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

[Aleksey Shipilev]

On 5/22/19 9:25 AM, Roman Kennke wrote:
> Where exactly does verification interfere with evacuation-in-progress?
> Because I don't see it. verify_before_evacuation() is done before
> activating the flag, and verify_after_evacuation() is done after
> clearing the flag.

There is also ShenandoahVerifier::verify_during_evacuation(). But anyhow, we want to skip any
barriers when doing verification, as we'd like to see the unaided view of the heap.

-Aleksey