RFR: JDK-8269651: ZGC: Optimize away gc locker in mark start

[Per Liden]

On Thu, 1 Jul 2021 01:38:05 GMT, Hamlin Li <mli at openjdk.org> wrote:

> GetXxxCritical returns the native address, and whether good or bad these addresses are valid address for native code (am I right here?), unless it's relocated later at Relocate.

We currently have the invariant that all accesses must go through the currently good heap view. While you can "cheat" and access objects through a bad heap view that will only work as long as you don't enable -XX:+ZVerifyViews, which will enforce this invariant. ZVerifyViews unmaps memory that isn't currently in the good heap view. It's a valuable debugging option to catch bad accesses, like when a load barrier is missing. ZVerifyViews is disabled by default because it's expensive.

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PR: https://git.openjdk.java.net/jdk/pull/4638