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

[Ivan Gerasimov]

Hi Martin!

Good point about the command line flags, thanks!

These variants are close to numberOfTrailingZeros_07 that I've already 
tested, though you did better by saving one arithmetic operation at the 
return line!

I'll rerun the benchmarks.

With kind regards,

Ivan


On 8/13/18 7:56 AM, Martin Buchholz wrote:
> The number of plausible variants is astonishing!
>
> ---
>
> Your use of -client and -server is outdated, which explains why you 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
>
> ---
>
> 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);
>     }
>

-- 
With kind regards,
Ivan Gerasimov