RFR: Shenandoah string deduplication

[Roman Kennke]

Am 06.12.2017 um 18:15 schrieb Zhengyu Gu:
> The implementation takes different approach from G1's implementation.
> 
> * It treats string dedup queue/table as weak roots.
> 
> * It identifies deduplication candidates during concurrent marking and 
> during full gc, to reduce impact on pause time.
> 
> * To properly identify the candidates, it manipulates string's age field 
> concurrently, to prevent from processing string too early or 
> deduplicating the same string over and over again. While it is 
> undesirable, it reduces the risk by only manipulating *regular* strings 
> - when they do not have displaced headers. Locked objects are relative 
> rare, especially for immutable objects, such as Strings.
> 
> * Deduplicate routine is implemented lock-free, in case we have to embed 
> it in barriers.
> 
> * Reverted all early changes to G1's implementation
> 
> Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/sh_strdedup/webrev.00/
> 
> Test:
>    hotspot_gc_shenandoah (fastdebug and release)
>    specJBB               (fastdebug and release)
> 
> 
> Thanks,
> 
> -Zhengyu

src/hotspot/share/classfile/stringTable.cpp:
/me makes a note that this warrants a GC interface

Other than that, I like it! :-)

Probably warrants another pair of eyes.

Roman