RFR 8203279 : Faster calculation of power of two (original) (raw)

Ivan Gerasimov ivan.gerasimov at oracle.com
Thu May 17 03:01:27 UTC 2018

On 5/16/18 7:44 PM, David Holmes wrote:

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 :) Of course it's not performance critical code, but why not optimize it by a tiny bit if possible?

Do you think it's good to go?

With kind regards, Ivan

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.MINVALUE to Integer.MAXVALUE (including), to make sure >>> the result is the same. >>>>>>> For TimSort.ensureCapacity() I checked all positive values of >>> minCapacity. >>>>>>>>>

With kind regards, Ivan Gerasimov

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