RFR: 8338011: CDS archived heap object support for 64-bit Windows [v2]

[Ioi Lam]

On Sat, 10 Aug 2024 16:50:46 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> But then a follow-up question, what you do now in `map_heap_region_impl` for Windows, would that not be the same as the `ArchiveHeapLoader::load_heap_region` path?

No. `ArchiveHeapLoader::can_load()` and `ArchiveHeapLoader::can_map()` are not capabilities of the platform, but capabilities of the GC. 

- `can_map()` is hard-coded for G1.
- `can_load()` returns `Universe::heap()->can_load_archived_objects() && UseCompressedOops`. This returns true only on Serial, Parallel and Epsilon.

So on Windows, when G1 is used, we go into this funny "mapping" mode except that the "mapping" is really implemented with `os::read()`.

> And another question, sorry, unrelated to this PR:
> 
> I see we always attempt to load the heap region regardless of here (note how its outside the INCLUDE_CDS_JAVA_HEAP block):
> 
> https://github.com/openjdk/jdk/blob/358d77dafbe0e35d5b20340fccddc0fb8f3db82a/src/hotspot/share/cds/metaspaceShared.cpp#L1200
> 
> I wonder whether this is wrong. If it is wrong, its benign? Do we even include heap region in CDS dumps on Windows?
> 
> (Sorry for loading this PR with questions)

It's benign. If the heap region is not in the CDS archive, the `static_mapinfo->map_or_load_heap_region()` call does nothing.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20514#discussion_r1713059295