RFR 8203279 : Faster calculation of power of two

[David Holmes]

On 17/05/2018 12:30 PM, Ivan Gerasimov wrote:
> Hi David!
> 
> 
> On 5/16/18 6:12 PM, David Holmes wrote:
>> Hi Ivan,
>>
>> Surely you need to back this up with some performance numbers! And 
>> verify not assume that numberOfLeadingZeroes is intrinsified!
>>
> Yes, good point.
> Please find the benchmark with the results here:
> http://cr.openjdk.java.net/~igerasim/8203279/00/MyBenchmark.java
> 
> It shows that the new version of HashMap. tableSizeFor() was 
> (approximately) two times faster.

Impressive. :)

Thanks,
David

> With kind regards,
> Ivan
> 
>> 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.
>>>
>>
>