RFR 8203279 : Faster calculation of power of two

[Martin Buchholz]

Hello Ivan,

I've wondered about this myself.
Probably the performance is architecture dependent.
Probably a new method should be added to Integer and Long with
@HotspotIntrinsicCandidate.
That gives David another chance to practice his blind aarch64 assembly
language coding skills.
It's hard to give a good name for this.  Hacker's delight calls it dclp2
and we can probably do better than that.
http://www.hackersdelight.org/hdcodetxt/clp2.c.txt
But I approve either way.


On Wed, May 16, 2018 at 5:32 PM, Ivan Gerasimov <ivan.gerasimov at oracle.com>
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.
>
> --
> With kind regards,
> Ivan Gerasimov
>
>