RFR: 8255662: ZGC: Unify nmethod closures in the heap iterator

[Per Liden]

On Mon, 2 Nov 2020 16:10:55 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> In the heap iterator, we use different nmethod closures to visit the on-stack nmethods and the rest that are visited if class unloading is turned off.
> 
> The rational is that the first set have already been processed and does not have to be fixed, so the code simply verifies that the nmethod has been processed and visits all the oops. The second set contains nmethods of the kind that has been entered and processed, and those that have not. Before visiting oops in those nmethods, we apply an nmethod barrier to ensure that it's safe to visit the oops.
> 
> The proposal is to get rid of this separation and simply apply the nmethod entry barrier on all visited nmethods. This will make it easier to reason about the safeness of visiting the oops.

Marked as reviewed by pliden (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/1011