Why the bounds check doesn't get eliminated?

Question

I wrote a simple benchmark in order to find out if bounds check can be eliminated when the array gets computed via bitwise and. This is basically what nearly all hash tables

Answer 1

1. No, this is evidently an effect of not enough smart bounds check elimination.

I've extended a benchmark by Marko Topolnik:

@OutputTimeUnit(TimeUnit.NANOSECONDS)
@BenchmarkMode(Mode.AverageTime)
@OperationsPerInvocation(BCElimination.N)
@Warmup(iterations = 5, time = 1)
@Measurement(iterations = 10, time = 1)
@State(Scope.Thread)
@Threads(1)
@Fork(2)
public class BCElimination {
    public static final int N = 1024;
    private static final Unsafe U;
    private static final long INT_BASE;
    private static final long INT_SCALE;
    static {
        try {
            Field f = Unsafe.class.getDeclaredField("theUnsafe");
            f.setAccessible(true);
            U = (Unsafe) f.get(null);
        } catch (Exception e) {
            throw new IllegalStateException(e);
        }

        INT_BASE = U.arrayBaseOffset(int[].class);
        INT_SCALE = U.arrayIndexScale(int[].class);
    }

    private final int[] table = new int[BCElimination.N];

    @Setup public void setUp() {
        final Random random = new Random();
        for (int i=0; i

Results:

Benchmark                                Mean   Mean error    Units
BCElimination.maskedIndex               1,235        0,004    ns/op
BCElimination.maskedIndexUnsafe         1,092        0,007    ns/op
BCElimination.normalIndex               1,071        0,008    ns/op

2. The second question is for hotspot-dev mailing lists rather than StackOverflow, IMHO.