RFR 8203279 : Faster calculation of power of two

[David Holmes]

Hi Ivan,

Surely you need to back this up with some performance numbers! And 
verify not assume that numberOfLeadingZeroes is intrinsified!

Cheers,
David

On 17/05/2018 10:32 AM, Ivan Gerasimov wrote:
> Hello!
> 
> In a few places we have code that rounds an integer up to the nearest 
> power of two.
> 
> It is done with a series of RSHOTFs and ORs, but it can possibly be done 
> faster with the use of Integer.numberOfLeadingZeros (assuming it is 
> intrinsified).
> 
> Would you please help review this trivial optimization:
> 
> BUGURL: https://bugs.openjdk.java.net/browse/JDK-8203279
> WEBREV: http://cr.openjdk.java.net/~igerasim/8203279/00/webrev/
> 
> For HashMap.tableSizeFor() I created a simple test with a loop from 
> Integer.MIN_VALUE to Integer.MAX_VALUE (including), to make sure the 
> result is the same.
> 
> For TimSort.ensureCapacity() I checked all positive values of minCapacity.
>