RFR(XXS) 8224525: Shenandoah: Eliminate shenandoah verifier's side-effects

[Aleksey Shipilev]

On 5/21/19 10:57 PM, Zhengyu Gu wrote:
> Since switching to LRB, verifier has side-effects of evacuating oops, if it is ran during evacuation
> phase.
> 
> The patch resets gc_state before LRB can be triggered by verifier, and restore gc_state when it is
> done.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8224525
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8224525/webrev.00/
> 
> Test:
>   hotspot_gc_shenandoah with -XX:+ShenandoahVerify (fastdebug and release)

The idea looks good.

Let's put the comment like:

 672   // Deactivate barriers temporarily: Verifier wants plain heap accesses
 673   ShenandoahGCStateResetter resetter;

You can common the use of ShenandoahHeap::heap() here (in fact you can probably just pull it into
the ShenandoahGCStateResetter const field):

 618   ~ShenandoahGCStateResetter() {
 619     ShenandoahHeap::heap()->_gc_state.set(_gc_state);
 620     assert(ShenandoahHeap::heap()->gc_state() == _gc_state, "Should be restored");
 621   }

-Aleksey