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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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|
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package jsr166e; |
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import java.util.Random; |
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import java.util.concurrent.atomic.AtomicInteger; |
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import java.util.concurrent.atomic.AtomicLong; |
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import java.io.IOException; |
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import java.io.Serializable; |
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import java.io.ObjectInputStream; |
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import java.io.ObjectOutputStream; |
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|
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/** |
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* A set of variables that together maintain a sum. When updates |
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* (method {@link #add}) are contended across threads, this set of |
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* adder variables may grow dynamically to reduce contention. Method |
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* {@link #sum} returns the current combined total across these |
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* adders. This value is <em>NOT</em> an atomic snapshot (concurrent |
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* updates may occur while the sum is being calculated), and so cannot |
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* be used alone for fine-grained synchronization control. |
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* |
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* <p> This class may be applicable when many threads frequently |
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* update a common sum that is used for purposes such as collecting |
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* statistics. In this case, performance may be significantly faster |
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* than using a shared {@link AtomicLong}, at the expense of using |
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* much more space. On the other hand, if it is known that only one |
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* thread can ever update the sum, performance may be significantly |
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* slower than just updating a local variable. |
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* |
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* <p>A StripedAdder may optionally be constructed with a given |
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* expected contention level; i.e., the number of threads that are |
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* expected to concurrently update the sum. Supplying an accurate |
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* value may improve performance by reducing the need for dynamic |
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* adjustment. |
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* |
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* @author Doug Lea |
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*/ |
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public class StripedAdder implements Serializable { |
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private static final long serialVersionUID = 7249069246863182397L; |
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|
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/* |
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* A StripedAdder maintains a table of Atomic long variables. The |
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* table is indexed by per-thread hash codes. |
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* |
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* Table entries are of class Adder; a variant of AtomicLong |
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* padded to reduce cache contention on most processors. Padding |
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* is overkill for most Atomics because they are usually |
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* irregularly scattered in memory and thus don't interfere much |
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* with each other. But Atomic objects residing in arrays will |
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* tend to be placed adjacent to each other, and so will most |
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* often share cache lines (with a huge negative performance |
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* impact) without this precaution. |
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* |
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* Because Adders are relatively large, we avoid creating them |
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* until they are needed. On the other hand, we try to create them |
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* on any sign of contention. |
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* |
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* Per-thread hash codes are initialized to random values. |
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* Collisions are indicated by failed CASes when performing an add |
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* operation (see method retryAdd). Upon a collision, if the table |
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* size is less than the capacity, it is doubled in size unless |
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* some other thread holds lock. If a hashed slot is empty, and |
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* lock is available, a new Adder is created. Otherwise, if the |
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* slot exists, a CAS is tried. Retries proceed by "double |
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* hashing", using a secondary hash (Marsaglia XorShift) to try to |
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* find a free slot. |
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* |
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* By default, the table is lazily initialized. Upon first use, |
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* the table is set to size 2 (the minimum non-empty size), but |
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* containing only a single Adder. The maximum table size is |
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* bounded by nearest power of two >= the number of CPUS. The |
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* table size is capped because, when there are more threads than |
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* CPUs, supposing that each thread were bound to a CPU, there |
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* would exist a perfect hash function mapping threads to slots |
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* that eliminates collisions. When we reach capacity, we search |
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* for this mapping by randomly varying the hash codes of |
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* colliding threads. Because search is random, and failures only |
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* become known via CAS failures, convergence will be slow, and |
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* because threads are typically not bound to CPUS forever, may |
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* not occur at all. However, despite these limitations, observed |
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* contention is typically low in these cases. |
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* |
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* A single spinlock is used for resizing the table as well as |
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* populating slots with new Adders. After initialization, there |
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* is no need for a blocking lock: Upon lock contention, threads |
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* try other slots rather than blocking. After initialization, at |
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* least one slot exists, so retries will eventually find a |
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* candidate Adder. During these retries, there is increased |
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* contention and reduced locality, which is still better than |
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* alternatives. |
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*/ |
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|
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private static final int NCPU = Runtime.getRuntime().availableProcessors(); |
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|
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/** |
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* Padded variant of AtomicLong. The value field is placed |
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* between pads, hoping that the JVM doesn't reorder them. |
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* Updates are via inlined CAS in methods add and retryAdd. |
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*/ |
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static final class Adder { |
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volatile long p0, p1, p2, p3, p4, p5, p6; |
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volatile long value; |
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volatile long q0, q1, q2, q3, q4, q5, q6; |
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Adder(long x) { value = x; } |
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} |
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|
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/** |
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* Holder for the thread-local hash code. The code is initially |
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* random, but may be set to a different value upon collisions. |
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*/ |
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static final class HashCode { |
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static final Random rng = new Random(); |
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int code; |
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HashCode() { |
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int h = rng.nextInt(); // Avoid zero, because of xorShift rehash |
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code = (h == 0) ? 1 : h; |
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} |
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} |
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|
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/** |
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* The corresponding ThreadLocal class |
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*/ |
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static final class ThreadHashCode extends ThreadLocal<HashCode> { |
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public HashCode initialValue() { return new HashCode(); } |
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} |
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|
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/** |
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* Static per-thread hash codes. Shared across all StripedAdders |
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* to reduce ThreadLocal pollution and because adjustments due to |
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* collisions in one table are likely to be appropriate for |
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* others. |
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*/ |
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static final ThreadHashCode threadHashCode = new ThreadHashCode(); |
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|
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/** |
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* Table of adders. When non-null, size is a power of 2, at least 2. |
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*/ |
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private transient volatile Adder[] adders; |
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|
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/** |
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* Spinlock (locked via CAS) used when resizing and/or creating Adders. |
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*/ |
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private volatile int busy; |
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|
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/** |
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* Creates a new adder with zero sum. |
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*/ |
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public StripedAdder() { |
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} |
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|
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/** |
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* Creates a new adder with zero sum, and with stripes presized |
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* for the given expected contention level. |
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* |
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* @param expectedContention the expected number of threads that |
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* will concurrently update the sum. |
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*/ |
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public StripedAdder(int expectedContention) { |
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int cap = (expectedContention < NCPU) ? expectedContention : NCPU; |
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int size = 2; |
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while (size < cap) |
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size <<= 1; |
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Adder[] as = new Adder[size]; |
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for (int i = 0; i < size; ++i) |
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as[i] = new Adder(0); |
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this.adders = as; |
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} |
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|
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/** |
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* Adds the given value. |
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* |
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* @param x the value to add |
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*/ |
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public void add(long x) { |
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Adder[] as; Adder a; int n; // locals to hold volatile reads |
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HashCode hc = threadHashCode.get(); |
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int h = hc.code; |
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boolean collide; |
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if ((as = adders) != null && (n = as.length) > 0 && |
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(a = as[(n - 1) & h]) != null) { |
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long v = a.value; |
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if (UNSAFE.compareAndSwapLong(a, valueOffset, v, v + x)) |
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return; |
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collide = true; |
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} |
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else |
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collide = false; |
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retryAdd(x, hc, collide); |
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} |
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|
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/** |
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* Handle cases of add involving initialization, resizing, |
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* creating new Adders, and/or contention. See above for |
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* explanation. This method suffers the usual non-modularity |
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* problems of optimistic retry code, relying on rechecked sets of |
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* reads. |
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*/ |
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private void retryAdd(long x, HashCode hc, boolean collide) { |
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int h = hc.code; |
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for (;;) { |
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Adder[] as; Adder a; int n; |
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if ((as = adders) != null && (n = as.length) > 0) { |
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if ((a = as[(n - 1) & h]) != null) { |
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boolean shared = true; // Slot exists |
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if (collide && n < NCPU && busy == 0 && |
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UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) { |
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try { |
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if (adders == as) { // Expand table |
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Adder[] rs = new Adder[n << 1]; |
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for (int i = 0; i < n; ++i) |
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rs[i] = as[i]; |
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adders = rs; |
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shared = false; |
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} |
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} finally { |
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busy = 0; |
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} |
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if (shared || (h & n) != 0) { |
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collide = false; |
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continue; // Array or index changed |
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} |
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} |
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long v = a.value; |
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if (UNSAFE.compareAndSwapLong(a, valueOffset, v, v + x)) |
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break; |
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collide = shared; |
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} |
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else { // Try to attach new Adder |
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if (busy == 0 && |
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UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) { |
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boolean created = false; |
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try { // Recheck under lock |
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Adder[] rs; int m, j; |
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if ((rs = adders) != null && (m = rs.length) > 0 && |
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rs[j = (m - 1) & h] == null) { |
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rs[j] = new Adder(x); |
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created = true; |
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} |
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} finally { |
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busy = 0; |
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} |
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if (created) |
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break; |
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continue; // Slot is now non-empty |
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} |
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collide = false; |
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} |
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h ^= h << 13; // Rehash |
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h ^= h >>> 17; |
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h ^= h << 5; |
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} |
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else if (busy == 0) { // Default-initialize |
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Adder r = new Adder(x); |
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Adder[] rs = new Adder[2]; |
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rs[h & 1] = r; |
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if (adders == as && busy == 0 && |
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UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1)) { |
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boolean init = false; |
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try { |
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if (adders == as) { |
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adders = rs; |
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init = true; |
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} |
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} finally { |
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busy = 0; |
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} |
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if (init) |
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break; |
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} |
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} |
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else if (adders == as) // Lost initialization race |
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Thread.yield(); |
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} |
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hc.code = h; // Record index for next time |
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} |
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|
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/** |
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* Returns an estimate of the current sum. The result is |
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* calculated by summing multiple variables, so may not be |
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* accurate if updates occur concurrently with this method. |
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* |
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* @return the estimated sum |
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*/ |
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public long sum() { |
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long sum = 0L; |
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Adder[] as = adders; |
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if (as != null) { |
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int n = as.length; |
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for (int i = 0; i < n; ++i) { |
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Adder a = as[i]; |
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if (a != null) |
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sum += a.value; |
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} |
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} |
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return sum; |
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} |
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|
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/** |
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* Resets each of the variables to zero. This is effective in |
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* fully resetting the sum only if there are no concurrent |
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* updates. |
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*/ |
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public void reset() { |
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Adder[] as = adders; |
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if (as != null) { |
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int n = as.length; |
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for (int i = 0; i < n; ++i) { |
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Adder a = as[i]; |
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if (a != null) |
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a.value = 0L; |
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} |
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} |
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} |
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|
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/** |
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* Equivalent to {@code add(1)}. |
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*/ |
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public void increment() { |
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add(1L); |
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} |
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|
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/** |
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* Equivalent to {@code add(-1)}. |
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*/ |
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public void decrement() { |
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add(-1L); |
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} |
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|
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/** |
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* Equivalent to {@link #sum} followed by {@link #reset}. |
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* |
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* @return the estimated sum |
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*/ |
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public long sumAndReset() { |
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long sum = 0L; |
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Adder[] as = adders; |
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if (as != null) { |
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int n = as.length; |
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for (int i = 0; i < n; ++i) { |
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Adder a = as[i]; |
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if (a != null) { |
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sum += a.value; |
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a.value = 0L; |
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} |
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} |
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} |
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return sum; |
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} |
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|
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private void writeObject(java.io.ObjectOutputStream s) |
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throws java.io.IOException { |
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s.defaultWriteObject(); |
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s.writeLong(sum()); |
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} |
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|
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private void readObject(ObjectInputStream s) |
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throws IOException, ClassNotFoundException { |
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s.defaultReadObject(); |
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busy = 0; |
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add(s.readLong()); |
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} |
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|
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// Unsafe mechanics |
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private static final sun.misc.Unsafe UNSAFE; |
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private static final long busyOffset; |
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private static final long valueOffset; |
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static { |
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try { |
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UNSAFE = getUnsafe(); |
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Class<?> sk = StripedAdder.class; |
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busyOffset = UNSAFE.objectFieldOffset |
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(sk.getDeclaredField("busy")); |
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Class<?> ak = Adder.class; |
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valueOffset = UNSAFE.objectFieldOffset |
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(ak.getDeclaredField("value")); |
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} catch (Exception e) { |
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throw new Error(e); |
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} |
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} |
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|
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/** |
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* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
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* Replace with a simple call to Unsafe.getUnsafe when integrating |
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* into a jdk. |
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* |
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* @return a sun.misc.Unsafe |
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*/ |
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private static sun.misc.Unsafe getUnsafe() { |
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try { |
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return sun.misc.Unsafe.getUnsafe(); |
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} catch (SecurityException se) { |
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try { |
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return java.security.AccessController.doPrivileged |
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(new java.security |
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.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
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public sun.misc.Unsafe run() throws Exception { |
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java.lang.reflect.Field f = sun.misc |
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.Unsafe.class.getDeclaredField("theUnsafe"); |
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f.setAccessible(true); |
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return (sun.misc.Unsafe) f.get(null); |
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}}); |
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} catch (java.security.PrivilegedActionException e) { |
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throw new RuntimeException("Could not initialize intrinsics", |
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e.getCause()); |
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} |
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} |
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} |
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|
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} |