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

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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package java.lang;

import java.util.WeakHashMap;
import java.lang.ref.WeakReference;
import java.util.concurrent.atomic.AtomicInteger;

import jdk.internal.misc.Unsafe;

import static java.lang.ClassValue.ClassValueMap.probeHomeLocation;
import static java.lang.ClassValue.ClassValueMap.probeBackupLocations;

/**
 * Lazily associate a computed value with any {@code Class} object.
 * 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.
 * <p>
 * The basic operation of a {@code ClassValue} is {@link #get get}, which
 * returns the associated value, initially created by an invocation to {@link
 * #computeValue computeValue}; multiple invocations may happen under race, but
 * exactly one value is associated to a {@code Class} and returned.
 * <p>
 * Another operation is {@link #remove remove}: it clears the associated value
 * (if it exists), and ensures the next associated value is computed with input
 * states up-to-date with the removal.
 * <p>
 * For a particular association, there is a total order for accesses to the
 * associated value.  Accesses are atomic; they include:
 * <ul>
 * <li>A read-only access by {@code get}</li>
 * <li>An attempt to associate the return value of a {@code computeValue} by
 * {@code get}</li>
 * <li>Clearing of an association by {@code remove}</li>
 * </ul>
 * A {@code get} call always include at least one access; a {@code remove} call
 * always has exactly one access; a {@code computeValue} call always happens
 * between two accesses.  This establishes the order of {@code computeValue}
 * calls with respect to {@code remove} calls and determines whether the
 * results of a {@code computeValue} can be successfully associated by a {@code
 * get}.
 *
 * @param <T> the type of the associated value
 * @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 value to associate to the given {@code Class}.
     * <p>
     * This method is invoked when the initial read-only access by {@link #get
     * get} finds no associated value.
     * <p>
     * If this method throws an exception, the initiating {@code get} call will
     * not attempt to associate a value, and may terminate by returning the
     * associated value if it exists, or by propagating that exception otherwise.
     * <p>
     * Otherwise, the value is computed and returned.  An attempt to associate
     * the return value happens, with one of the following outcomes:
     * <ul>
     * <li>The associated value is present; it is returned and no association
     * is done.</li>
     * <li>The most recent {@link #remove remove} call, if it exists, does not
     * happen-before (JLS {@jls 17.4.5}) the finish of the {@code computeValue}
     * that computed the value to associate.  A new invocation to {@code
     * computeValue}, which that {@code remove} call happens-before, will
     * re-establish this happens-before relationship.</li>
     * <li>Otherwise, this value is successfully associated and returned.</li>
     * </ul>
     *
     * @apiNote
     * A {@code computeValue} call may, due to class loading or other
     * circumstances, recursively call {@code get} or {@code remove} for the
     * same {@code type}.  The recursive {@code get}, if the recursion stops,
     * successfully finishes and this initiating {@code get} observes the
     * associated value from recursion.  The recursive {@code remove} is no-op,
     * since being on the same thread, the {@code remove} already happens-before
     * the finish of this {@code computeValue}; the result from this {@code
     * computeValue} still may be associated.
     *
     * @param type the {@code Class} to associate a value to
     * @return the newly computed value to associate
     * @see #get
     * @see #remove
     */
    protected abstract T computeValue(Class<?> type);

    /**
     * {@return the value associated to the given {@code Class}}
     * <p>
     * This method first performs a read-only access, and returns the associated
     * value if it exists.  Otherwise, this method tries to associate a value
     * from a {@link #computeValue computeValue} invocation until the associated
     * value exists, which could be associated by a competing thread.
     * <p>
     * This method may throw an exception from a {@code computeValue} invocation.
     * In this case, no association happens.
     *
     * @param type the {@code Class} to retrieve the associated value for
     * @throws NullPointerException if the argument is {@code 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 {@code Class} and invalidates
     * all out-of-date computations.  If this association is subsequently
     * {@linkplain #get accessed}, this removal happens-before (JLS {@jls
     * 17.4.5}) the finish of the {@link #computeValue computeValue} call that
     * returned the associated value.
     *
     * @param type the type whose class value must be removed
     * @throws NullPointerException if the argument is {@code null}
     */
    public void remove(Class<?> type) {
        ClassValueMap map = getMap(type);
        map.removeAccess(this);
    }

    // Possible functionality for JSR 292 MR 1
    /*public*/ void put(Class<?> type, T value) {
        ClassValueMap map = getMap(type);
        map.forcedAssociateAccess(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;
        // reads non-null due to StoreStore barrier in critical section in initializeMap
        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);
        var accessed = map.readAccess(this);
        if (accessed instanceof Entry) {
            @SuppressWarnings("unchecked")
            var cast = (Entry<T>) accessed;
            return cast.value;
        }

        RemovalToken token = (RemovalToken) accessed; // nullable
        for (; ; ) {
            T value;
            try {
                value = computeValue(type);
            } catch (Throwable ex) {
                // no value is associated, but there may be already associated
                // value. Return that if it exists.
                accessed = map.readAccess(this);
                if (accessed instanceof Entry) {
                    @SuppressWarnings("unchecked")
                    var cast = (Entry<T>) accessed;
                    return cast.value;
                }
                // report failure here, but allow other callers to try again
                if (ex instanceof RuntimeException rte) {
                    throw rte;
                } else {
                    throw ex instanceof Error err ? err : new Error(ex);
                }
            }
            // computeValue succeed, proceed to associate
            accessed = map.associateAccess(this, token, value);
            if (accessed instanceof Entry) {
                @SuppressWarnings("unchecked")
                var cast = (Entry<T>) accessed;
                return cast.value;
            } else {
                token = (RemovalToken) accessed;
                // repeat
            }
        }
    }

    /** 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.version() == 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 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);

    void bumpVersion() { version = new Version<>(this); }

    record Version<T>(/* Object identity, */ClassValue<T> classValue) {
        boolean isLive() {
            return classValue.version == this;
        }
    }

    /**
     * Besides a value (represented by an Entry), a "removal token" object,
     * including the value {@code null}, can be present at a ClassValue-Class
     * coordinate.  A removal token indicates whether the value from a
     * computation is up-to-date; the value is up-to-date if the token is the
     * same before and after computation (no removal during this period), or if
     * the token is from the same thread (removed during computeValue).
     * {@code null} is the initial state, meaning all computations are valid.
     * Later tokens are always non-null, no matter if they replace existing
     * entries or outdated tokens.
     */
    private static final class RemovalToken {
        // Use thread ID, which presumably don't duplicate and is cheaper than WeakReference
        private final long actorId;

        private RemovalToken() {
            this.actorId = Thread.currentThread().threadId();
        }

        // Arguments are intentionally nullable, to allow initial tokens
        private static boolean allowsAssociation(RemovalToken current, RemovalToken start) {
            // No removal token after the initial can be null
            assert current != null || start == null : current + " : " + start;
            return current == start || current.actorId == Thread.currentThread().threadId();
        }
    }

    /** One binding of a value to a class via a ClassValue.
     *  Shared for the map and the cache array.
     *  The version is only meaningful for the cache array; whatever in the map
     *  is authentic, but state informs the cache an entry may be out-of-date.
     *  States are:<ul>
     *  <li> dead if version == null
     *  <li> stale if version != classValue.version
     *  <li> else live </ul>
     *  Once an entry goes stale, it can be reset at any time
     *  into the dead state.
     */
    static final class Entry<T> {
        final T value;
        final WeakReference<Version<T>> version; // The version exists only for cache invalidation

        Entry(Version<T> version, T value) {
            this.value = value;
            this.version = new WeakReference<>(version);
        }

        Version<T> version() { return version.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;
            version.clear();
            return false;
        }
        Entry<T> refreshVersion(Version<T> v2) {
            return version.refersTo(v2) ? this : new Entry<>(v2, value);
        }
        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 final Unsafe UNSAFE = Unsafe.getUnsafe();
    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) {
                map = new ClassValueMap();
                // getCacheCarefully anticipates entry array to be non-null when
                // a ClassValueMap is published to it.  However, ClassValueMap
                // has no final field, so compiler does not emit a fence, and
                // we must manually issue a Store-Store barrier to prevent
                // the assignment below to be reordered with the store to
                // entry array in the constructor above
                UNSAFE.storeFence();

                type.classValueMap = 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.
     *  Gives a fully serialized "true state" for each pair (ClassValue cv, Class type).
     *  The state may be assigned value or unassigned token.
     *  Also manages an unserialized fast-path cache.
     */
    static final class ClassValueMap extends WeakHashMap<ClassValue.Identity, Object> {
        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.
         *  Also, create an empty cache array and install it on the class.
         */
        ClassValueMap() {
            sizeCache(INITIAL_ENTRIES);
        }

        Entry<?>[] getCache() { return cacheArray; }

        // A simple read access to this map, for the initial step of get or failure recovery.
        // This may refresh the entry for the cache, but the associated value always stays the same.
        synchronized <T> Object readAccess(ClassValue<T> classValue) {
            var item = get(classValue.identity);
            if (item instanceof Entry) {
                @SuppressWarnings("unchecked")
                var entry = (Entry<T>) item;
                // cache refresh
                var updated = entry.refreshVersion(classValue.version);
                if (updated != entry) {
                    put(classValue.identity, updated);
                }
                // Add to the cache, to enable the fast path, next time.
                checkCacheLoad();
                addToCache(classValue, updated);
            }
            return item;
        }

        // An association attempt, for when a computeValue returns a value.
        synchronized <T> Object associateAccess(ClassValue<T> classValue, RemovalToken startToken, T value) {
            var item = readAccess(classValue);
            if (item instanceof Entry)
                return item; // value already associated
            var currentToken = (RemovalToken) item;
            if (RemovalToken.allowsAssociation(currentToken, startToken)) {
                var entry = makeEntry(classValue.version, value);
                put(classValue.identity, entry);
                // Add to the cache, to enable the fast path, next time.
                checkCacheLoad();
                addToCache(classValue, entry);
                return entry;
            }
            return currentToken;
        }

        // A removal, requiring subsequent associations to be up-to-date with it.
        synchronized void removeAccess(ClassValue<?> classValue) {
            // Always put in a token to invalidate ongoing computations
            put(classValue.identity, new RemovalToken());
            classValue.bumpVersion();
            removeStaleEntries(classValue);
        }

        // A forced association, requires cache to flush.
        synchronized <T> void forcedAssociateAccess(ClassValue<T> classValue, T value) {
            classValue.bumpVersion();
            removeStaleEntries();
            var entry = makeEntry(classValue.version, value);
            put(classValue.identity, entry);
            // Add to the cache, to enable the fast path, next time.
            checkCacheLoad();
            addToCache(classValue, entry);
        }

        //| --------
        //| 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.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.
    }
}