RFR: 8274024: Use regular accessors to internal fields of oopDesc

[Coleen Phillimore]

On Mon, 20 Sep 2021 18:59:34 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> Currently, we are using 'raw' accessors to initialize the mark, Klass*, (array-)length and klass_gap of oops. This is ugly and confusing and we should just use the regular accessors.
> 
> Testing:
>  - [ ] tier1
>  - [ ] tier2
>  - [ ] hotspot_gc

The places that Roman changed are the boundaries of where HeapWord* becomes oops so the cast_to_oops() here are appropriate. This is a nice cleanup and changes the sources to what they were before the shenandoah changes.

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PR: https://git.openjdk.java.net/jdk/pull/5585