RFR: 8301106 allow cds interned strings to be moved by gc

[David Holmes]

On Thu, 16 Feb 2023 19:14:15 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> **Background:**
> 
> Currently, the archived java strings are mapped in the G1 "[closed archive](https://github.com/openjdk/jdk/blob/574b48c6925ebfb31345fc46c7d23aa4153f99b0/src/hotspot/share/gc/g1/heapRegionType.hpp#L80-L92)" region. This essentially pins all the strings in memory.
> 
> As a prerequisite for  ([JDK-8296263](https://bugs.openjdk.org/browse/JDK-8296263)), this PR removes the requirement of pinning the archived strings. This will allow the CDS archive heap to be mapped in garbage collectors that do not support object pinning.
> 
> **Code changes:**
> 
> - The archived strings are referenced through an objArray (`_shared_strings_array`) to keep them alive. As a result, it's no longer necessary to pin them.
> - Since it's possible for the GC to move these strings, the `_shared_table` in stringTable.cpp is modified to store a 32-bit index for each archived string. This index is used to retrieve the archived string from `_shared_strings_array` at runtime.
> 
> Note that CDS has a limit on the size of archived objArrays. When there's a large number of strings, we use a two-level table. See the comments around  `_shared_strings_array` in the header file.
> 
> **Testing**
> 
> Tiers 1 - 4

src/hotspot/share/cds/heapShared.cpp line 426:

> 424:   oop shared_strings_array = StringTable::init_shared_table(_dumped_interned_strings);
> 425:   bool success = archive_reachable_objects_from(1, _default_subgraph_info, shared_strings_array, /*is_closed_archive=*/ false);
> 426:   guarantee(success, "shared strings array should not point to any unachivable objects");

typo: unachivable

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PR: https://git.openjdk.org/jdk/pull/12607