java/17 : java.base/jdk/internal/util/ArraysSupport.java

ArraysSupport
https://openjdk.java.net/
GPLv2 + Classpath Exception
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 *
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package jdk.internal.util;

import jdk.internal.misc.Unsafe;
import jdk.internal.vm.annotation.IntrinsicCandidate;

Utility methods to work with arrays.  This includes a set of methods
to find a mismatch between two primitive arrays.  Also included is
a method to calculate the new length of an array to be reallocated.
Array equality and lexicographical comparison can be built on top of
array mismatch functionality.
The mismatch method implementation, vectorizedMismatch, leverages vector-based techniques to access and compare the contents of two arrays. The Java implementation uses Unsafe.getLongUnaligned to access the content of an array, thus access is supported on platforms that do not support unaligned access. For a byte[] array, 8 bytes (64 bits) can be accessed and compared as a unit rather than individually, which increases the performance when the method is compiled by the HotSpot VM. On supported platforms the mismatch implementation is intrinsified to leverage SIMD instructions. So for a byte[] array, 16 bytes (128 bits), 32 bytes (256 bits), and perhaps in the future even 64 bytes (512 bits), platform permitting, can be accessed and compared as a unit, which further increases the performance over the Java implementation. 
None of the mismatch methods perform array bounds checks.  It is the
responsibility of the caller (direct or otherwise) to perform such checks
before calling this method.
/**
 * Utility methods to work with arrays.  This includes a set of methods
 * to find a mismatch between two primitive arrays.  Also included is
 * a method to calculate the new length of an array to be reallocated.
 *
 * <p>Array equality and lexicographical comparison can be built on top of
 * array mismatch functionality.
 *
 * <p>The mismatch method implementation, {@link #vectorizedMismatch}, leverages
 * vector-based techniques to access and compare the contents of two arrays.
 * The Java implementation uses {@code Unsafe.getLongUnaligned} to access the
 * content of an array, thus access is supported on platforms that do not
 * support unaligned access.  For a byte[] array, 8 bytes (64 bits) can be
 * accessed and compared as a unit rather than individually, which increases
 * the performance when the method is compiled by the HotSpot VM.  On supported
 * platforms the mismatch implementation is intrinsified to leverage SIMD
 * instructions.  So for a byte[] array, 16 bytes (128 bits), 32 bytes
 * (256 bits), and perhaps in the future even 64 bytes (512 bits), platform
 * permitting, can be accessed and compared as a unit, which further increases
 * the performance over the Java implementation.
 *
 * <p>None of the mismatch methods perform array bounds checks.  It is the
 * responsibility of the caller (direct or otherwise) to perform such checks
 * before calling this method.
 */
public class ArraysSupport {
    static final Unsafe U = Unsafe.getUnsafe();

    private static final boolean BIG_ENDIAN = U.isBigEndian();

    public static final int LOG2_ARRAY_BOOLEAN_INDEX_SCALE = exactLog2(Unsafe.ARRAY_BOOLEAN_INDEX_SCALE);
    public static final int LOG2_ARRAY_BYTE_INDEX_SCALE = exactLog2(Unsafe.ARRAY_BYTE_INDEX_SCALE);
    public static final int LOG2_ARRAY_CHAR_INDEX_SCALE = exactLog2(Unsafe.ARRAY_CHAR_INDEX_SCALE);
    public static final int LOG2_ARRAY_SHORT_INDEX_SCALE = exactLog2(Unsafe.ARRAY_SHORT_INDEX_SCALE);
    public static final int LOG2_ARRAY_INT_INDEX_SCALE = exactLog2(Unsafe.ARRAY_INT_INDEX_SCALE);
    public static final int LOG2_ARRAY_LONG_INDEX_SCALE = exactLog2(Unsafe.ARRAY_LONG_INDEX_SCALE);
    public static final int LOG2_ARRAY_FLOAT_INDEX_SCALE = exactLog2(Unsafe.ARRAY_FLOAT_INDEX_SCALE);
    public static final int LOG2_ARRAY_DOUBLE_INDEX_SCALE = exactLog2(Unsafe.ARRAY_DOUBLE_INDEX_SCALE);

    private static final int LOG2_BYTE_BIT_SIZE = exactLog2(Byte.SIZE);

    private static int exactLog2(int scale) {
        if ((scale & (scale - 1)) != 0)
            throw new Error("data type scale not a power of two");
        return Integer.numberOfTrailingZeros(scale);
    }

    private ArraysSupport() {}

    Find the relative index of the first mismatching pair of elements in two
primitive arrays of the same component type.  Pairs of elements will be
tested in order relative to given offsets into both arrays.
This method does not perform type checks or bounds checks.  It is the
responsibility of the caller to perform such checks before calling this
method.
The given offsets, in bytes, need not be aligned according to the
given log₂ size the array elements.  More specifically, an
offset modulus the size need not be zero.
Params: a – the first array to be tested for mismatch, or null for direct memory access
aOffset – the relative offset, in bytes, from the base address of the first array to test from, otherwise if the first array is null, an absolute address pointing to the first element to test.
b – the second array to be tested for mismatch, or null for direct memory access
bOffset – the relative offset, in bytes, from the base address of the second array to test from, otherwise if the second array is null, an absolute address pointing to the first element to test.
length – the number of array elements to test
log2ArrayIndexScale – log₂ of the array index scale, that
corresponds to the size, in bytes, of an array element.
Returns: if a mismatch is found a relative index, between 0 (inclusive) and length (exclusive), of the first mismatching pair of elements in the two arrays. Otherwise, if a mismatch is not found the bitwise compliment of the number of remaining pairs of elements to be checked in the tail of the two arrays./**
     * Find the relative index of the first mismatching pair of elements in two
     * primitive arrays of the same component type.  Pairs of elements will be
     * tested in order relative to given offsets into both arrays.
     *
     * <p>This method does not perform type checks or bounds checks.  It is the
     * responsibility of the caller to perform such checks before calling this
     * method.
     *
     * <p>The given offsets, in bytes, need not be aligned according to the
     * given log<sub>2</sub> size the array elements.  More specifically, an
     * offset modulus the size need not be zero.
     *
     * @param a the first array to be tested for mismatch, or {@code null} for
     * direct memory access
     * @param aOffset the relative offset, in bytes, from the base address of
     * the first array to test from, otherwise if the first array is
     * {@code null}, an absolute address pointing to the first element to test.
     * @param b the second array to be tested for mismatch, or {@code null} for
     * direct memory access
     * @param bOffset the relative offset, in bytes, from the base address of
     * the second array to test from, otherwise if the second array is
     * {@code null}, an absolute address pointing to the first element to test.
     * @param length the number of array elements to test
     * @param log2ArrayIndexScale log<sub>2</sub> of the array index scale, that
     * corresponds to the size, in bytes, of an array element.
     * @return if a mismatch is found a relative index, between 0 (inclusive)
     * and {@code length} (exclusive), of the first mismatching pair of elements
     * in the two arrays.  Otherwise, if a mismatch is not found the bitwise
     * compliment of the number of remaining pairs of elements to be checked in
     * the tail of the two arrays.
     */
    @IntrinsicCandidate
    public static int vectorizedMismatch(Object a, long aOffset,
                                         Object b, long bOffset,
                                         int length,
                                         int log2ArrayIndexScale) {
        // assert a.getClass().isArray();
        // assert b.getClass().isArray();
        // assert 0 <= length <= sizeOf(a)
        // assert 0 <= length <= sizeOf(b)
        // assert 0 <= log2ArrayIndexScale <= 3

        int log2ValuesPerWidth = LOG2_ARRAY_LONG_INDEX_SCALE - log2ArrayIndexScale;
        int wi = 0;
        for (; wi < length >> log2ValuesPerWidth; wi++) {
            long bi = ((long) wi) << LOG2_ARRAY_LONG_INDEX_SCALE;
            long av = U.getLongUnaligned(a, aOffset + bi);
            long bv = U.getLongUnaligned(b, bOffset + bi);
            if (av != bv) {
                long x = av ^ bv;
                int o = BIG_ENDIAN
                        ? Long.numberOfLeadingZeros(x) >> (LOG2_BYTE_BIT_SIZE + log2ArrayIndexScale)
                        : Long.numberOfTrailingZeros(x) >> (LOG2_BYTE_BIT_SIZE + log2ArrayIndexScale);
                return (wi << log2ValuesPerWidth) + o;
            }
        }

        // Calculate the tail of remaining elements to check
        int tail = length - (wi << log2ValuesPerWidth);

        if (log2ArrayIndexScale < LOG2_ARRAY_INT_INDEX_SCALE) {
            int wordTail = 1 << (LOG2_ARRAY_INT_INDEX_SCALE - log2ArrayIndexScale);
            // Handle 4 bytes or 2 chars in the tail using int width
            if (tail >= wordTail) {
                long bi = ((long) wi) << LOG2_ARRAY_LONG_INDEX_SCALE;
                int av = U.getIntUnaligned(a, aOffset + bi);
                int bv = U.getIntUnaligned(b, bOffset + bi);
                if (av != bv) {
                    int x = av ^ bv;
                    int o = BIG_ENDIAN
                            ? Integer.numberOfLeadingZeros(x) >> (LOG2_BYTE_BIT_SIZE + log2ArrayIndexScale)
                            : Integer.numberOfTrailingZeros(x) >> (LOG2_BYTE_BIT_SIZE + log2ArrayIndexScale);
                    return (wi << log2ValuesPerWidth) + o;
                }
                tail -= wordTail;
            }
            return ~tail;
        }
        else {
            return ~tail;
        }
    }

    // Booleans
    // Each boolean element takes up one byte

    public static int mismatch(boolean[] a,
                               boolean[] b,
                               int length) {
        int i = 0;
        if (length > 7) {
            if (a[0] != b[0])
                return 0;
            i = vectorizedMismatch(
                    a, Unsafe.ARRAY_BOOLEAN_BASE_OFFSET,
                    b, Unsafe.ARRAY_BOOLEAN_BASE_OFFSET,
                    length, LOG2_ARRAY_BOOLEAN_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[i] != b[i])
                return i;
        }
        return -1;
    }

    public static int mismatch(boolean[] a, int aFromIndex,
                               boolean[] b, int bFromIndex,
                               int length) {
        int i = 0;
        if (length > 7) {
            if (a[aFromIndex] != b[bFromIndex])
                return 0;
            int aOffset = Unsafe.ARRAY_BOOLEAN_BASE_OFFSET + aFromIndex;
            int bOffset = Unsafe.ARRAY_BOOLEAN_BASE_OFFSET + bFromIndex;
            i = vectorizedMismatch(
                    a, aOffset,
                    b, bOffset,
                    length, LOG2_ARRAY_BOOLEAN_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[aFromIndex + i] != b[bFromIndex + i])
                return i;
        }
        return -1;
    }


    // Bytes

    Find the index of a mismatch between two arrays.
This method does not perform bounds checks. It is the responsibility
of the caller to perform such bounds checks before calling this method.
Params: a – the first array to be tested for a mismatch
b – the second array to be tested for a mismatch
length – the number of bytes from each array to check
Returns: the index of a mismatch between the two arrays, otherwise -1 if
no mismatch.  The index will be within the range of (inclusive) 0 to
(exclusive) the smaller of the two array lengths./**
     * Find the index of a mismatch between two arrays.
     *
     * <p>This method does not perform bounds checks. It is the responsibility
     * of the caller to perform such bounds checks before calling this method.
     *
     * @param a the first array to be tested for a mismatch
     * @param b the second array to be tested for a mismatch
     * @param length the number of bytes from each array to check
     * @return the index of a mismatch between the two arrays, otherwise -1 if
     * no mismatch.  The index will be within the range of (inclusive) 0 to
     * (exclusive) the smaller of the two array lengths.
     */
    public static int mismatch(byte[] a,
                               byte[] b,
                               int length) {
        // ISSUE: defer to index receiving methods if performance is good
        // assert length <= a.length
        // assert length <= b.length

        int i = 0;
        if (length > 7) {
            if (a[0] != b[0])
                return 0;
            i = vectorizedMismatch(
                    a, Unsafe.ARRAY_BYTE_BASE_OFFSET,
                    b, Unsafe.ARRAY_BYTE_BASE_OFFSET,
                    length, LOG2_ARRAY_BYTE_INDEX_SCALE);
            if (i >= 0)
                return i;
            // Align to tail
            i = length - ~i;
//            assert i >= 0 && i <= 7;
        }
        // Tail < 8 bytes
        for (; i < length; i++) {
            if (a[i] != b[i])
                return i;
        }
        return -1;
    }

    Find the relative index of a mismatch between two arrays starting from
given indexes.
This method does not perform bounds checks. It is the responsibility
of the caller to perform such bounds checks before calling this method.
Params: a – the first array to be tested for a mismatch
aFromIndex – the index of the first element (inclusive) in the first
array to be compared
b – the second array to be tested for a mismatch
bFromIndex – the index of the first element (inclusive) in the
second array to be compared
length – the number of bytes from each array to check
Returns: the relative index of a mismatch between the two arrays,
otherwise -1 if no mismatch.  The index will be within the range of
(inclusive) 0 to (exclusive) the smaller of the two array bounds./**
     * Find the relative index of a mismatch between two arrays starting from
     * given indexes.
     *
     * <p>This method does not perform bounds checks. It is the responsibility
     * of the caller to perform such bounds checks before calling this method.
     *
     * @param a the first array to be tested for a mismatch
     * @param aFromIndex the index of the first element (inclusive) in the first
     * array to be compared
     * @param b the second array to be tested for a mismatch
     * @param bFromIndex the index of the first element (inclusive) in the
     * second array to be compared
     * @param length the number of bytes from each array to check
     * @return the relative index of a mismatch between the two arrays,
     * otherwise -1 if no mismatch.  The index will be within the range of
     * (inclusive) 0 to (exclusive) the smaller of the two array bounds.
     */
    public static int mismatch(byte[] a, int aFromIndex,
                               byte[] b, int bFromIndex,
                               int length) {
        // assert 0 <= aFromIndex < a.length
        // assert 0 <= aFromIndex + length <= a.length
        // assert 0 <= bFromIndex < b.length
        // assert 0 <= bFromIndex + length <= b.length
        // assert length >= 0

        int i = 0;
        if (length > 7) {
            if (a[aFromIndex] != b[bFromIndex])
                return 0;
            int aOffset = Unsafe.ARRAY_BYTE_BASE_OFFSET + aFromIndex;
            int bOffset = Unsafe.ARRAY_BYTE_BASE_OFFSET + bFromIndex;
            i = vectorizedMismatch(
                    a, aOffset,
                    b, bOffset,
                    length, LOG2_ARRAY_BYTE_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[aFromIndex + i] != b[bFromIndex + i])
                return i;
        }
        return -1;
    }


    // Chars

    public static int mismatch(char[] a,
                               char[] b,
                               int length) {
        int i = 0;
        if (length > 3) {
            if (a[0] != b[0])
                return 0;
            i = vectorizedMismatch(
                    a, Unsafe.ARRAY_CHAR_BASE_OFFSET,
                    b, Unsafe.ARRAY_CHAR_BASE_OFFSET,
                    length, LOG2_ARRAY_CHAR_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[i] != b[i])
                return i;
        }
        return -1;
    }

    public static int mismatch(char[] a, int aFromIndex,
                               char[] b, int bFromIndex,
                               int length) {
        int i = 0;
        if (length > 3) {
            if (a[aFromIndex] != b[bFromIndex])
                return 0;
            int aOffset = Unsafe.ARRAY_CHAR_BASE_OFFSET + (aFromIndex << LOG2_ARRAY_CHAR_INDEX_SCALE);
            int bOffset = Unsafe.ARRAY_CHAR_BASE_OFFSET + (bFromIndex << LOG2_ARRAY_CHAR_INDEX_SCALE);
            i = vectorizedMismatch(
                    a, aOffset,
                    b, bOffset,
                    length, LOG2_ARRAY_CHAR_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[aFromIndex + i] != b[bFromIndex + i])
                return i;
        }
        return -1;
    }


    // Shorts

    public static int mismatch(short[] a,
                               short[] b,
                               int length) {
        int i = 0;
        if (length > 3) {
            if (a[0] != b[0])
                return 0;
            i = vectorizedMismatch(
                    a, Unsafe.ARRAY_SHORT_BASE_OFFSET,
                    b, Unsafe.ARRAY_SHORT_BASE_OFFSET,
                    length, LOG2_ARRAY_SHORT_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[i] != b[i])
                return i;
        }
        return -1;
    }

    public static int mismatch(short[] a, int aFromIndex,
                               short[] b, int bFromIndex,
                               int length) {
        int i = 0;
        if (length > 3) {
            if (a[aFromIndex] != b[bFromIndex])
                return 0;
            int aOffset = Unsafe.ARRAY_SHORT_BASE_OFFSET + (aFromIndex << LOG2_ARRAY_SHORT_INDEX_SCALE);
            int bOffset = Unsafe.ARRAY_SHORT_BASE_OFFSET + (bFromIndex << LOG2_ARRAY_SHORT_INDEX_SCALE);
            i = vectorizedMismatch(
                    a, aOffset,
                    b, bOffset,
                    length, LOG2_ARRAY_SHORT_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[aFromIndex + i] != b[bFromIndex + i])
                return i;
        }
        return -1;
    }


    // Ints

    public static int mismatch(int[] a,
                               int[] b,
                               int length) {
        int i = 0;
        if (length > 1) {
            if (a[0] != b[0])
                return 0;
            i = vectorizedMismatch(
                    a, Unsafe.ARRAY_INT_BASE_OFFSET,
                    b, Unsafe.ARRAY_INT_BASE_OFFSET,
                    length, LOG2_ARRAY_INT_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[i] != b[i])
                return i;
        }
        return -1;
    }

    public static int mismatch(int[] a, int aFromIndex,
                               int[] b, int bFromIndex,
                               int length) {
        int i = 0;
        if (length > 1) {
            if (a[aFromIndex] != b[bFromIndex])
                return 0;
            int aOffset = Unsafe.ARRAY_INT_BASE_OFFSET + (aFromIndex << LOG2_ARRAY_INT_INDEX_SCALE);
            int bOffset = Unsafe.ARRAY_INT_BASE_OFFSET + (bFromIndex << LOG2_ARRAY_INT_INDEX_SCALE);
            i = vectorizedMismatch(
                    a, aOffset,
                    b, bOffset,
                    length, LOG2_ARRAY_INT_INDEX_SCALE);
            if (i >= 0)
                return i;
            i = length - ~i;
        }
        for (; i < length; i++) {
            if (a[aFromIndex + i] != b[bFromIndex + i])
                return i;
        }
        return -1;
    }


    // Floats

    public static int mismatch(float[] a,
                               float[] b,
                               int length) {
        return mismatch(a, 0, b, 0, length);
    }

    public static int mismatch(float[] a, int aFromIndex,
                               float[] b, int bFromIndex,
                               int length) {
        int i = 0;
        if (length > 1) {
            if (Float.floatToRawIntBits(a[aFromIndex]) == Float.floatToRawIntBits(b[bFromIndex])) {
                int aOffset = Unsafe.ARRAY_FLOAT_BASE_OFFSET + (aFromIndex << LOG2_ARRAY_FLOAT_INDEX_SCALE);
                int bOffset = Unsafe.ARRAY_FLOAT_BASE_OFFSET + (bFromIndex << LOG2_ARRAY_FLOAT_INDEX_SCALE);
                i = vectorizedMismatch(
                        a, aOffset,
                        b, bOffset,
                        length, LOG2_ARRAY_FLOAT_INDEX_SCALE);
            }
            // Mismatched
            if (i >= 0) {
                // Check if mismatch is not associated with two NaN values
                if (!Float.isNaN(a[aFromIndex + i]) || !Float.isNaN(b[bFromIndex + i]))
                    return i;

                // Mismatch on two different NaN values that are normalized to match
                // Fall back to slow mechanism
                // ISSUE: Consider looping over vectorizedMismatch adjusting ranges
                // However, requires that returned value be relative to input ranges
                i++;
            }
            // Matched
            else {
                i = length - ~i;
            }
        }
        for (; i < length; i++) {
            if (Float.floatToIntBits(a[aFromIndex + i]) != Float.floatToIntBits(b[bFromIndex + i]))
                return i;
        }
        return -1;
    }

    // 64 bit sizes

    // Long

    public static int mismatch(long[] a,
                               long[] b,
                               int length) {
        if (length == 0) {
            return -1;
        }
        if (a[0] != b[0])
            return 0;
        int i = vectorizedMismatch(
                a, Unsafe.ARRAY_LONG_BASE_OFFSET,
                b, Unsafe.ARRAY_LONG_BASE_OFFSET,
                length, LOG2_ARRAY_LONG_INDEX_SCALE);
        return i >= 0 ? i : -1;
    }

    public static int mismatch(long[] a, int aFromIndex,
                               long[] b, int bFromIndex,
                               int length) {
        if (length == 0) {
            return -1;
        }
        if (a[aFromIndex] != b[bFromIndex])
            return 0;
        int aOffset = Unsafe.ARRAY_LONG_BASE_OFFSET + (aFromIndex << LOG2_ARRAY_LONG_INDEX_SCALE);
        int bOffset = Unsafe.ARRAY_LONG_BASE_OFFSET + (bFromIndex << LOG2_ARRAY_LONG_INDEX_SCALE);
        int i = vectorizedMismatch(
                a, aOffset,
                b, bOffset,
                length, LOG2_ARRAY_LONG_INDEX_SCALE);
        return i >= 0 ? i : -1;
    }


    // Double

    public static int mismatch(double[] a,
                               double[] b,
                               int length) {
        return mismatch(a, 0, b, 0, length);
    }

    public static int mismatch(double[] a, int aFromIndex,
                               double[] b, int bFromIndex,
                               int length) {
        if (length == 0) {
            return -1;
        }
        int i = 0;
        if (Double.doubleToRawLongBits(a[aFromIndex]) == Double.doubleToRawLongBits(b[bFromIndex])) {
            int aOffset = Unsafe.ARRAY_DOUBLE_BASE_OFFSET + (aFromIndex << LOG2_ARRAY_DOUBLE_INDEX_SCALE);
            int bOffset = Unsafe.ARRAY_DOUBLE_BASE_OFFSET + (bFromIndex << LOG2_ARRAY_DOUBLE_INDEX_SCALE);
            i = vectorizedMismatch(
                    a, aOffset,
                    b, bOffset,
                    length, LOG2_ARRAY_DOUBLE_INDEX_SCALE);
        }
        if (i >= 0) {
            // Check if mismatch is not associated with two NaN values
            if (!Double.isNaN(a[aFromIndex + i]) || !Double.isNaN(b[bFromIndex + i]))
                return i;

            // Mismatch on two different NaN values that are normalized to match
            // Fall back to slow mechanism
            // ISSUE: Consider looping over vectorizedMismatch adjusting ranges
            // However, requires that returned value be relative to input ranges
            i++;
            for (; i < length; i++) {
                if (Double.doubleToLongBits(a[aFromIndex + i]) != Double.doubleToLongBits(b[bFromIndex + i]))
                    return i;
            }
        }

        return -1;
    }

    A soft maximum array length imposed by array growth computations.
Some JVMs (such as HotSpot) have an implementation limit that will cause
    OutOfMemoryError("Requested array size exceeds VM limit")
to be thrown if a request is made to allocate an array of some length near
Integer.MAX_VALUE, even if there is sufficient heap available. The actual
limit might depend on some JVM implementation-specific characteristics such
as the object header size. The soft maximum value is chosen conservatively so
as to be smaller than any implementation limit that is likely to be encountered.
/**
     * A soft maximum array length imposed by array growth computations.
     * Some JVMs (such as HotSpot) have an implementation limit that will cause
     *
     *     OutOfMemoryError("Requested array size exceeds VM limit")
     *
     * to be thrown if a request is made to allocate an array of some length near
     * Integer.MAX_VALUE, even if there is sufficient heap available. The actual
     * limit might depend on some JVM implementation-specific characteristics such
     * as the object header size. The soft maximum value is chosen conservatively so
     * as to be smaller than any implementation limit that is likely to be encountered.
     */
    public static final int SOFT_MAX_ARRAY_LENGTH = Integer.MAX_VALUE - 8;

    Computes a new array length given an array's current length, a minimum growth
amount, and a preferred growth amount. The computation is done in an overflow-safe
fashion.
This method is used by objects that contain an array that might need to be grown
in order to fulfill some immediate need (the minimum growth amount) but would also
like to request more space (the preferred growth amount) in order to accommodate
potential future needs. The returned length is usually clamped at the soft maximum
length in order to avoid hitting the JVM implementation limit. However, the soft
maximum will be exceeded if the minimum growth amount requires it.
If the preferred growth amount is less than the minimum growth amount, the
minimum growth amount is used as the preferred growth amount.
The preferred length is determined by adding the preferred growth amount to the
current length. If the preferred length does not exceed the soft maximum length
(SOFT_MAX_ARRAY_LENGTH) then the preferred length is returned.
If the preferred length exceeds the soft maximum, we use the minimum growth
amount. The minimum required length is determined by adding the minimum growth
amount to the current length. If the minimum required length exceeds Integer.MAX_VALUE,
then this method throws OutOfMemoryError. Otherwise, this method returns the greater of
the soft maximum or the minimum required length.
Note that this method does not do any array allocation itself; it only does array
length growth computations. However, it will throw OutOfMemoryError as noted above.
Note also that this method cannot detect the JVM's implementation limit, and it
may compute and return a length value up to and including Integer.MAX_VALUE that
might exceed the JVM's implementation limit. In that case, the caller will likely
attempt an array allocation with that length and encounter an OutOfMemoryError.
Of course, regardless of the length value returned from this method, the caller
may encounter OutOfMemoryError if there is insufficient heap to fulfill the request.
Params: oldLength –   current length of the array (must be nonnegative)
minGrowth –   minimum required growth amount (must be positive)
prefGrowth –  preferred growth amount
Throws: OutOfMemoryError – if the new length would exceed Integer.MAX_VALUE
Returns: the new array length/**
     * Computes a new array length given an array's current length, a minimum growth
     * amount, and a preferred growth amount. The computation is done in an overflow-safe
     * fashion.
     *
     * This method is used by objects that contain an array that might need to be grown
     * in order to fulfill some immediate need (the minimum growth amount) but would also
     * like to request more space (the preferred growth amount) in order to accommodate
     * potential future needs. The returned length is usually clamped at the soft maximum
     * length in order to avoid hitting the JVM implementation limit. However, the soft
     * maximum will be exceeded if the minimum growth amount requires it.
     *
     * If the preferred growth amount is less than the minimum growth amount, the
     * minimum growth amount is used as the preferred growth amount.
     *
     * The preferred length is determined by adding the preferred growth amount to the
     * current length. If the preferred length does not exceed the soft maximum length
     * (SOFT_MAX_ARRAY_LENGTH) then the preferred length is returned.
     *
     * If the preferred length exceeds the soft maximum, we use the minimum growth
     * amount. The minimum required length is determined by adding the minimum growth
     * amount to the current length. If the minimum required length exceeds Integer.MAX_VALUE,
     * then this method throws OutOfMemoryError. Otherwise, this method returns the greater of
     * the soft maximum or the minimum required length.
     *
     * Note that this method does not do any array allocation itself; it only does array
     * length growth computations. However, it will throw OutOfMemoryError as noted above.
     *
     * Note also that this method cannot detect the JVM's implementation limit, and it
     * may compute and return a length value up to and including Integer.MAX_VALUE that
     * might exceed the JVM's implementation limit. In that case, the caller will likely
     * attempt an array allocation with that length and encounter an OutOfMemoryError.
     * Of course, regardless of the length value returned from this method, the caller
     * may encounter OutOfMemoryError if there is insufficient heap to fulfill the request.
     *
     * @param oldLength   current length of the array (must be nonnegative)
     * @param minGrowth   minimum required growth amount (must be positive)
     * @param prefGrowth  preferred growth amount
     * @return the new array length
     * @throws OutOfMemoryError if the new length would exceed Integer.MAX_VALUE
     */
    public static int newLength(int oldLength, int minGrowth, int prefGrowth) {
        // preconditions not checked because of inlining
        // assert oldLength >= 0
        // assert minGrowth > 0

        int prefLength = oldLength + Math.max(minGrowth, prefGrowth); // might overflow
        if (0 < prefLength && prefLength <= SOFT_MAX_ARRAY_LENGTH) {
            return prefLength;
        } else {
            // put code cold in a separate method
            return hugeLength(oldLength, minGrowth);
        }
    }

    private static int hugeLength(int oldLength, int minGrowth) {
        int minLength = oldLength + minGrowth;
        if (minLength < 0) { // overflow
            throw new OutOfMemoryError(
                "Required array length " + oldLength + " + " + minGrowth + " is too large");
        } else if (minLength <= SOFT_MAX_ARRAY_LENGTH) {
            return SOFT_MAX_ARRAY_LENGTH;
        } else {
            return minLength;
        }
    }
}
Params:	a – the first array to be tested for a mismatch b – the second array to be tested for a mismatch length – the number of bytes from each array to check
Returns:	the index of a mismatch between the two arrays, otherwise -1 if no mismatch. The index will be within the range of (inclusive) 0 to (exclusive) the smaller of the two array lengths.
Params:	oldLength – current length of the array (must be nonnegative) minGrowth – minimum required growth amount (must be positive) prefGrowth – preferred growth amount
Throws:	OutOfMemoryError – if the new length would exceed Integer.MAX_VALUE
Returns:	the new array length
/

java/ 17/ java.base/jdk/internal/util/ArraysSupport.java
