Use SharedSecrets for ThreadLocalRandomProbe; other tweaks

Author: DougLea

src/java.base/share/classes/java/util/concurrent/ForkJoinPool.java

@@ -1273,15 +1273,17 @@ final int queueSize() {
         /**
          * Pushes a task. Called only by owner or if already locked
          *
-         * @param task the task. Caller must ensure non-null.
+         * @param task the task; no-op if null
          * @param pool the pool to signal if was previously empty, else null
          * @param internal if caller owns this queue
          * @throws RejectedExecutionException if array could not be resized
          */
         final void push(ForkJoinTask<?> task, ForkJoinPool pool,
                         boolean internal) {
             int s = top, b = base, m, cap, room; ForkJoinTask<?>[] a;
-            if ((a = array) != null && (cap = a.length) > 0) { // else disabled
+            if ((a = array) != null && (cap = a.length) > 0 && // else disabled
+                task != null) {
+                int pk = task.noUserHelp() + 1;             // prev slot offset
                 if ((room = (m = cap - 1) - (s - b)) >= 0) {
                     top = s + 1;
                     long pos = slotOffset(m & s);
@@ -1296,9 +1298,7 @@ final void push(ForkJoinTask<?> task, ForkJoinPool pool,
                     unlockPhase();
                 if (room < 0)
                     throw new RejectedExecutionException("Queue capacity exceeded");
-                if ((room == 0 ||        // pad if no caller-run
-                     a[m & (s - ((internal || task == null ||
-                                  task.noUserHelp() == 0) ? 1 : 2))] == null) &&
+                if ((room == 0 || a[m & (s - pk)] == null) &&
                     pool != null)
                     pool.signalWork();   // may have appeared empty
             }
@@ -2016,14 +2016,13 @@ final void runWorker(WorkQueue w) {
                             }
                             else {
                                 boolean propagate;
-                                int nb = q.base = b + 1;
+                                int nb = q.base = b + 1, prevSrc = src;
                                 w.nsteals = ++nsteals;
-                                int ts = t.status;
-                                w.source = j;             // volatile
+                                w.source = src = j;       // volatile
                                 rescan = true;
+                                int nh = t.noUserHelp();
                                 if (propagate =
-                                    ((src != (src = j) || t.noUserHelp() != 0) &&
-                                     a[nb & m] != null))
+                                    (prevSrc != src || nh != 0) && a[nb & m] != null)
                                     signalWork();
                                 w.topLevelExec(t, fifo);
                                 if ((b = q.base) != nb && !propagate)
@@ -2062,8 +2061,8 @@ private int deactivate(WorkQueue w, int phase) {
             ((e & SHUTDOWN) != 0L && ac == 0 && quiescent() > 0) ||
             (qs = queues) == null || (n = qs.length) <= 0)
             return IDLE;                      // terminating
-        int prechecks = 3;                    // reactivation threshold
-        for (int k = Math.max(n << 2, SPIN_WAITS << 1);;) {
+        int k = Math.max(n << 2, SPIN_WAITS << 1);
+        for (int prechecks = k / n;;) {       // reactivation threshold
             WorkQueue q; int cap; ForkJoinTask<?>[] a; long c;
             if (w.phase == activePhase)
                 return activePhase;
@@ -2072,7 +2071,7 @@ private int deactivate(WorkQueue w, int phase) {
             if ((q = qs[k & (n - 1)]) == null)
                 Thread.onSpinWait();
             else if ((a = q.array) != null && (cap = a.length) > 0 &&
-                     a[q.base & (cap - 1)] != null && --prechecks < 0 &&
+                     a[q.base & (cap - 1)] != null && --prechecks <= 0 &&
                      (int)(c = ctl) == activePhase &&
                      compareAndSetCtl(c, (sp & LMASK) | ((c + RC_UNIT) & UMASK)))
                 return w.phase = activePhase; // reactivate

src/java.base/share/classes/java/util/concurrent/ForkJoinTask.java

@@ -273,7 +273,8 @@ final boolean casNext(Aux c, Aux v) { // used only in cancellation
     static final int ABNORMAL       = 1 << 16;
     static final int THROWN         = 1 << 17;
     static final int HAVE_EXCEPTION = DONE | ABNORMAL | THROWN;
-    static final int NO_USER_HELP   = 1 << 24; // no external caller-run helping
+    static final int NUH_BIT        = 24;      // no external caller helping
+    static final int NO_USER_HELP   = 1 << NUH_BIT;
     static final int MARKER         = 1 << 30; // utility marker
     static final int SMASK          = 0xffff;  // short bits for tags
     static final int UNCOMPENSATE   = 1 << 16; // helpJoin sentinel
@@ -293,8 +294,8 @@ private int getAndBitwiseOrStatus(int v) {
     private boolean casStatus(int c, int v) {
         return U.compareAndSetInt(this, STATUS, c, v);
     }
-    final int noUserHelp() {    // nonvolatile read
-        return U.getInt(this, STATUS) & NO_USER_HELP;
+    final int noUserHelp() {     // nonvolatile read; return 0 or 1
+        return (U.getInt(this, STATUS) & NO_USER_HELP) >>> NUH_BIT;
     }
     final void setNoUserHelp() { // for use in constructors only
         U.putInt(this, STATUS, NO_USER_HELP);

src/java.base/share/classes/java/util/concurrent/ThreadLocalRandom.java

@@ -242,16 +242,18 @@ protected int next(int bits) {
      * the classes that use them. Briefly, a thread's "probe" value is
      * a non-zero hash code that (probably) does not collide with
      * other existing threads with respect to any power of two
-     * collision space. When it does collide, it is pseudo-randomly
-     * adjusted (using a Marsaglia XorShift). The nextSecondarySeed
-     * method is used in the same contexts as ThreadLocalRandom, but
-     * only for transient usages such as random adaptive spin/block
-     * sequences for which a cheap RNG suffices and for which it could
-     * in principle disrupt user-visible statistical properties of the
-     * main ThreadLocalRandom if we were to use it.
+     * collision space, based on carrier threads in the case of
+     * VirtualThreads to reduce the expected collision rate. When it
+     * does collide, it is pseudo-randomly adjusted (using a Marsaglia
+     * XorShift). The nextSecondarySeed method is used in the same
+     * contexts as ThreadLocalRandom, but only for transient usages
+     * such as random adaptive spin/block sequences for which a cheap
+     * RNG suffices and for which it could in principle disrupt
+     * user-visible statistical properties of the main
+     * ThreadLocalRandom if we were to use it.
      *
-     * Note: Because of package-protection issues, versions of some
-     * these methods also appear in some subpackage classes.
+     * Note: jdk SharedSecrets are used enable use in jdk classes
+     * outside this package.
      */
 
     /**
@@ -397,6 +399,12 @@ private static class Access {
                     public int nextSecondaryThreadLocalRandomSeed() {
                         return nextSecondarySeed();
                     }
+                    public int getThreadLocalRandomProbe() {
+                        return getProbe();
+                    }
+                    public int advanceThreadLocalRandomProbe(int r) {
+                        return advanceProbe(r);
+                    }
                 }
             );
         }

src/java.base/share/classes/java/util/concurrent/atomic/Striped64.java

@@ -43,6 +43,8 @@
 import java.util.concurrent.ThreadLocalRandom;
 import java.util.function.DoubleBinaryOperator;
 import java.util.function.LongBinaryOperator;
+import jdk.internal.access.SharedSecrets;
+import jdk.internal.access.JavaUtilConcurrentTLRAccess;
 
 /**
  * A package-local class holding common representation and mechanics
@@ -188,24 +190,18 @@ final boolean casCellsBusy() {
     }
 
     /**
-     * Returns the probe value for the current thread.
-     * Duplicated from ThreadLocalRandom because of packaging restrictions.
+     * Returns the ThreadLocalRandom probe value for the current thread.
      */
     static final int getProbe() {
-        return (int) THREAD_PROBE.get(Thread.currentThread());
+        return TLR.getThreadLocalRandomProbe();
     }
 
     /**
      * Pseudo-randomly advances and records the given probe value for the
      * given thread.
-     * Duplicated from ThreadLocalRandom because of packaging restrictions.
      */
     static final int advanceProbe(int probe) {
-        probe ^= probe << 13;   // xorshift
-        probe ^= probe >>> 17;
-        probe ^= probe << 5;
-        THREAD_PROBE.set(Thread.currentThread(), probe);
-        return probe;
+        return TLR.advanceThreadLocalRandomProbe(probe);
     }
 
     /**
@@ -371,21 +367,17 @@ else if (casBase(v = base, apply(fn, v, x)))
         }
     }
 
+    private static final JavaUtilConcurrentTLRAccess TLR =
+        SharedSecrets.getJavaUtilConcurrentTLRAccess();
+
     // VarHandle mechanics
     private static final VarHandle BASE;
     private static final VarHandle CELLSBUSY;
-    private static final VarHandle THREAD_PROBE;
     static {
         MethodHandles.Lookup l1 = MethodHandles.lookup();
 
         BASE = MhUtil.findVarHandle(l1, "base", long.class);
         CELLSBUSY = MhUtil.findVarHandle(l1, "cellsBusy", int.class);
-        try {
-            MethodHandles.Lookup l2 = MethodHandles.privateLookupIn(Thread.class, l1);
-            THREAD_PROBE = MhUtil.findVarHandle(l2, "threadLocalRandomProbe", int.class);
-        } catch (ReflectiveOperationException e) {
-            throw new ExceptionInInitializerError(e);
-        }
     }
 
 }

src/java.base/share/classes/jdk/internal/access/JavaUtilConcurrentTLRAccess.java

@@ -27,4 +27,6 @@
 
 public interface JavaUtilConcurrentTLRAccess {
     int nextSecondaryThreadLocalRandomSeed();
+    int getThreadLocalRandomProbe();
+    int advanceThreadLocalRandomProbe(int r);
 }