RFR: 8297914: Remove java_lang_Class::process_archived_mirror()

[Ashutosh Mehra]

On Wed, 4 Jan 2023 21:01:28 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> This is another prerequisite for [JDK-8296344](https://bugs.openjdk.org/browse/JDK-8296344).
> 
> Before this PR, when archiving mirror objects (i.e., instances of `java.lang.Class`):
> - We first allocate a copy of the mirror inside a safepoint. 
> - We then reinitialize the contents of the copy to the desired state (so that it can be used by `Klass::restore_unshareable_info()`
> 
> This copy-and-modify operation inside the safepoint makes it difficult to implement [JDK-8296344](https://bugs.openjdk.org/browse/JDK-8296344). It violates the requirements [1] and [2] as stated in [JDK-8298600](https://bugs.openjdk.org/browse/JDK-8298600). Also, the reinitialization code is complicated.
> 
> After this PR:
> - During the creation of each regular mirror object, we allocate a "scratch mirror" object, which has the states as expected by `Klass::restore_unshareable_info()`. See `java_lang_Class::create_scratch_mirror()`.
> - When the archive heap is dumped inside a safepoint, we use the scratch mirror, so we can avoid the copy-and-modify operation. See `HeapShared::archive_java_mirrors()`.
> 
> Testing: tiers 1-4

src/hotspot/share/cds/heapShared.cpp line 379:

> 377: };
> 378: 
> 379: static KlassToOopHandleTable* _scratch_java_mirror_table = NULL;

Any reason why `_scratch_basic_type_mirrors` is a static member of `HeapShared` but `_scratch_java_mirror_table` is not?

src/hotspot/share/classfile/javaClasses.cpp line 1347:

> 1345:     assert(java_class == Universe::void_mirror(), "only valid non-array primitive");
> 1346:   }
> 1347:   assert(Universe::java_mirror(type) == java_class ||

If we need the condition `HeapShared::scratch_java_mirror(type) == java_class` here, shouldn't the previous assert be also updated to check for scratch java mirror for void type?

  `assert(java_class == Universe::void_mirror() |, "only valid non-array primitive");`

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PR: https://git.openjdk.org/jdk/pull/11853