[lworld] RFR: 8366093: [lworld] Add preview mode to C++ classloader [v2]

[David Beaumont]

On Tue, 14 Oct 2025 12:34:14 GMT, David Beaumont <duke at openjdk.org> wrote:

>> C++ changes for supporting preview mode when preview mode resources (with new location flags) are available.
>> 
>> At the moment, this code will operate on non-preview jimage files (1.0) and act as if no preview resources are available by virtue of the default value for missing attributes being zero (which matches location flags for "normal" entries).
>
> David Beaumont has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Reorganising to catch preview-only cases

src/java.base/share/native/libjimage/jimage.cpp line 145:

> 143:     }
> 144: 
> 145:     // find_location_index() returns the data "offset", not an index.

I reworked this code slightly and added more comments explaining why the "do two lookups if the resource doesn't exist" should not risk performance issues (we should only be being asked for things that are expected to exist). Of course if anyone reviewing this thinks this is not a correct statement, please speak up!!

Right now, the flags are definitely zero (the new attribute is never written and defaults to zero) so the preview specific code is unreachable code until the jimage creation is modified (even if `is_preview_mode` were true, which it isn't).

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1618#discussion_r2429322847