RFR: Fix recycled regions zapping

[Zhengyu Gu]

Okay.

-Zhengyu

On 06/14/2017 12:52 PM, Aleksey Shipilev wrote:
> Turns out, it is not enough to call ContiguousSpace::clear(true) to get zapping.
> The default version calls into zap_unused, this zaps everything between top and
> high watermark. And we do not set any watermark. So, as suggested in the docs,
> we need to call mangle_unused_area_complete() after resetting top. This will
> mangle everything between top() and end().
>
> Fix:
>   http://cr.openjdk.java.net/~shade/shenandoah/zapping-enable/webrev.01/
>
> ZapUnusedHeapArea is true in debug, which makes tests run around 20% longer.
>
> Testing: hotspot_gc_shenandoah, failing tests
>
> Thanks,
> -Aleksey
>
>