src/jsr166e/StripedAdder.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 high update contention, throughput of this class is
 * expected to be significantly higher, at the expense of higher space
 * consumption.  Under low contention, this class imposes very little
 * time and space overhead compared to AtomicLong.  On the other hand,
 * in contexts where it is statically known that only one thread can
 * ever update a sum, time and space overhead is noticeably greater
 * than just updating a local variable.
 *
 * <p> Method {@link #sum} returns the current combined total across
 * the variables maintaining the sum.  This value is <em>NOT</em> an
 * atomic snapshot: Concurrent updates may occur while the sum is
 * being calculated.  However, updates cannot be "lost", so invocation
 * of <code>sum</code> in the absence of concurrent updates always
 * returns an accurate result.  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 StripedAdder implements Serializable {
    private static final long serialVersionUID = 7249069246863182397L;

    /*
     * A StripedAdder 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.
     *
     */

    private static final int NCPU = Runtime.getRuntime().availableProcessors();

    /**
     * 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 StripedAdders
     * 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();

    /**
     * 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 StripedAdder() {
    }

    /**
     * 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; boolean contended;
        if ((as = cells) != null ||
            !UNSAFE.compareAndSwapLong(this, baseOffset, v = base, v + x)) {
            int h = (hc = threadHashCode.get()).code;
            if (as != null && (n = as.length) > 0 &&
                (a = as[(n - 1) & h]) != null) {
                if (UNSAFE.compareAndSwapLong(a, valueOffset,
                                              v = a.value, v + x))
                    return;
                contended = true;
            }
            else
                contended = false;
            retryAdd(x, hc, contended);
        }
    }

    /**
     * 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 precontended true if CAS failed before call
     */
    private void retryAdd(long x, HashCode hc, boolean precontended) {
        int h = hc.code;
        boolean collide = false; // true if last slot nonempty
        for (;;) {
            Cell[] as; Cell a; int n;
            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 (precontended)          // CAS already known to fail
                    precontended = false;       // Continue after rehash
                else {
                    long v = a.value;
                    if (UNSAFE.compareAndSwapLong(a, valueOffset, v, v + x))
                        break;
                    if (!collide)
                        collide = true;
                    else if (n >= NCPU || cells != as)
                        collide = false;        // Can't expand
                    else if (busy == 0 &&
                             UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
                        collide = false;
                        try {
                            if (cells == as) { // Expand table
                                Cell[] rs = new Cell[n << 1];
                                for (int i = 0; i < n; ++i)
                                    rs[i] = as[i];
                                cells = rs;
                            }
                        } finally {
                            busy = 0;
                        }
                        continue;
                    }
                }
                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
                    if (cells == as) {
                        Cell r = new Cell(x);
                        Cell[] rs = new Cell[2];
                        rs[h & 1] = r;
                        cells = rs;
                        init = true;
                    }
                } finally {
                    busy = 0;
                }
                if (init)
                    break;
            }
            else {                              // Lost initialization race
                long b = base;                  // Fall back on using base
                if (UNSAFE.compareAndSwapLong(this, baseOffset, b, b + x))
                    break;
            }
        }
        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 = StripedAdder.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:	1.12
Committed:	Sat Jul 30 16:26:34 2011 UTC (12 years, 10 months ago) by dl
Branch:	MAIN
Changes since 1.11:	+179 -149 lines
Log Message:	API, spec and implementation improvements
#	User	Rev	Content
1	dl	1.1	/*
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	dl	1.12	* 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	jsr166	1.2	*
21	dl	1.12	* <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 high update contention, throughput of this class is
25			* expected to be significantly higher, at the expense of higher space
26			* consumption. Under low contention, this class imposes very little
27			* time and space overhead compared to AtomicLong. On the other hand,
28			* in contexts where it is statically known that only one thread can
29			* ever update a sum, time and space overhead is noticeably greater
30	dl	1.10	* than just updating a local variable.
31	dl	1.1	*
32	dl	1.12	* <p> Method {@link #sum} returns the current combined total across
33			* the variables maintaining the sum. This value is <em>NOT</em> an
34			* atomic snapshot: Concurrent updates may occur while the sum is
35			* being calculated. However, updates cannot be "lost", so invocation
36			* of <code>sum</code> in the absence of concurrent updates always
37			* returns an accurate result. The sum may also be <code>reset</code>
38			* to zero, as an alternative to creating a new adder. However,
39			* method {@link #reset} is intrinsically racy, so should only be used
40			* when it is known that no threads are concurrently updating the sum.
41			*
42			* <p><em>jsr166e note: This class is targeted to be placed in
43			* java.util.concurrent.atomic<em>
44	dl	1.3	*
45	jsr166	1.2	* @author Doug Lea
46	dl	1.1	*/
47			public class StripedAdder implements Serializable {
48			private static final long serialVersionUID = 7249069246863182397L;
49
50			/*
51	dl	1.12	* A StripedAdder maintains a lazily-initialized table of
52			* atomically updated variables, plus an extra "base" field. The
53			* table size is a power of two. Indexing uses masked per-thread
54			* hash codes
55			*
56			* Table entries are of class Cell; a variant of AtomicLong padded
57			* to reduce cache contention on most processors. Padding is
58			* overkill for most Atomics because they are usually irregularly
59			* scattered in memory and thus don't interfere much with each
60			* other. But Atomic objects residing in arrays will tend to be
61			* placed adjacent to each other, and so will most often share
62			* cache lines (with a huge negative performance impact) without
63			* this precaution.
64			*
65			* In part because Cells are relatively large, we avoid creating
66			* them until they are needed. When there is no contention, all
67			* updates are made to the base field. Upon first contention (a
68			* failed CAS on base update), the table is initialized to size 2.
69			* The table size is doubled upon further contention until
70			* reaching the nearest power of two greater than or equal to the
71			* number of CPUS.
72	dl	1.3	*
73	dl	1.12	* Per-thread hash codes are initialized to random values.
74			* Contention and/or table collisions are indicated by failed
75			* CASes when performing an add operation (see method
76			* retryAdd). Upon a collision, if the table size is less than the
77			* capacity, it is doubled in size unless some other thread holds
78			* the lock. If a hashed slot is empty, and lock is available, a
79			* new Cell is created. Otherwise, if the slot exists, a CAS is
80			* tried. Retries proceed by "double hashing", using a secondary
81			* hash (Marsaglia XorShift) to try to find a free slot.
82			*
83			* The table size is capped because, when there are more threads
84			* than CPUs, supposing that each thread were bound to a CPU,
85			* there would exist a perfect hash function mapping threads to
86			* slots that eliminates collisions. When we reach capacity, we
87			* search for this mapping by randomly varying the hash codes of
88			* colliding threads. Because search is random, and collisions
89			* only become known via CAS failures, convergence can be slow,
90			* and because threads are typically not bound to CPUS forever,
91			* may not occur at all. However, despite these limitations,
92			* observed contention rates are typically low in these cases.
93			*
94			* A single spinlock is used for initializing and resizing the
95			* table, as well as populating slots with new Cells. There is no
96			* need for a blocking lock: Upon lock contention, threads try
97			* other slots (or the base) rather than blocking. During these
98			* retries, there is increased contention and reduced locality,
99			* which is still better than alternatives.
100			*
101			* It is possible for a Cell to become unused when threads that
102			* once hashed to it terminate, as well as in the case where
103			* doubling the table causes no thread to hash to it under
104			* expanded mask. We do not try to detect or remove such cells,
105			* under the assumption that for long-running adders, observed
106			* contention levels will recur, so the cells will eventually be
107			* needed again; and for short-lived ones, it does not matter.
108	dl	1.6	*
109	dl	1.1	*/
110
111	dl	1.6	private static final int NCPU = Runtime.getRuntime().availableProcessors();
112	dl	1.5
113			/**
114	dl	1.8	* Padded variant of AtomicLong. The value field is placed
115			* between pads, hoping that the JVM doesn't reorder them.
116			* Updates are via inlined CAS in methods add and retryAdd.
117			*/
118	dl	1.12	static final class Cell {
119	dl	1.8	volatile long p0, p1, p2, p3, p4, p5, p6;
120			volatile long value;
121			volatile long q0, q1, q2, q3, q4, q5, q6;
122	dl	1.12	Cell(long x) { value = x; }
123	dl	1.8	}
124	dl	1.1
125	jsr166	1.2	/**
126	dl	1.6	* Holder for the thread-local hash code. The code is initially
127			* random, but may be set to a different value upon collisions.
128	dl	1.1	*/
129			static final class HashCode {
130	dl	1.6	static final Random rng = new Random();
131	dl	1.1	int code;
132	dl	1.6	HashCode() {
133	dl	1.12	int h = rng.nextInt(); // Avoid zero to allow xorShift rehash
134	dl	1.8	code = (h == 0) ? 1 : h;
135	dl	1.6	}
136	dl	1.1	}
137
138			/**
139			* The corresponding ThreadLocal class
140			*/
141			static final class ThreadHashCode extends ThreadLocal<HashCode> {
142	dl	1.6	public HashCode initialValue() { return new HashCode(); }
143	dl	1.1	}
144
145			/**
146			* Static per-thread hash codes. Shared across all StripedAdders
147	dl	1.6	* to reduce ThreadLocal pollution and because adjustments due to
148			* collisions in one table are likely to be appropriate for
149			* others.
150	dl	1.1	*/
151			static final ThreadHashCode threadHashCode = new ThreadHashCode();
152
153			/**
154	dl	1.12	* Table of cells. When non-null, size is a power of 2.
155	dl	1.1	*/
156	dl	1.12	private transient volatile Cell[] cells;
157	dl	1.1
158			/**
159	dl	1.12	* Base sum, used mainly when there is no contention, but also as
160			* a fallback during table initializion races. Updated via CAS.
161	dl	1.1	*/
162	dl	1.12	private transient volatile long base;
163	dl	1.1
164			/**
165	dl	1.12	* Spinlock (locked via CAS) used when resizing and/or creating Cells.
166	dl	1.1	*/
167	dl	1.12	private transient volatile int busy;
168	dl	1.3
169			/**
170	dl	1.12	* Creates a new adder with initial sum of zero.
171	dl	1.3	*/
172	dl	1.12	public StripedAdder() {
173	dl	1.1	}
174
175			/**
176			* Adds the given value.
177			*
178			* @param x the value to add
179			*/
180			public void add(long x) {
181	dl	1.12	Cell[] as; long v; HashCode hc; Cell a; int n; boolean contended;
182			if ((as = cells) != null \|\|
183			!UNSAFE.compareAndSwapLong(this, baseOffset, v = base, v + x)) {
184			int h = (hc = threadHashCode.get()).code;
185			if (as != null && (n = as.length) > 0 &&
186			(a = as[(n - 1) & h]) != null) {
187			if (UNSAFE.compareAndSwapLong(a, valueOffset,
188			v = a.value, v + x))
189			return;
190			contended = true;
191			}
192			else
193			contended = false;
194			retryAdd(x, hc, contended);
195	dl	1.8	}
196	dl	1.4	}
197
198			/**
199			* Handle cases of add involving initialization, resizing,
200	dl	1.12	* creating new Cells, and/or contention. See above for
201	dl	1.8	* explanation. This method suffers the usual non-modularity
202			* problems of optimistic retry code, relying on rechecked sets of
203			* reads.
204	dl	1.10	*
205			* @param x the value to add
206			* @param hc the hash code holder
207			* @param precontended true if CAS failed before call
208	dl	1.4	*/
209	dl	1.10	private void retryAdd(long x, HashCode hc, boolean precontended) {
210	dl	1.4	int h = hc.code;
211	dl	1.10	boolean collide = false; // true if last slot nonempty
212	dl	1.6	for (;;) {
213	dl	1.12	Cell[] as; Cell a; int n;
214			if ((as = cells) != null && (n = as.length) > 0) {
215	dl	1.10	if ((a = as[(n - 1) & h]) == null) {
216	dl	1.12	if (busy == 0) { // Try to attach new Cell
217			Cell r = new Cell(x); // Optimistically create
218	dl	1.11	if (busy == 0 &&
219			UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
220			boolean created = false;
221			try { // Recheck under lock
222	dl	1.12	Cell[] rs; int m, j;
223			if ((rs = cells) != null &&
224	dl	1.11	(m = rs.length) > 0 &&
225			rs[j = (m - 1) & h] == null) {
226			rs[j] = r;
227			created = true;
228			}
229			} finally {
230			busy = 0;
231	dl	1.8	}
232	dl	1.11	if (created)
233			break;
234			continue; // Slot is now non-empty
235	dl	1.8	}
236	dl	1.6	}
237	dl	1.8	collide = false;
238	dl	1.6	}
239	dl	1.10	else if (precontended) // CAS already known to fail
240			precontended = false; // Continue after rehash
241			else {
242			long v = a.value;
243			if (UNSAFE.compareAndSwapLong(a, valueOffset, v, v + x))
244			break;
245			if (!collide)
246			collide = true;
247	dl	1.12	else if (n >= NCPU \|\| cells != as)
248	dl	1.11	collide = false; // Can't expand
249	dl	1.10	else if (busy == 0 &&
250			UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
251			collide = false;
252			try {
253	dl	1.12	if (cells == as) { // Expand table
254			Cell[] rs = new Cell[n << 1];
255	dl	1.10	for (int i = 0; i < n; ++i)
256			rs[i] = as[i];
257	dl	1.12	cells = rs;
258	dl	1.10	}
259			} finally {
260			busy = 0;
261			}
262			continue;
263			}
264			}
265	dl	1.8	h ^= h << 13; // Rehash
266			h ^= h >>> 17;
267			h ^= h << 5;
268	dl	1.6	}
269	dl	1.12	else if (busy == 0 && cells == as &&
270			UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) {
271	dl	1.10	boolean init = false;
272	dl	1.12	try { // Initialize
273			if (cells == as) {
274			Cell r = new Cell(x);
275			Cell[] rs = new Cell[2];
276			rs[h & 1] = r;
277			cells = rs;
278			init = true;
279	dl	1.5	}
280	dl	1.12	} finally {
281			busy = 0;
282	dl	1.1	}
283	dl	1.10	if (init)
284			break;
285	dl	1.5	}
286	dl	1.12	else { // Lost initialization race
287			long b = base; // Fall back on using base
288			if (UNSAFE.compareAndSwapLong(this, baseOffset, b, b + x))
289			break;
290			}
291	dl	1.1	}
292	dl	1.8	hc.code = h; // Record index for next time
293	dl	1.1	}
294
295			/**
296	dl	1.9	* Equivalent to {@code add(1)}.
297			*/
298			public void increment() {
299			add(1L);
300			}
301
302			/**
303			* Equivalent to {@code add(-1)}.
304			*/
305			public void decrement() {
306			add(-1L);
307			}
308
309			/**
310	dl	1.12	* Returns the current sum. The result is only guaranteed to be
311			* accurate in the absence of concurrent updates. Otherwise, it
312			* may fail to reflect one or more updates occuring while
313			* calculating the result.
314	jsr166	1.2	*
315	dl	1.12	* @return the sum
316	dl	1.1	*/
317			public long sum() {
318	dl	1.12	Cell[] as = cells;
319			long sum = base;
320	dl	1.4	if (as != null) {
321			int n = as.length;
322			for (int i = 0; i < n; ++i) {
323	dl	1.12	Cell a = as[i];
324	dl	1.4	if (a != null)
325	dl	1.8	sum += a.value;
326	dl	1.4	}
327	dl	1.1	}
328			return sum;
329			}
330
331			/**
332	dl	1.12	* Resets variables maintaining the sum to zero. This is
333			* effective in setting the sum to zero only if there are no
334			* concurrent updates.
335			*/
336			public void reset() {
337			Cell[] as = cells;
338			base = 0L;
339			if (as != null) {
340			int n = as.length;
341			for (int i = 0; i < n; ++i) {
342			Cell a = as[i];
343			if (a != null)
344			a.value = 0L;
345			}
346			}
347			}
348
349			/**
350			* Equivalent in effect to {@link #sum} followed by {@link
351			* #reset}. This method may apply for example during quiescent
352			* points between multithreaded computations. If there are
353			* updates concurrent with this method, the returned value is
354			* <em>not</em> guaranteed to be the final sum occurring before
355			* the reset.
356			*
357			* @return the sum
358			*/
359			public long sumThenReset() {
360			Cell[] as = cells;
361			long sum = base;
362			base = 0L;
363	dl	1.4	if (as != null) {
364			int n = as.length;
365			for (int i = 0; i < n; ++i) {
366	dl	1.12	Cell a = as[i];
367	dl	1.4	if (a != null) {
368	dl	1.8	sum += a.value;
369			a.value = 0L;
370	dl	1.4	}
371	dl	1.1	}
372			}
373			return sum;
374			}
375
376			private void writeObject(java.io.ObjectOutputStream s)
377			throws java.io.IOException {
378			s.defaultWriteObject();
379			s.writeLong(sum());
380			}
381
382			private void readObject(ObjectInputStream s)
383			throws IOException, ClassNotFoundException {
384			s.defaultReadObject();
385	dl	1.8	busy = 0;
386	dl	1.12	cells = null;
387			base = s.readLong();
388	dl	1.1	}
389
390	dl	1.8	// Unsafe mechanics
391			private static final sun.misc.Unsafe UNSAFE;
392	dl	1.12	private static final long baseOffset;
393	dl	1.8	private static final long busyOffset;
394			private static final long valueOffset;
395			static {
396			try {
397			UNSAFE = getUnsafe();
398			Class<?> sk = StripedAdder.class;
399	dl	1.12	baseOffset = UNSAFE.objectFieldOffset
400			(sk.getDeclaredField("base"));
401	dl	1.8	busyOffset = UNSAFE.objectFieldOffset
402			(sk.getDeclaredField("busy"));
403	dl	1.12	Class<?> ak = Cell.class;
404	dl	1.8	valueOffset = UNSAFE.objectFieldOffset
405			(ak.getDeclaredField("value"));
406			} catch (Exception e) {
407			throw new Error(e);
408			}
409			}
410
411			/**
412			* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
413			* Replace with a simple call to Unsafe.getUnsafe when integrating
414			* into a jdk.
415			*
416			* @return a sun.misc.Unsafe
417			*/
418			private static sun.misc.Unsafe getUnsafe() {
419			try {
420			return sun.misc.Unsafe.getUnsafe();
421			} catch (SecurityException se) {
422			try {
423			return java.security.AccessController.doPrivileged
424			(new java.security
425			.PrivilegedExceptionAction<sun.misc.Unsafe>() {
426			public sun.misc.Unsafe run() throws Exception {
427			java.lang.reflect.Field f = sun.misc
428			.Unsafe.class.getDeclaredField("theUnsafe");
429			f.setAccessible(true);
430			return (sun.misc.Unsafe) f.get(null);
431			}});
432			} catch (java.security.PrivilegedActionException e) {
433			throw new RuntimeException("Could not initialize intrinsics",
434			e.getCause());
435			}
436			}
437			}
438
439	dl	1.1	}