RFR: 8310428: Add CollectedHeap::reserved_range() API to return reserved heap memory range

[Stefan Karlsson]

On Wed, 21 Jun 2023 15:30:17 GMT, Ashutosh Mehra <duke at openjdk.org> wrote:

> Please review this patch to add a new API `CollectedHeap::reserved_range()`. The changes are extracted out from https://github.com/openjdk/jdk/pull/14520 as per the suggestion [here](https://github.com/openjdk/jdk/pull/14520#discussion_r1234991165). Support for ZGC is not implemented yet as it can have discontiguous heap regions and unlike other collectors, it does not set `CollectedHeap::_reserved`.

This change completely undoes the removal of CollectedHeap::reserved_region() we did in:
8224815: Remove non-GC uses of CollectedHeap::is_in_reserved()

We removed it because it is a false abstraction that makes it easy for people to accidentally start to use this API without realizing that it will not work for ZGC.

I'd prefer to see a plan on how to implement heap archiving for ZGC before we decide on making abstractions in CollectedHeap that doesn't work for ZGC. I know that @fisk has had earlier discussions with  @iklam about other ways to make the heap archiving GC-agnostic. In #14520 we saw that another approach was taken and @fisk stepped in and voiced again that we have concerns about this approach. Given that we haven't completed that discussion I'd like to pause this PR until we have come to an agreement on how to proceed.

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Changes requested by stefank (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/14595#pullrequestreview-1491489633