RFR: 8298601: Refactor archiving of java.lang.Module objects

[Calvin Cheung]

On Mon, 19 Dec 2022 05:28:39 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> This RFE is a prerequisite for [JDK-8296344: Remove dependency on G1 for writing the CDS archive heap](https://bugs.openjdk.org/browse/JDK-8296344). 
> 
> The main change of this RFE is in `HeapShared::archive_reachable_objects_from()`. We avoid the use of "archived objects" in the interface between CDS and the module system, and use integer "root indices" instead. (See note [1] in [JDK-8298600](https://bugs.openjdk.org/browse/JDK-8298600).)
> 
> Also, for better encapsulation, the module-specific code has been moved from `HeapShared::check_module_oop()` to the three new functions in modules.cpp.
> 
> ----
> 
> Besides the refactoring, this RFE also tightens up the code that deals with `java.lang.Module` oops and the corresponding C++ `ModuleEntry` objects.
> 
> As this is admittedly an obscured area of the CDS archive heap, I've added the following:
> 
> - Assertions to make sure that when an oop is archived, the corresponding C++ object is also archived, and vice-versa.
> - More comments

I have a few minor comments.

src/hotspot/share/classfile/moduleEntry.cpp line 399:

> 397:   ModuleEntry* archived_entry = (ModuleEntry*)ArchiveBuilder::rw_region_alloc(sizeof(ModuleEntry));
> 398:   memcpy((void*)archived_entry, (void*)this, sizeof(ModuleEntry));
> 399:   archived_entry->_archived_module_index = -2;

Why not use -1 as the initial value?

src/hotspot/share/classfile/moduleEntry.cpp line 516:

> 514:   } else if (SystemDictionary::is_system_class_loader(loader_data()->class_loader())) {
> 515:     info.print("system");
> 516:   }

I think this could be simplified to
`info.print(loader_data()->loader_name());`

src/hotspot/share/classfile/modules.hpp line 56:

> 54:                             jstring location, jobjectArray packages, TRAPS);
> 55: 
> 56:   static bool check_module_oop(oop orig_module_obj);

I think this needs `NOT_CDS_JAVA_HEAP_RETURN_(false)` at the end.

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PR: https://git.openjdk.org/jdk/pull/11715