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streamsupport-atomic is a backport of the java.util.concurrent.atomic classes
added in Java 8 (Double/Long Accumulator/Adder) for Java 6 / 7 and Android developers
/*
* 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 java8.util.concurrent.atomic;
import java.lang.reflect.Method;
import java.util.Arrays;
import java8.util.function.LongBinaryOperator;
import java8.util.concurrent.ThreadLocalRandom;
import java8.util.function.DoubleBinaryOperator;
/**
* A package-local class holding common representation and mechanics
* for classes supporting dynamic striping on 64bit values. The class
* extends Number so that concrete subclasses must publicly do so.
*/
@SuppressWarnings("serial")
abstract class Striped64 extends Number {
// CVS rev. 1.26
/*
* This class 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.
* Nearly all declarations in this class are package-private,
* accessed directly by subclasses.
*
* Table entries are of class Cell; a variant of AtomicLong padded
* (via @sun.misc.Contended) to reduce cache contention. 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. Table slots remain empty (null) until they are
* needed.
*
* A single spinlock ("cellsBusy") is used for initializing and
* resizing the table, as well as populating slots with new Cells.
* There is no need for a blocking lock; when the lock is not
* available, threads try other slots (or the base). During these
* retries, there is increased contention and reduced locality,
* which is still better than alternatives.
*
* The Thread probe fields maintained via ThreadLocalRandom serve
* as per-thread hash codes. We let them remain uninitialized as
* zero (if they come in this way) until they contend at slot
* 0. They are then initialized to values that typically do not
* often conflict with others. Contention and/or table collisions
* are indicated by failed CASes when performing an update
* operation. 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.
*
* 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 instances, observed
* contention levels will recur, so the cells will eventually be
* needed again; and for short-lived ones, it does not matter.
*/
/**
* Padded variant of AtomicLong supporting only raw accesses plus CAS.
* The value field is placed between pads, hoping that the JVM doesn't
* reorder them.
*
* JVM intrinsics note: It would be possible to use a release-only
* form of CAS here, if it were provided.
*/
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; }
final boolean cas(long cmp, long val) {
return U.compareAndSwapLong(this, VALUE, cmp, val);
}
final void reset() {
U.putLongVolatile(this, VALUE, 0L);
}
final void reset(long identity) {
U.putLongVolatile(this, VALUE, identity);
}
final long getAndSet(long val) {
return getAndSetLong(this, VALUE, val);
}
// Unsafe mechanics
private static final sun.misc.Unsafe U = UnsafeAccess.unsafe;
private static final long VALUE;
static {
try {
VALUE = U.objectFieldOffset(Cell.class
.getDeclaredField("value"));
} catch (Exception e) {
throw new ExceptionInInitializerError(e);
}
}
}
/** Number of CPUS, to place bound on table size */
static final int NCPU = Runtime.getRuntime().availableProcessors();
/**
* Table of cells. When non-null, size is a power of 2.
*/
transient volatile Cell[] cells;
/**
* Base value, used mainly when there is no contention, but also as
* a fallback during table initialization races. Updated via CAS.
*/
transient volatile long base;
/**
* Spinlock (locked via CAS) used when resizing and/or creating Cells.
*/
transient volatile int cellsBusy;
/**
* Package-private default constructor.
*/
Striped64() {
}
/**
* CASes the base field.
*/
final boolean casBase(long cmp, long val) {
return U.compareAndSwapLong(this, BASE, cmp, val);
}
final long getAndSetBase(long val) {
return getAndSetLong(this, BASE, val);
}
/**
* CASes the cellsBusy field from 0 to 1 to acquire lock.
*/
final boolean casCellsBusy() {
return U.compareAndSwapInt(this, CELLSBUSY, 0, 1);
}
/**
* Returns the probe value for the current thread.
* Duplicated from ThreadLocalRandom because of packaging restrictions.
*/
static int getProbe() {
try {
return ((Integer) GET_PROBE_METHOD.invoke(null)).intValue();
} catch (Exception e) {
throw new Error(e);
}
}
private static void setProbe(int probe) {
try {
SET_PROBE_METHOD.invoke(null, probe);
} catch (Exception e) {
throw new Error(e);
}
}
/**
* Pseudo-randomly advances and records the given probe value for the
* given thread.
* Duplicated from ThreadLocalRandom because of packaging restrictions.
*/
static int advanceProbe(int probe) {
probe ^= probe << 13; // xorshift
probe ^= probe >>> 17;
probe ^= probe << 5;
setProbe(probe);
return probe;
}
/**
* Handles cases of updates 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
* @param fn the update function, or null for add (this convention
* avoids the need for an extra field or function in LongAdder).
* @param wasUncontended false if CAS failed before call
*/
final void longAccumulate(long x, LongBinaryOperator fn,
boolean wasUncontended) {
int h;
if ((h = getProbe()) == 0) {
ThreadLocalRandom.current(); // force initialization
h = getProbe();
wasUncontended = true;
}
boolean collide = false; // True if last slot nonempty
done: for (;;) {
Cell[] cs; Cell c; int n; long v;
if ((cs = cells) != null && (n = cs.length) > 0) {
if ((c = cs[(n - 1) & h]) == null) {
if (cellsBusy == 0) { // Try to attach new Cell
Cell r = new Cell(x); // Optimistically create
if (cellsBusy == 0 && casCellsBusy()) {
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;
break done;
}
} finally {
cellsBusy = 0;
}
continue; // Slot is now non-empty
}
}
collide = false;
}
else if (!wasUncontended) // CAS already known to fail
wasUncontended = true; // Continue after rehash
else if (c.cas(v = c.value,
((fn == null) ? v + x : fn.applyAsLong(v, x))))
break;
else if (n >= NCPU || cells != cs)
collide = false; // At max size or stale
else if (!collide)
collide = true;
else if (cellsBusy == 0 && casCellsBusy()) {
try {
if (cells == cs) { // Expand table unless stale
cells = Arrays.copyOf(cs, n << 1);
}
} finally {
cellsBusy = 0;
}
collide = false;
continue; // Retry with expanded table
}
h = advanceProbe(h);
}
else if (cellsBusy == 0 && cells == cs && casCellsBusy()) {
try { // Initialize table
if (cells == cs) {
Cell[] rs = new Cell[2];
rs[h & 1] = new Cell(x);
cells = rs;
break done;
}
} finally {
cellsBusy = 0;
}
}
// Fall back on using base
else if (casBase(v = base,
((fn == null) ? v + x : fn.applyAsLong(v, x)))) {
break done;
}
}
}
private static long apply(DoubleBinaryOperator fn, long v, double x) {
double d = Double.longBitsToDouble(v);
d = (fn == null) ? d + x : fn.applyAsDouble(d, x);
return Double.doubleToRawLongBits(d);
}
/**
* Same as longAccumulate, but injecting long/double conversions
* in too many places to sensibly merge with long version, given
* the low-overhead requirements of this class. So must instead be
* maintained by copy/paste/adapt.
*/
final void doubleAccumulate(double x, DoubleBinaryOperator fn,
boolean wasUncontended) {
int h;
if ((h = getProbe()) == 0) {
ThreadLocalRandom.current(); // force initialization
h = getProbe();
wasUncontended = true;
}
boolean collide = false; // True if last slot nonempty
done: for (;;) {
Cell[] cs; Cell c; int n; long v;
if ((cs = cells) != null && (n = cs.length) > 0) {
if ((c = cs[(n - 1) & h]) == null) {
if (cellsBusy == 0) { // Try to attach new Cell
Cell r = new Cell(Double.doubleToRawLongBits(x));
if (cellsBusy == 0 && casCellsBusy()) {
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;
break done;
}
} finally {
cellsBusy = 0;
}
continue; // Slot is now non-empty
}
}
collide = false;
}
else if (!wasUncontended) // CAS already known to fail
wasUncontended = true; // Continue after rehash
else if (c.cas(v = c.value, apply(fn, v, x)))
break;
else if (n >= NCPU || cells != cs)
collide = false; // At max size or stale
else if (!collide)
collide = true;
else if (cellsBusy == 0 && casCellsBusy()) {
try {
if (cells == cs) { // Expand table unless stale
cells = Arrays.copyOf(cs, n << 1);
}
} finally {
cellsBusy = 0;
}
collide = false;
continue; // Retry with expanded table
}
h = advanceProbe(h);
}
else if (cellsBusy == 0 && cells == cs && casCellsBusy()) {
try { // Initialize table
if (cells == cs) {
Cell[] rs = new Cell[2];
rs[h & 1] = new Cell(Double.doubleToRawLongBits(x));
cells = rs;
break done;
}
} finally {
cellsBusy = 0;
}
}
// Fall back on using base
else if (casBase(v = base, apply(fn, v, x))) {
break done;
}
}
}
static long getAndSetLong(Object o, long offset, long newValue) {
long v;
do {
v = U.getLongVolatile(o, offset);
} while (!U.compareAndSwapLong(o, offset, v, newValue));
return v;
}
// Unsafe mechanics
private static final sun.misc.Unsafe U = UnsafeAccess.unsafe;
private static final long BASE;
private static final long CELLSBUSY;
private static final Method GET_PROBE_METHOD;
private static final Method SET_PROBE_METHOD;
static {
try {
BASE = U.objectFieldOffset(Striped64.class.getDeclaredField("base"));
CELLSBUSY = U.objectFieldOffset(Striped64.class
.getDeclaredField("cellsBusy"));
// reflective access to TLRandom
Class tlrk = Class.forName("java8.util.concurrent.TLRandom");
Method getProbe = tlrk
.getDeclaredMethod("getThreadLocalRandomProbe");
getProbe.setAccessible(true);
Method setProbe = tlrk.getDeclaredMethod(
"setThreadLocalRandomProbe", int.class);
setProbe.setAccessible(true);
GET_PROBE_METHOD = getProbe;
SET_PROBE_METHOD = setProbe;
} catch (Exception e) {
throw new ExceptionInInitializerError(e);
}
}
}
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