RFR: 8153713 : aarch64: improve short array clearing using store pair

[Andrew Haley]

On 04/07/2016 04:01 PM, Felix Yang wrote:
>     Please review webrev: http://cr.openjdk.java.net/~fyang/8153713/webrev.00/
>     JIRA Issue: https://bugs.openjdk.java.net/browse/JDK-8153713
> 
>     Currently, C2 compiler generate independent stores to clear
>     short arrays whose size is no bigger than parameter
>     InitArrayShortSize (refer to ClearArrayNode::Ideal function).
>     For the aarch64 port, we have store pair instruction which can
>     zero two memory words at a time and this will be good for code
>     size and maybe performance for some micro-archs.
> 
>     For Spark Terasort, an extra of 550 stp (xzr, xzr) instructions
>     are generated with the patch, which mean about 2KB reduction in
>     codesize.
> 
>     Tested with JTreg hotspot, langtools and jdk.  Is it OK?

It looks reasonable.  It's rather a big slab of code for aarch64.ad,
and I think that it should be in MacroAssembler.  Long Chen created
MacroAssembler::zero_words, and you should create an overload of
zero_words which takes a constant int as an argument.

Andrew.