RFR JDK-7143928 : (coll) Optimize for Empty ArrayList and HashMap (original) (raw)

Patrick Wright pdoubleya at gmail.com
Tue Apr 2 09:12:49 UTC 2013

On Tue, Apr 2, 2013 at 9:27 AM, Laurent Bourgès <bourges.laurent at gmail.com>wrote:

> --- > > 604 Arrays.fill(elementData, newSize, size, null); > > In performance-critical code I would avoid Arrays.fill because it adds a > bit of overhead (unless it's intrinsified, which I don't think it is). >

Last week, I sent few benchmarks I did on array cleaning (zero fill) comparing Arrays.fill, System.arraycopy, Unsafe.setMemory ... Arrays.fill is the winner (always faster than arraycopy which use native code) by 10 - 20% ! I suspect aggressive hotspot optimizations (native code ?) because I agree Arrays.fill looks like a stupid for-loop ! Does somebody have clues explaining the Arrays.fill performance ?

There was at least one round of optimization done by the HotSpot team in mid-2010 - "This adds new logic to recognize fill idioms and convert them into a call to an optimized fill routine. Loop predication creates easily matched loops that are simply replaced with calls to the new assembly stubs. Currently only 1,2 and 4 byte primitive types are supported. Objects and longs/double will be supported in a later putback. Tested with runthese, nsk and ctw plus jbb2005. "

see http://openjdk.5641.n7.nabble.com/review-M-for-4809552-Optimize-Arrays-fill-td10322.html

Looks like the change was part of 6u23 http://download.java.net/jdk6/6u23/promoted/b03/changes/JDK6u23.b03.list.html

Could not find anything more recent than that (on a quick mail search)

Cheers Patrick

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