src/jsr166e/LongAdder.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package jsr166e;
import java.util.Random;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import java.io.IOException;
import java.io.Serializable;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

/**
 * One or more variables that together maintain an initially zero sum.
 * When updates (method {@link #add}) are contended across threads,
 * the set of variables may grow dynamically to reduce contention.
 *
 * <p> This class is usually preferable to {@link AtomicLong} when
 * multiple threads update a common sum that is used for purposes such
 * as collecting statistics, not for fine-grained synchronization
 * control.  Under low update contention, the two classes have similar
 * characteristics. But under high contention, expected throughput of
 * this class is significantly higher, at the expense of higher space
 * consumption.
 *
 * <p> Method {@link #sum} returns the current combined total across
 * the variables maintaining the sum.  This value is <em>NOT</em> an
 * atomic snapshot: Invocation of <code>sum</code> in the absence of
 * concurrent updates returns an accurate result, but concurrent
 * updates that occur while the sum is being calculated might not be
 * incorporated.  The sum may also be <code>reset</code> to zero, as
 * an alternative to creating a new adder.  However, method {@link
 * #reset} is intrinsically racy, so should only be used when it is
 * known that no threads are concurrently updating the sum.
 *
 * <p><em>jsr166e note: This class is targeted to be placed in
 * java.util.concurrent.atomic<em>
 *
 * @author Doug Lea
 */
public class LongAdder implements Serializable {
    private static final long serialVersionUID = 7249069246863182397L;

    /*
     * A LongAdder maintains a lazily-initialized table of atomically
     * updated variables, plus an extra "base" field. The table size
     * is a power of two. Indexing uses masked per-thread hash codes
     *
     * Table entries are of class Cell; a variant of AtomicLong padded
     * to reduce cache contention on most processors. Padding is
     * overkill for most Atomics because they are usually irregularly
     * scattered in memory and thus don't interfere much with each
     * other. But Atomic objects residing in arrays will tend to be
     * placed adjacent to each other, and so will most often share
     * cache lines (with a huge negative performance impact) without
     * this precaution.
     *
     * In part because Cells are relatively large, we avoid creating
     * them until they are needed.  When there is no contention, all
     * updates are made to the base field.  Upon first contention (a
     * failed CAS on base update), the table is initialized to size 2.
     * The table size is doubled upon further contention until
     * reaching the nearest power of two greater than or equal to the
     * number of CPUS.
     *
     * Per-thread hash codes are initialized to random values.
     * Contention and/or table collisions are indicated by failed
     * CASes when performing an add operation (see method
     * retryAdd). Upon a collision, if the table size is less than the
     * capacity, it is doubled in size unless some other thread holds
     * the lock. If a hashed slot is empty, and lock is available, a
     * new Cell is created. Otherwise, if the slot exists, a CAS is
     * tried.  Retries proceed by "double hashing", using a secondary
     * hash (Marsaglia XorShift) to try to find a free slot.
     *
     * The table size is capped because, when there are more threads
     * than CPUs, supposing that each thread were bound to a CPU,
     * there would exist a perfect hash function mapping threads to
     * slots that eliminates collisions. When we reach capacity, we
     * search for this mapping by randomly varying the hash codes of
     * colliding threads.  Because search is random, and collisions
     * only become known via CAS failures, convergence can be slow,
     * and because threads are typically not bound to CPUS forever,
     * may not occur at all. However, despite these limitations,
     * observed contention rates are typically low in these cases.
     *
     * A single spinlock is used for initializing and resizing the
     * table, as well as populating slots with new Cells.  There is no
     * need for a blocking lock: Upon lock contention, threads try
     * other slots (or the base) rather than blocking.  During these
     * retries, there is increased contention and reduced locality,
     * which is still better than alternatives.
     *
     * It is possible for a Cell to become unused when threads that
     * once hashed to it terminate, as well as in the case where
     * doubling the table causes no thread to hash to it under
     * expanded mask.  We do not try to detect or remove such cells,
     * under the assumption that for long-running adders, observed
     * contention levels will recur, so the cells will eventually be
     * needed again; and for short-lived ones, it does not matter.
     *
     * JVM intrinsics note: It would be possible to use a release-only
     * form of CAS here, if it were provided.
     */

    /**
     * Padded variant of AtomicLong.  The value field is placed
     * between pads, hoping that the JVM doesn't reorder them.
     * Updates are via inlined CAS in methods add and retryAdd.
     */
    static final class Cell {
        volatile long p0, p1, p2, p3, p4, p5, p6;
        volatile long value;
        volatile long q0, q1, q2, q3, q4, q5, q6;
        Cell(long x) { value = x; }
    }

    /**
     * Holder for the thread-local hash code. The code is initially
     * random, but may be set to a different value upon collisions.
     */
    static final class HashCode {
        static final Random rng = new Random();
        int code;
        HashCode() {
            int h = rng.nextInt(); // Avoid zero to allow xorShift rehash
            code = (h == 0) ? 1 : h;
        }
    }

    /**
     * The corresponding ThreadLocal class
     */
    static final class ThreadHashCode extends ThreadLocal<HashCode> {
        public HashCode initialValue() { return new HashCode(); }
    }

    /**
     * Static per-thread hash codes. Shared across all LongAdders
     * to reduce ThreadLocal pollution and because adjustments due to
     * collisions in one table are likely to be appropriate for
     * others.
     */
    static final ThreadHashCode threadHashCode = new ThreadHashCode();

    /** Nomber of CPUS, to place bound on table size */
    private static final int NCPU = Runtime.getRuntime().availableProcessors();

    /**
     * Table of cells. When non-null, size is a power of 2.
     */
    private transient volatile Cell[] cells;

    /**
     * Base sum, used mainly when there is no contention, but also as
     * a fallback during table initializion races. Updated via CAS.
     */
    private transient volatile long base;

    /**
     * Spinlock (locked via CAS) used when resizing and/or creating Cells.
     */
    private transient volatile int busy;

    /**
     * Creates a new adder with initial sum of zero.
     */
    public LongAdder() {
    }

    /**
     * Adds the given value.
     *
     * @param x the value to add
     */
    public void add(long x) {
        Cell[] as; long v; HashCode hc; Cell a; int n;
        if ((as = cells) != null ||
            !UNSAFE.compareAndSwapLong(this, baseOffset, v = base, v + x)) {
            boolean uncontended = true;
            int h = (hc = threadHashCode.get()).code;
            if (as == null || (n = as.length) < 1 ||
                (a = as[(n - 1) & h]) == null ||
                !(uncontended = UNSAFE.compareAndSwapLong(a, valueOffset,
                                                          v = a.value, v + x)))
                retryAdd(x, hc, uncontended);
        }
    }

    /**
     * Handle cases of add involving initialization, resizing,
     * creating new Cells, and/or contention. See above for
     * explanation. This method suffers the usual non-modularity
     * problems of optimistic retry code, relying on rechecked sets of
     * reads.
     *
     * @param x the value to add
     * @param hc the hash code holder
     * @param wasUncontended false if CAS failed before call
     */
    private void retryAdd(long x, HashCode hc, boolean wasUncontended) {
        int h = hc.code;
        boolean collide = false;                // True if last slot nonempty
        for (;;) {
            Cell[] as; Cell a; int n; long v;
            if ((as = cells) != null && (n = as.length) > 0) {
                if ((a = as[(n - 1) & h]) == null) {
                    if (busy == 0) {            // Try to attach new Cell
                        Cell r = new Cell(x);   // Optimistically create
                        if (busy == 0 &&
                            UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
                            boolean created = false;
                            try {               // Recheck under lock
                                Cell[] rs; int m, j;
                                if ((rs = cells) != null &&
                                    (m = rs.length) > 0 &&
                                    rs[j = (m - 1) & h] == null) {
                                    rs[j] = r;
                                    created = true;
                                }
                            } finally {
                                busy = 0;
                            }
                            if (created)
                                break;
                            continue;           // Slot is now non-empty
                        }
                    }
                    collide = false;
                }
                else if (!wasUncontended)       // CAS already known to fail
                    wasUncontended = true;      // Continue after rehash
                else if (UNSAFE.compareAndSwapLong(a, valueOffset,
                                                   v = a.value, v + x))
                    break;
                else if (n >= NCPU || cells != as)
                    collide = false;            // At max size or stale
                else if (!collide)
                    collide = true;
                else if (busy == 0 &&           // Try to expand table
                         UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
                    try {
                        if (cells == as) {
                            Cell[] rs = new Cell[n << 1];
                            for (int i = 0; i < n; ++i)
                                rs[i] = as[i];
                            cells = rs;
                        }
                    } finally {
                        busy = 0;
                    }
                    collide = false;
                    continue;                   // Retry with expanded table
                }
                h ^= h << 13;                   // Rehash
                h ^= h >>> 17;
                h ^= h << 5;
            }
            else if (busy == 0 && cells == as &&
                     UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
                boolean init = false;
                try {                           // Initialize table
                    if (cells == as) {
                        Cell[] rs = new Cell[2];
                        rs[h & 1] = new Cell(x);
                        cells = rs;
                        init = true;
                    }
                } finally {
                    busy = 0;
                }
                if (init)
                    break;
            }
            else if (UNSAFE.compareAndSwapLong(this, baseOffset,
                                               v = base, v + x))
                break;                          // Fall back on using base
        }
        hc.code = h;                            // Record index for next time
    }

    /**
     * Equivalent to {@code add(1)}.
     */
    public void increment() {
        add(1L);
    }

    /**
     * Equivalent to {@code add(-1)}.
     */
    public void decrement() {
        add(-1L);
    }

    /**
     * Returns the current sum.  The result is only guaranteed to be
     * accurate in the absence of concurrent updates. Otherwise, it
     * may fail to reflect one or more updates occuring while
     * calculating the result.
     *
     * @return the sum
     */
    public long sum() {
        Cell[] as = cells;
        long sum = base;
        if (as != null) {
            int n = as.length;
            for (int i = 0; i < n; ++i) {
                Cell a = as[i];
                if (a != null)
                    sum += a.value;
            }
        }
        return sum;
    }

    /**
     * Resets variables maintaining the sum to zero.  This is
     * effective in setting the sum to zero only if there are no
     * concurrent updates.
     */
    public void reset() {
        Cell[] as = cells;
        base = 0L;
        if (as != null) {
            int n = as.length;
            for (int i = 0; i < n; ++i) {
                Cell a = as[i];
                if (a != null)
                    a.value = 0L;
            }
        }
    }

    /**
     * Equivalent in effect to {@link #sum} followed by {@link
     * #reset}. This method may apply for example during quiescent
     * points between multithreaded computations.  If there are
     * updates concurrent with this method, the returned value is
     * <em>not</em> guaranteed to be the final sum occurring before
     * the reset.
     *
     * @return the sum
     */
    public long sumThenReset() {
        Cell[] as = cells;
        long sum = base;
        base = 0L;
        if (as != null) {
            int n = as.length;
            for (int i = 0; i < n; ++i) {
                Cell a = as[i];
                if (a != null) {
                    sum += a.value;
                    a.value = 0L;
                }
            }
        }
        return sum;
    }

    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        s.defaultWriteObject();
        s.writeLong(sum());
    }

    private void readObject(ObjectInputStream s)
        throws IOException, ClassNotFoundException {
        s.defaultReadObject();
        busy = 0;
        cells = null;
        base = s.readLong();
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long baseOffset;
    private static final long busyOffset;
    private static final long valueOffset;
    static {
        try {
            UNSAFE = getUnsafe();
            Class<?> sk = LongAdder.class;
            baseOffset = UNSAFE.objectFieldOffset
                (sk.getDeclaredField("base"));
            busyOffset = UNSAFE.objectFieldOffset
                (sk.getDeclaredField("busy"));
            Class<?> ak = Cell.class;
            valueOffset = UNSAFE.objectFieldOffset
                (ak.getDeclaredField("value"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }

    /**
     * Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
     * Replace with a simple call to Unsafe.getUnsafe when integrating
     * into a jdk.
     *
     * @return a sun.misc.Unsafe
     */
    private static sun.misc.Unsafe getUnsafe() {
        try {
            return sun.misc.Unsafe.getUnsafe();
        } catch (SecurityException se) {
            try {
                return java.security.AccessController.doPrivileged
                    (new java.security
                     .PrivilegedExceptionAction<sun.misc.Unsafe>() {
                        public sun.misc.Unsafe run() throws Exception {
                            java.lang.reflect.Field f = sun.misc
                                .Unsafe.class.getDeclaredField("theUnsafe");
                            f.setAccessible(true);
                            return (sun.misc.Unsafe) f.get(null);
                        }});
            } catch (java.security.PrivilegedActionException e) {
                throw new RuntimeException("Could not initialize intrinsics",
                                           e.getCause());
            }
        }
    }

}
Revision:	1.2
Committed:	Mon Aug 1 04:18:59 2011 UTC (12 years, 9 months ago) by jsr166
Branch:	MAIN
Changes since 1.1:	+3 -3 lines
Log Message:	whitespace
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/publicdomain/zero/1.0/
5	*/
6
7	package jsr166e;
8	import java.util.Random;
9	import java.util.concurrent.atomic.AtomicInteger;
10	import java.util.concurrent.atomic.AtomicLong;
11	import java.io.IOException;
12	import java.io.Serializable;
13	import java.io.ObjectInputStream;
14	import java.io.ObjectOutputStream;
15
16	/**
17	* One or more variables that together maintain an initially zero sum.
18	* When updates (method {@link #add}) are contended across threads,
19	* the set of variables may grow dynamically to reduce contention.
20	*
21	* <p> This class is usually preferable to {@link AtomicLong} when
22	* multiple threads update a common sum that is used for purposes such
23	* as collecting statistics, not for fine-grained synchronization
24	* control. Under low update contention, the two classes have similar
25	* characteristics. But under high contention, expected throughput of
26	* this class is significantly higher, at the expense of higher space
27	* consumption.
28	*
29	* <p> Method {@link #sum} returns the current combined total across
30	* the variables maintaining the sum. This value is <em>NOT</em> an
31	* atomic snapshot: Invocation of <code>sum</code> in the absence of
32	* concurrent updates returns an accurate result, but concurrent
33	* updates that occur while the sum is being calculated might not be
34	* incorporated. The sum may also be <code>reset</code> to zero, as
35	* an alternative to creating a new adder. However, method {@link
36	* #reset} is intrinsically racy, so should only be used when it is
37	* known that no threads are concurrently updating the sum.
38	*
39	* <p><em>jsr166e note: This class is targeted to be placed in
40	* java.util.concurrent.atomic<em>
41	*
42	* @author Doug Lea
43	*/
44	public class LongAdder implements Serializable {
45	private static final long serialVersionUID = 7249069246863182397L;
46
47	/*
48	* A LongAdder maintains a lazily-initialized table of atomically
49	* updated variables, plus an extra "base" field. The table size
50	* is a power of two. Indexing uses masked per-thread hash codes
51	*
52	* Table entries are of class Cell; a variant of AtomicLong padded
53	* to reduce cache contention on most processors. Padding is
54	* overkill for most Atomics because they are usually irregularly
55	* scattered in memory and thus don't interfere much with each
56	* other. But Atomic objects residing in arrays will tend to be
57	* placed adjacent to each other, and so will most often share
58	* cache lines (with a huge negative performance impact) without
59	* this precaution.
60	*
61	* In part because Cells are relatively large, we avoid creating
62	* them until they are needed. When there is no contention, all
63	* updates are made to the base field. Upon first contention (a
64	* failed CAS on base update), the table is initialized to size 2.
65	* The table size is doubled upon further contention until
66	* reaching the nearest power of two greater than or equal to the
67	* number of CPUS.
68	*
69	* Per-thread hash codes are initialized to random values.
70	* Contention and/or table collisions are indicated by failed
71	* CASes when performing an add operation (see method
72	* retryAdd). Upon a collision, if the table size is less than the
73	* capacity, it is doubled in size unless some other thread holds
74	* the lock. If a hashed slot is empty, and lock is available, a
75	* new Cell is created. Otherwise, if the slot exists, a CAS is
76	* tried. Retries proceed by "double hashing", using a secondary
77	* hash (Marsaglia XorShift) to try to find a free slot.
78	*
79	* The table size is capped because, when there are more threads
80	* than CPUs, supposing that each thread were bound to a CPU,
81	* there would exist a perfect hash function mapping threads to
82	* slots that eliminates collisions. When we reach capacity, we
83	* search for this mapping by randomly varying the hash codes of
84	* colliding threads. Because search is random, and collisions
85	* only become known via CAS failures, convergence can be slow,
86	* and because threads are typically not bound to CPUS forever,
87	* may not occur at all. However, despite these limitations,
88	* observed contention rates are typically low in these cases.
89	*
90	* A single spinlock is used for initializing and resizing the
91	* table, as well as populating slots with new Cells. There is no
92	* need for a blocking lock: Upon lock contention, threads try
93	* other slots (or the base) rather than blocking. During these
94	* retries, there is increased contention and reduced locality,
95	* which is still better than alternatives.
96	*
97	* It is possible for a Cell to become unused when threads that
98	* once hashed to it terminate, as well as in the case where
99	* doubling the table causes no thread to hash to it under
100	* expanded mask. We do not try to detect or remove such cells,
101	* under the assumption that for long-running adders, observed
102	* contention levels will recur, so the cells will eventually be
103	* needed again; and for short-lived ones, it does not matter.
104	*
105	* JVM intrinsics note: It would be possible to use a release-only
106	* form of CAS here, if it were provided.
107	*/
108
109	/**
110	* Padded variant of AtomicLong. The value field is placed
111	* between pads, hoping that the JVM doesn't reorder them.
112	* Updates are via inlined CAS in methods add and retryAdd.
113	*/
114	static final class Cell {
115	volatile long p0, p1, p2, p3, p4, p5, p6;
116	volatile long value;
117	volatile long q0, q1, q2, q3, q4, q5, q6;
118	Cell(long x) { value = x; }
119	}
120
121	/**
122	* Holder for the thread-local hash code. The code is initially
123	* random, but may be set to a different value upon collisions.
124	*/
125	static final class HashCode {
126	static final Random rng = new Random();
127	int code;
128	HashCode() {
129	int h = rng.nextInt(); // Avoid zero to allow xorShift rehash
130	code = (h == 0) ? 1 : h;
131	}
132	}
133
134	/**
135	* The corresponding ThreadLocal class
136	*/
137	static final class ThreadHashCode extends ThreadLocal<HashCode> {
138	public HashCode initialValue() { return new HashCode(); }
139	}
140
141	/**
142	* Static per-thread hash codes. Shared across all LongAdders
143	* to reduce ThreadLocal pollution and because adjustments due to
144	* collisions in one table are likely to be appropriate for
145	* others.
146	*/
147	static final ThreadHashCode threadHashCode = new ThreadHashCode();
148
149	/** Nomber of CPUS, to place bound on table size */
150	private static final int NCPU = Runtime.getRuntime().availableProcessors();
151
152	/**
153	* Table of cells. When non-null, size is a power of 2.
154	*/
155	private transient volatile Cell[] cells;
156
157	/**
158	* Base sum, used mainly when there is no contention, but also as
159	* a fallback during table initializion races. Updated via CAS.
160	*/
161	private transient volatile long base;
162
163	/**
164	* Spinlock (locked via CAS) used when resizing and/or creating Cells.
165	*/
166	private transient volatile int busy;
167
168	/**
169	* Creates a new adder with initial sum of zero.
170	*/
171	public LongAdder() {
172	}
173
174	/**
175	* Adds the given value.
176	*
177	* @param x the value to add
178	*/
179	public void add(long x) {
180	Cell[] as; long v; HashCode hc; Cell a; int n;
181	if ((as = cells) != null \|\|
182	!UNSAFE.compareAndSwapLong(this, baseOffset, v = base, v + x)) {
183	boolean uncontended = true;
184	int h = (hc = threadHashCode.get()).code;
185	if (as == null \|\| (n = as.length) < 1 \|\|
186	(a = as[(n - 1) & h]) == null \|\|
187	!(uncontended = UNSAFE.compareAndSwapLong(a, valueOffset,
188	v = a.value, v + x)))
189	retryAdd(x, hc, uncontended);
190	}
191	}
192
193	/**
194	* Handle cases of add involving initialization, resizing,
195	* creating new Cells, and/or contention. See above for
196	* explanation. This method suffers the usual non-modularity
197	* problems of optimistic retry code, relying on rechecked sets of
198	* reads.
199	*
200	* @param x the value to add
201	* @param hc the hash code holder
202	* @param wasUncontended false if CAS failed before call
203	*/
204	private void retryAdd(long x, HashCode hc, boolean wasUncontended) {
205	int h = hc.code;
206	boolean collide = false; // True if last slot nonempty
207	for (;;) {
208	Cell[] as; Cell a; int n; long v;
209	if ((as = cells) != null && (n = as.length) > 0) {
210	if ((a = as[(n - 1) & h]) == null) {
211	if (busy == 0) { // Try to attach new Cell
212	Cell r = new Cell(x); // Optimistically create
213	if (busy == 0 &&
214	UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
215	boolean created = false;
216	try { // Recheck under lock
217	Cell[] rs; int m, j;
218	if ((rs = cells) != null &&
219	(m = rs.length) > 0 &&
220	rs[j = (m - 1) & h] == null) {
221	rs[j] = r;
222	created = true;
223	}
224	} finally {
225	busy = 0;
226	}
227	if (created)
228	break;
229	continue; // Slot is now non-empty
230	}
231	}
232	collide = false;
233	}
234	else if (!wasUncontended) // CAS already known to fail
235	wasUncontended = true; // Continue after rehash
236	else if (UNSAFE.compareAndSwapLong(a, valueOffset,
237	v = a.value, v + x))
238	break;
239	else if (n >= NCPU \|\| cells != as)
240	collide = false; // At max size or stale
241	else if (!collide)
242	collide = true;
243	else if (busy == 0 && // Try to expand table
244	UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
245	try {
246	if (cells == as) {
247	Cell[] rs = new Cell[n << 1];
248	for (int i = 0; i < n; ++i)
249	rs[i] = as[i];
250	cells = rs;
251	}
252	} finally {
253	busy = 0;
254	}
255	collide = false;
256	continue; // Retry with expanded table
257	}
258	h ^= h << 13; // Rehash
259	h ^= h >>> 17;
260	h ^= h << 5;
261	}
262	else if (busy == 0 && cells == as &&
263	UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
264	boolean init = false;
265	try { // Initialize table
266	if (cells == as) {
267	Cell[] rs = new Cell[2];
268	rs[h & 1] = new Cell(x);
269	cells = rs;
270	init = true;
271	}
272	} finally {
273	busy = 0;
274	}
275	if (init)
276	break;
277	}
278	else if (UNSAFE.compareAndSwapLong(this, baseOffset,
279	v = base, v + x))
280	break; // Fall back on using base
281	}
282	hc.code = h; // Record index for next time
283	}
284
285	/**
286	* Equivalent to {@code add(1)}.
287	*/
288	public void increment() {
289	add(1L);
290	}
291
292	/**
293	* Equivalent to {@code add(-1)}.
294	*/
295	public void decrement() {
296	add(-1L);
297	}
298
299	/**
300	* Returns the current sum. The result is only guaranteed to be
301	* accurate in the absence of concurrent updates. Otherwise, it
302	* may fail to reflect one or more updates occuring while
303	* calculating the result.
304	*
305	* @return the sum
306	*/
307	public long sum() {
308	Cell[] as = cells;
309	long sum = base;
310	if (as != null) {
311	int n = as.length;
312	for (int i = 0; i < n; ++i) {
313	Cell a = as[i];
314	if (a != null)
315	sum += a.value;
316	}
317	}
318	return sum;
319	}
320
321	/**
322	* Resets variables maintaining the sum to zero. This is
323	* effective in setting the sum to zero only if there are no
324	* concurrent updates.
325	*/
326	public void reset() {
327	Cell[] as = cells;
328	base = 0L;
329	if (as != null) {
330	int n = as.length;
331	for (int i = 0; i < n; ++i) {
332	Cell a = as[i];
333	if (a != null)
334	a.value = 0L;
335	}
336	}
337	}
338
339	/**
340	* Equivalent in effect to {@link #sum} followed by {@link
341	* #reset}. This method may apply for example during quiescent
342	* points between multithreaded computations. If there are
343	* updates concurrent with this method, the returned value is
344	* <em>not</em> guaranteed to be the final sum occurring before
345	* the reset.
346	*
347	* @return the sum
348	*/
349	public long sumThenReset() {
350	Cell[] as = cells;
351	long sum = base;
352	base = 0L;
353	if (as != null) {
354	int n = as.length;
355	for (int i = 0; i < n; ++i) {
356	Cell a = as[i];
357	if (a != null) {
358	sum += a.value;
359	a.value = 0L;
360	}
361	}
362	}
363	return sum;
364	}
365
366	private void writeObject(java.io.ObjectOutputStream s)
367	throws java.io.IOException {
368	s.defaultWriteObject();
369	s.writeLong(sum());
370	}
371
372	private void readObject(ObjectInputStream s)
373	throws IOException, ClassNotFoundException {
374	s.defaultReadObject();
375	busy = 0;
376	cells = null;
377	base = s.readLong();
378	}
379
380	// Unsafe mechanics
381	private static final sun.misc.Unsafe UNSAFE;
382	private static final long baseOffset;
383	private static final long busyOffset;
384	private static final long valueOffset;
385	static {
386	try {
387	UNSAFE = getUnsafe();
388	Class<?> sk = LongAdder.class;
389	baseOffset = UNSAFE.objectFieldOffset
390	(sk.getDeclaredField("base"));
391	busyOffset = UNSAFE.objectFieldOffset
392	(sk.getDeclaredField("busy"));
393	Class<?> ak = Cell.class;
394	valueOffset = UNSAFE.objectFieldOffset
395	(ak.getDeclaredField("value"));
396	} catch (Exception e) {
397	throw new Error(e);
398	}
399	}
400
401	/**
402	* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
403	* Replace with a simple call to Unsafe.getUnsafe when integrating
404	* into a jdk.
405	*
406	* @return a sun.misc.Unsafe
407	*/
408	private static sun.misc.Unsafe getUnsafe() {
409	try {
410	return sun.misc.Unsafe.getUnsafe();
411	} catch (SecurityException se) {
412	try {
413	return java.security.AccessController.doPrivileged
414	(new java.security
415	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
416	public sun.misc.Unsafe run() throws Exception {
417	java.lang.reflect.Field f = sun.misc
418	.Unsafe.class.getDeclaredField("theUnsafe");
419	f.setAccessible(true);
420	return (sun.misc.Unsafe) f.get(null);
421	}});
422	} catch (java.security.PrivilegedActionException e) {
423	throw new RuntimeException("Could not initialize intrinsics",
424	e.getCause());
425	}
426	}
427	}
428
429	}