RFR 8209171 : Simplify Java implementation of Integer/Long.numberOfTrailingZeros()

[Martin Buchholz]

The number of plausible variants is astonishing!

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Your use of -client and -server is outdated, which explains why you get the
same results for both (-client is ignored).

I'm not sure what's blessed by hotspot team, but for C1 I
use -XX:+TieredCompilation  -XX:TieredStopAtLevel=1 and for C2 I
use -XX:-TieredCompilation -server

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Now I understand the advantage of using ~i & (i - 1): the subsequent zero
check is a short-circuit for all odd numbers, better than i & -i, which
explains your results - they depend on being able to short-circuit.

So just use a more faithful inlining of nlz without trying to improve on it.

    static int ntz_inlineNlz5(int i) {
        i = ~i & (i - 1);
        if (i <= 0)
            return (i == 0) ? 0 : 32;
        int n = 1;
        if (i >= 1 << 16) { n += 16; i >>>= 16; }
        if (i >= 1 <<  8) { n +=  8; i >>>=  8; }
        if (i >= 1 <<  4) { n +=  4; i >>>=  4; }
        if (i >= 1 <<  2) { n +=  2; i >>>=  2; }
        return n + (i >>> 1);
    }

But it's hard to resist the urge to optimize out a branch:

    static int ntz_inlineNlz6(int i) {
        i = ~i & (i - 1);
        if (i <= 0) return i & 32;
        int n = 1;
        if (i >= 1 << 16) { n += 16; i >>>= 16; }
        if (i >= 1 <<  8) { n +=  8; i >>>=  8; }
        if (i >= 1 <<  4) { n +=  4; i >>>=  4; }
        if (i >= 1 <<  2) { n +=  2; i >>>=  2; }
        return n + (i >>> 1);
    }