diff -r 000000000000 -r 98bdfed1a6e9 rt/emul/compact/src/main/java/java/lang/ClassValue.java --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/rt/emul/compact/src/main/java/java/lang/ClassValue.java Sun Aug 10 05:55:55 2014 +0200 @@ -0,0 +1,760 @@ +/* + * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved. + * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. + * + * This code is free software; you can redistribute it and/or modify it + * under the terms of the GNU General Public License version 2 only, as + * published by the Free Software Foundation. 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 + * 2 along with this work; if not, write to the Free Software Foundation, + * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. + * + * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA + * or visit www.oracle.com if you need additional information or have any + * questions. + */ + +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 { + /** + * Sole constructor. (For invocation by subclass constructors, typically + * implicit.) + */ + protected ClassValue() { + } + + /** + * Computes the given class's derived value for this {@code ClassValue}. + *

+ * This method will be invoked within the first thread that accesses + * the value with the {@link #get get} method. + *

+ * 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}. + *

+ * 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. + *

+ * 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. + *

+ * 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}. + *

+ * 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 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. + *

+ * 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. + *

+ * 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}. + *

+ * 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. + *

+ * 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}. + *

+ * Otherwise, whether the new state is even or odd, + * {@code T} will discard the newly computed value + * and retry the {@code get} operation. + *

+ * Discarding and retrying is an important proviso, + * since otherwise {@code T} could potentially install + * a disastrously stale value. For example: + *

+ * 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. + * + * @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 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 castEntry(Entry e) { return (Entry) 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 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.) + } + + /** 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. + *

+ * 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. + *

+ * 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. + *

+ * 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 version = new Version<>(this); + Version version() { return version; } + void bumpVersion() { version = new Version<>(this); } + static class Version { + private final ClassValue classValue; + private final Entry promise = new Entry<>(this); + Version(ClassValue classValue) { this.classValue = classValue; } + ClassValue classValue() { return classValue; } + Entry promise() { return promise; } + boolean isLive() { return classValue.version() == this; } + } + + /** One binding of a value to a class via a ClassValue. + * States are:

    + *
  • promise if value == Entry.this + *
  • else dead if version == null + *
  • else stale if version != classValue.version + *
  • else live
+ * 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 extends WeakReference> { + final Object value; // usually of type T, but sometimes (Entry)this + Entry(Version 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 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 version() { return get(); } + ClassValue classValueOrNull() { + Version v = version(); + return (v == null) ? null : v.classValue(); + } + boolean isLive() { + Version v = version(); + if (v == null) return false; + if (v.isLive()) return true; + clear(); + return false; + } + Entry refreshVersion(Version v2) { + assertNotPromise(); + @SuppressWarnings("unchecked") // if !isPromise, type is T + Entry 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); + } + return map; + } + + static Entry makeEntry(Version 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. + * 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> { + 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); + } + + Entry[] getCache() { return cacheArray; } + + /** Initiate a query. Store a promise (placeholder) if there is no value yet. */ + synchronized + Entry startEntry(ClassValue classValue) { + @SuppressWarnings("unchecked") // one map has entries for all value types + Entry e = (Entry) get(classValue.identity); + Version 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 + Entry finishEntry(ClassValue classValue, Entry e) { + @SuppressWarnings("unchecked") // one map has entries for all value types + Entry e0 = (Entry) 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 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 + void changeEntry(ClassValue classValue, T value) { + @SuppressWarnings("unchecked") // one map has entries for all value types + Entry e0 = (Entry) get(classValue.identity); + Version 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 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 Entry probeHomeLocation(Entry[] cache, ClassValue classValue) { + return classValue.castEntry(loadFromCache(cache, classValue.hashCodeForCache)); + } + + /** Given that first probe was a collision, retry at nearby locations. */ + static Entry probeBackupLocations(Entry[] cache, ClassValue 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 void addToCache(Entry e) { + ClassValue classValue = e.classValueOrNull(); + if (classValue != null) + addToCache(classValue, e); + } + + /** Add the given entry to the cache, in its home location. */ + private void addToCache(ClassValue classValue, Entry 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 Entry overwrittenEntry(Entry 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. + } +}