RFR: 8299079: Better interface nmethod oop accesses

[Erik Österlund]

On Wed, 21 Dec 2022 19:07:38 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Today when oops are read from nmethods, we use the same interfacing that we would use to read any other IN_NATIVE oops. That isn't enough for generational ZGC. The nmethod oops are rather different from other IN_NATIVE oops in that they are encoded in the machine code as direct pointers. Normally, IN_NATIVE oops with generational ZGC are not direct pointers, but are colored pointered that can be converted to direct pointers by going through a load barrier. For nmethod oops, that conversion requires external knowledge from the surrounding nmethod. I propose a new NMethodAccess to interface better with the GC.
>
> Good.

Thanks for the reviews, @vnkozlov and @dholmes-ora! Will fix the nits before integrating.

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PR: https://git.openjdk.org/jdk/pull/11738