ADDITIONAL SYSTEM INFORMATION :
OS：Win10
jdk: jdk-15.0.2
jre: jdk-15.0.2

A DESCRIPTION OF THE PROBLEM :
Creates a {@code ForkJoinPool} with the given parameters. If we make parallelism equal to 0x7ffe，it's less than MAX_CAP(Obviously,0x7ffe<0x7fff). When calculating the length of the array in the constructor，the main code that （int n = (parallelism > 1) ? parallelism - 1 : 1; //At this time，n=parallelism - 1，it‘s 0x7ffd(0111 1111 1111 1101).
                    n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8; n |= n >>> 16;
                    n = (n + 1) << 1；).
The last,n+1=1000 0000 0000 0000, [(n + 1) << 1]=1 0000 0000 0000 0000，that is 17bits!!!,more than 16.We didn't realize that near the limit, doubling the capacity would take another bit。So i think the MAX_CAP shuold be 0x3fff,not to be (1<<15)-1.Only in this way can we meet the requirements of the notes“To enable this packing, we restrict maximum parallelism to (1<<15)-1 (which is far in excess of normal operating range) to allow ids, counts, and their negations (used for thresholding) to fit into 16bit subfields.”
The constructor code is as follows：
********************************************************************************************************************

static final int MAX_CAP = 0x7fff; // max #workers - 1

public ForkJoinPool(int parallelism,
                        ForkJoinWorkerThreadFactory factory,
                        UncaughtExceptionHandler handler,
                        boolean asyncMode,
                        int corePoolSize,
                        int maximumPoolSize,
                        int minimumRunnable,
                        Predicate<? super ForkJoinPool> saturate,
                        long keepAliveTime,
                        TimeUnit unit) {
        // check, encode, pack parameters
        if (parallelism <= 0 || parallelism > MAX_CAP ||
            maximumPoolSize < parallelism || keepAliveTime <= 0L)
            throw new IllegalArgumentException();
        if (factory == null)
            throw new NullPointerException();
        long ms = Math.max(unit.toMillis(keepAliveTime), TIMEOUT_SLOP);

        int corep = Math.min(Math.max(corePoolSize, parallelism), MAX_CAP);
        long c = ((((long)(-corep) << TC_SHIFT) & TC_MASK) |
                  (((long)(-parallelism) << RC_SHIFT) & RC_MASK));
        int m = parallelism | (asyncMode ? FIFO : 0);
        int maxSpares = Math.min(maximumPoolSize, MAX_CAP) - parallelism;
        int minAvail = Math.min(Math.max(minimumRunnable, 0), MAX_CAP);
        int b = ((minAvail - parallelism) & SMASK) | (maxSpares << SWIDTH);
        int n = (parallelism > 1) ? parallelism - 1 : 1; // at least 2 slots
        n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8; n |= n >>> 16;
        n = (n + 1) << 1; // power of two, including space for submission queues

        this.workerNamePrefix = "ForkJoinPool-" + nextPoolId() + "-worker-";
        this.workQueues = new WorkQueue[n];
        this.factory = factory;
        this.ueh = handler;
        this.saturate = saturate;
        this.keepAlive = ms;
        this.bounds = b;
        this.mode = m;
        this.ctl = c;
        checkPermission();
    }
*************************************************************************************************************

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
step1:If we make parallelism equal to 0x7ffe，it's less than MAX_CAP(Obviously,0x7ffe<0x7fff).
It's mean that we give parallelism=0111 1111 1111 1110, capacity calculation：
int n = (parallelism > 1) ? parallelism - 1 : 1；
step2:
n=0111 1111 1111 1101;(n >>> 1)=0011 1111 1111 1110;
(n |= n >>> 1)=0111 1111 1111 1111
.........
(n |= n >>> 16)=0111 1111 1111 1111
step3: n+1=1000 0000 0000 0000
step4:[(n+1)<<1]= 1 0000 0000 0000 0000


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
[(n+1)<<1]= 1000 0000 0000 0000 Maximum poolIndex is 2^15-1
ACTUAL -
[(n+1)<<1]= 1 0000 0000 0000 0000 that is 17bits,exceed the last 16bits of the "ctl".
2^16-1

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Because this involves the maximum parallelism, which can be up to 32767. I have no equipment condition. But this error occurs in three lines of capacity code. The example I gave fully proves max_ Cap has errors in setting value, which is only a pure mathematical problem demonstration to find errors.I hope you can confirm this problem！
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CUSTOMER SUBMITTED WORKAROUND :
make the MAX_CAP = 0x7ffe，That's binary 0011 1111 1111 1111

FREQUENCY : always