[lworld] RFR: 8368467: [lworld] Add new flag generation for jimage to support preview mode

[David Beaumont]

On Tue, 4 Nov 2025 21:12:36 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

>> Adds support for writing preview related flags into jimage files.
>> 
>> Preview mode is complex. It's not nearly as simple as "does something in /modules/xxx/... have an entry in /modules/xxx/META-INF/preview/...".
>> 
>> Specific issues include:
>> 
>> Supporting preview-only resources without forcing a double lookup on everything.
>> Changing the set of entries in /packages/xxx directories to account for preview only packages in some modules.
>> Minimising the work done during image reader initialization to only need to process the small number of preview resources (rather than scanning the whole file to look for them).
>> The new flags added by this code address these issues, but calculating them correctly with only minor adjustments to the existing code was not feasible, it just became a lot larger and very complex.
>> 
>> To address this, a new type (ModuleReference) is introduced to track and then merge information about packages seen in each module. This allows a much simpler inner loop for processing resource paths when building the node tree, combined with a subsequent merging stage to produce the final package information for each module.
>> 
>> Not that since ModuleReference is needed during jimage reading, that class is already present in the previous PR on which this is based, but it starts to be used to calculate the module flags in this PR.
>> 
>> This PR can also adds the ImageReader unit tests for preview mode, which rely on being able to generate jimage files with preview mode flags in.
>
> test/jdk/tools/jlink/whitebox/ImageResourcesTreeTestDriver.java line 24:
> 
>> 22:  */
>> 23: 
>> 24: /*
> 
> Why is this separate from the test itself?

I don't understand the question.
This is needed because the test is a whitebox test and accesses package visible things.
Is that what you mean?

> test/micro/org/openjdk/bench/jdk/internal/jrtfs/ImageReaderBenchmark.java line 318:
> 
>> 316:     }
>> 317: 
>> 318:     /// Note: This list is inherently a little fragile and may end up being more
> 
> Yes, this kind of test/benchmark will bit rot very fast.
> The existing startup measurements are more stable and reflect actual startup times, not a simulation thereof.

Indeed. Mas talking to Michel yesterday and we were saying that, once this is in and the dust settles, it might be good to either convert this to an Aurora benchmark or just delete it.

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1718#discussion_r2494829885
PR Review Comment: https://git.openjdk.org/valhalla/pull/1718#discussion_r2494822167