head of a non-null run, which would allow them
          *  to be found via reprobes.  Find an entry after cache[begin]
          *  to plug into the hole, or return null if none is needed.
          */
         private Entry<?> findReplacement(Entry<?>[] cache, int home1) {
             Entry<?> replacement = null;
             int haveReplacement = -1, replacementPos = 0;
             int mask = (cache.length-1);
             for (int i2 = home1 + 1; i2 < home1 + PROBE_LIMIT; i2++) {
                 Entry<?> e2 = cache[i2 & mask];
                 if (e2 == null)  break;  // End of non-null run.
                 if (!e2.isLive())  continue;  // Doomed anyway.
                 int dis2 = entryDislocation(cache, i2, e2);
                 if (dis2 == 0)  continue;  // e2 already optimally placed
                 int home2 = i2 - dis2;
                 if (home2 <= home1) {
                     // e2 can replace entry at cache[home1]
                     if (home2 == home1) {
                         // Put e2 exactly where he belongs.
                         haveReplacement = 1;
                         replacementPos = i2;
                         replacement = e2;
                     } else if (haveReplacement <= 0) {
                         haveReplacement = 0;
                         replacementPos = i2;
                         replacement = e2;
                     }
                     // And keep going, so we can favor larger dislocations.
                 }
             }
             if (haveReplacement >= 0) {
                 if (cache[(replacementPos+1) & mask] != null) {
                     // Be conservative, to avoid breaking up a non-null run.
                     cache[replacementPos & mask] = (Entry<?>) Entry.DEAD_ENTRY;
                 } else {
                     cache[replacementPos & mask] = null;
                     cacheLoad -= 1;
                 }
             }
             return replacement;
         }
 
         /** Remove stale entries in the range near classValue. */
         private void removeStaleEntries(ClassValue<?> classValue) {
             removeStaleEntries(getCache(), classValue.hashCodeForCache, PROBE_LIMIT);
         }
 
         /** Remove all stale entries, everywhere. */
         private void removeStaleEntries() {
             Entry<?>[] cache = getCache();
             removeStaleEntries(cache, 0, cache.length + PROBE_LIMIT - 1);
         }
 
         /** Add the given entry to the cache, in its home location, unless it is out of date. */
         private <T> void addToCache(Entry<T> e) {
             ClassValue<T> classValue = e.classValueOrNull();
             if (classValue != null)
                 addToCache(classValue, e);
         }
 
         /** Add the given entry to the cache, in its home location. */
         private <T> void addToCache(ClassValue<T> classValue, Entry<T> e) {
             if (PROBE_LIMIT <= 0)  return;  // do not fill cache
             // Add e to the cache.
             Entry<?>[] cache = getCache();
             int mask = (cache.length-1);
             int home = classValue.hashCodeForCache & mask;
             Entry<?> e2 = placeInCache(cache, home, e, false);
             if (e2 == null)  return;  // done
             if (PROBE_LIMIT > 1) {
                 // try to move e2 somewhere else in his probe range
                 int dis2 = entryDislocation(cache, home, e2);
                 int home2 = home - dis2;
                 for (int i2 = home2; i2 < home2 + PROBE_LIMIT; i2++) {
                     if (placeInCache(cache, i2 & mask, e2, true) == null) {
                         return;
                     }
                 }
             }
             // Note:  At this point, e2 is just dropped from the cache.
         }
 
         /** Store the given entry.  Update cacheLoad, and return any live victim.
          *  'Gently' means return self rather than dislocating a live victim.
          */
         private Entry<?> placeInCache(Entry<?>[] cache, int pos, Entry<?> e, boolean gently) {
             Entry<?> e2 = overwrittenEntry(cache[pos]);
             if (gently && e2 != null) {
                 // do not overwrite a live entry
                 return e;
             } else {
                 cache[pos] = e;
                 return e2;
             }
         }
 
         /** Note an entry that is about to be overwritten.
          *  If it is not live, quietly replace it by null.
          *  If it is an actual null, increment cacheLoad,
          *  because the caller is going to store something
          *  in its place.
          */
         private <T> Entry<T> overwrittenEntry(Entry<T> e2) {
             if (e2 == null)  cacheLoad += 1;
             else if (e2.isLive())  return e2;
             return null;
         }
 
         /** Percent loading of cache before resize. */
         private static final int CACHE_LOAD_LIMIT = 67;  // 0..100
         /** Maximum number of probes to attempt. */
         private static final int PROBE_LIMIT      =  6;       // 1..
         // N.B.  Set PROBE_LIMIT=0 to disable all fast paths.
     }
 }