changeset 4606:6fb7e795664e

7030453: JSR 292 ClassValue.get method is too slow Summary: Implement ClassValue cooperatively with Class like ThreadLocal with Thread. Reviewed-by: twisti, mduigou
author jrose
date Wed, 18 Jan 2012 17:34:32 -0800
parents 98f844077547
children e22dda65a42c
files src/share/classes/java/lang/Class.java src/share/classes/java/lang/ClassValue.java test/java/lang/invoke/ClassValueTest.java
diffstat 3 files changed, 595 insertions(+), 76 deletions(-) [+]
line wrap: on
line diff
--- a/src/share/classes/java/lang/Class.java	Wed Jan 18 17:34:30 2012 -0800
+++ b/src/share/classes/java/lang/Class.java	Wed Jan 18 17:34:32 2012 -0800
@@ -3114,4 +3114,9 @@
     AnnotationType getAnnotationType() {
         return annotationType;
     }
+
+    /* Backing store of user-defined values pertaining to this class.
+     * Maintained by the ClassValue class.
+     */
+    transient ClassValue.ClassValueMap classValueMap;
 }
--- a/src/share/classes/java/lang/ClassValue.java	Wed Jan 18 17:34:30 2012 -0800
+++ b/src/share/classes/java/lang/ClassValue.java	Wed Jan 18 17:34:32 2012 -0800
@@ -25,9 +25,14 @@
 
 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
@@ -92,14 +97,22 @@
      * @see #computeValue
      */
     public T get(Class<?> type) {
-        ClassValueMap map = getMap(type);
-        if (map != null) {
-            Object x = map.get(this);
-            if (x != null) {
-                return (T) map.unmaskNull(x);
-            }
-        }
-        return setComputedValue(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);
     }
 
     /**
@@ -157,83 +170,582 @@
      */
     public void remove(Class<?> type) {
         ClassValueMap map = getMap(type);
-        if (map != null) {
-            synchronized (map) {
-                map.remove(this);
+        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)
         }
     }
 
-    /// Implementation...
-    // FIXME: Use a data structure here similar that of ThreadLocal (7030453).
-
-    private static final AtomicInteger STORE_BARRIER = new AtomicInteger();
-
-    /** Slow path for {@link #get}. */
-    private T setComputedValue(Class<?> type) {
-        ClassValueMap map = getMap(type);
-        if (map == null) {
-            map = initializeMap(type);
-        }
-        T value = computeValue(type);
-        STORE_BARRIER.lazySet(0);
-        // All stores pending from computeValue are completed.
-        synchronized (map) {
-            // Warm up the table with a null entry.
-            map.preInitializeEntry(this);
-        }
-        STORE_BARRIER.lazySet(0);
-        // All stores pending from table expansion are completed.
-        synchronized (map) {
-            value = (T) map.initializeEntry(this, value);
-            // One might fear a possible race condition here
-            // if the code for map.put has flushed the write
-            // to map.table[*] before the writes to the Map.Entry
-            // are done.  This is not possible, since we have
-            // warmed up the table with an empty entry.
-        }
-        return value;
+    /** 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>)
     }
 
-    // Replace this map by a per-class slot.
-    private static final WeakHashMap<Class<?>, ClassValueMap> ROOT
-        = new WeakHashMap<Class<?>, ClassValueMap>();
+    /** Internal hash code for accessing Class.classValueMap.cacheArray. */
+    final int hashCodeForCache = nextHashCode.getAndAdd(HASH_INCREMENT) & HASH_MASK;
 
-    private static ClassValueMap getMap(Class<?> type) {
-        type.getClass();  // test for null
-        return ROOT.get(type);
+    /** 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
+     * (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) {
-        synchronized (ClassValue.class) {
-            ClassValueMap map = ROOT.get(type);
-            if (map == null)
-                ROOT.put(type, map = new ClassValueMap());
+        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);
+        }
             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.
     }
 
-    static class ClassValueMap extends WeakHashMap<ClassValue, Object> {
-        /** Make sure this table contains an Entry for the given key, even if it is empty. */
-        void preInitializeEntry(ClassValue key) {
-            if (!this.containsKey(key))
-                this.put(key, null);
-        }
-        /** Make sure this table contains a non-empty Entry for the given key. */
-        Object initializeEntry(ClassValue key, Object value) {
-            Object prior = this.get(key);
-            if (prior != null) {
-                return unmaskNull(prior);
-            }
-            this.put(key, maskNull(value));
-            return value;
+    // The following class could also be top level and non-public:
+
+    /** A backing map for all ClassValues, relative a single given type.
+     *  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.
+         *  Also, create an empty cache array and install it on the class.
+         */
+        ClassValueMap(Class<?> type) {
+            this.type = type;
+            sizeCache(INITIAL_ENTRIES);
         }
 
-        Object maskNull(Object x) {
-            return x == null ? this : x;
+        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;
+            }
         }
-        Object unmaskNull(Object x) {
-            return x == this ? null : x;
+
+        /** 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) {
+            // make all cache elements for this guy go stale:
+            if (remove(classValue.identity) != null) {
+                classValue.bumpVersion();
+                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.
     }
 }
--- a/test/java/lang/invoke/ClassValueTest.java	Wed Jan 18 17:34:30 2012 -0800
+++ b/test/java/lang/invoke/ClassValueTest.java	Wed Jan 18 17:34:32 2012 -0800
@@ -61,7 +61,7 @@
         }
     }
 
-    static final Class[] CLASSES = {
+    static final Class<?>[] CLASSES = {
         String.class,
         Integer.class,
         int.class,
@@ -73,11 +73,11 @@
     @Test
     public void testGet() {
         countForCV1 = 0;
-        for (Class c : CLASSES) {
+        for (Class<?> c : CLASSES) {
             assertEquals(nameForCV1(c), CV1.get(c));
         }
         assertEquals(CLASSES.length, countForCV1);
-        for (Class c : CLASSES) {
+        for (Class<?> c : CLASSES) {
             assertEquals(nameForCV1(c), CV1.get(c));
         }
         assertEquals(CLASSES.length, countForCV1);
@@ -85,7 +85,7 @@
 
     @Test
     public void testRemove() {
-        for (Class c : CLASSES) {
+        for (Class<?> c : CLASSES) {
             CV1.get(c);
         }
         countForCV1 = 0;
@@ -94,7 +94,7 @@
             CV1.remove(CLASSES[i]);
         }
         assertEquals(0, countForCV1);  // no change
-        for (Class c : CLASSES) {
+        for (Class<?> c : CLASSES) {
             assertEquals(nameForCV1(c), CV1.get(c));
         }
         assertEquals(REMCOUNT, countForCV1);
@@ -124,7 +124,7 @@
         for (int pass = 0; pass <= 2; pass++) {
             for (int i1 = 0; i1 < CVN_COUNT1; i1++) {
                 eachClass:
-                for (Class c : CLASSES) {
+                for (Class<?> c : CLASSES) {
                     for (int i2 = 0; i2 < CVN_COUNT2; i2++) {
                         int n = i1*CVN_COUNT2 + i2;
                         assertEquals(0, countForCVN);
@@ -156,8 +156,10 @@
             }
         }
         assertEquals(countForCVN, 0);
-        for (int n = 0; n < cvns.length; n++) {
-            for (Class c : CLASSES) {
+        System.out.println("[rechecking values]");
+        for (int i = 0; i < cvns.length * 10; i++) {
+            int n = i % cvns.length;
+            for (Class<?> c : CLASSES) {
                 assertEquals(nameForCVN(c, n), cvns[n].get(c));
             }
         }