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

[Mike Duigou]

On Apr 2 2013, at 10:55 , Martin Buchholz wrote:

> Thanks for the research.
> It seems like hotspot is recognizing and optimizing fill loops, rather than
> intrinsifying calls to Arrays.fill itself (good!).

Why wouldn't doing both be better?

> Anyways, I'd still like the "simple" fill loops in ArrayList to stay
> unchanged.  Using Arrays.fill is only slightly more readable.

Part of the goal of the change was to make the intent clearer. I'll improve the comments instead.

>   There would
> be more of a case for fill if it was an actual array class instance method.

Maybe for Java 9....

Mike

> 
> 
> 
> On Tue, Apr 2, 2013 at 2:12 AM, Patrick Wright <pdoubleya at gmail.com> wrote:
> 
>> 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
>>