RFR: 8264271: Avoid creating non_oop_word oops [v2]

[Per Liden]

On Mon, 29 Mar 2021 09:46:57 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)
>
> Stefan Karlsson has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:
> 
>  - Merge branch '8264268_dervied_pointer_types' into 8264271_avoid_creating_non_oop_word_oops
>  - 8264271: Avoid creating non_oop_word oops

Looks good!

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Marked as reviewed by pliden (Reviewer).

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