RFR: 8279500: Remove FileMapHeader::_heap_obj_roots

[Calvin Cheung]

On Wed, 5 Jan 2022 22:29:33 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> `FileMapHeader::_heap_obj_roots` is an `narrowOop` that's used to remember the `objArray` that stores all the roots of archived heap objects. Reading/writing of this field requires some ad-hoc code in many places.
> 
> There's already a standard way of reading/writing oops in the CDS archive -- `SerializeClosure::do_oop()`. See here for an example:
> 
> - https://github.com/iklam/jdk/blame/ad1dc9c2ae5463363aff20072a3f2ca4ea23acd2/src/hotspot/share/memory/universe.cpp#L243
> 
> We should use the same pattern in `HeapShared.seralize()` to write/restore `HeapShared::_roots`. That way we can remove `FileMapHeader::_heap_obj_roots` and the associated ad-hoc code.

LGTM.

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Marked as reviewed by ccheung (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/6973