RFR: 8264271: Avoid creating non_oop_word oops

[Kim Barrett]

On Fri, 26 Mar 2021 12:01:35 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> Some parts of the JVM puts an marker to show that a location does not contain a valid oop. The code that handles this typically look like this:
> 
> oop* p = ...
> if (*p != Universe::non_oop_word())
> 
> This means that sometimes the *p will create an oop that contains the non_oop_word. This makes it problematic to add stricter oop verification. I propose that we add a new function that checks the value of locations without converting it to an oop. 
> 
> (Note: I'm testing the new dependent pull Skara feature with this PR. It builds depends on the pr/3214 branch)

Looks good.

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Marked as reviewed by kbarrett (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/3215