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

[Mike Duigou]

Hello all;

This is a review for optimization work that came out of internal analysis of Oracle's Java applications. It's based upon analysis that shows that in large applications as much as 10% of maps and lists are initialized but never receive any entries. A smaller number spend a large proportion of their lifetime empty. We've found similar results across other workloads as well. This patch is not a substitute for pre-sizing your collections and maps--doing so will *always* have better results.

This patch extends HashMap and ArrayList to provide special handling for newly created instances that avoids creating the backing array until needed. There is a very small additional cost for detecting when to inflate the map or list that is measurable in interpreted tests but disappears in JITed code. 

http://cr.openjdk.java.net/~mduigou/JDK-7143928/0/webrev/

We expect that should this code prove successful in Java 8 it will be backported to Java 7 updates.

The unit test may appear to be somewhat unrelated. It was created after resolving a bug in an early version of this patch to detect the issue encountered (LinkedHashMap.init() was not being called in readObject() when the map was empty).

Mike