RFR 8203279 : Faster calculation of power of two

[Claes Redestad]

On 2018-05-17 22:44, Ivan Gerasimov wrote:
> The following variant showed slightly better performance on my machine:
>
>     static final int numberOfLeadingZeros(int i) {
>         if (i <= 0)
>             return i == 0 ? 32 : 0;
>         int n = 31;
>         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);
>     }

Nice, this version also wins on all of -Xint and 
-XX:TieredStopAtLevel=1-3 (my version lost out slightly versus
the baseline on -Xint), so is potentially a startup enhancement even on 
platforms with C2 intrinsics.

>
> I agree that improving Java implementation of numberOfLeadingZeros() 
> can be done as a separate RFE. 

I filed https://bugs.openjdk.java.net/browse/JDK-8203352 for this.

/Claes