[lworld] RFR: 8368875: [lworld] UseParallelGC fails null narrow klass assertion failure

[Coleen Phillimore]

On Fri, 3 Oct 2025 14:18:09 GMT, Paul Hübner <phubner at openjdk.org> wrote:

>> This seems to pass multiple stressHierarchy test invocations with -XX:+UseParallelGC, and runtime/valhalla/inlinetypes tests. I think there's probably further investigation that needs to be done but hopefully this helps with the parallel GC test failures.
>> Testing with tier1-4.
>
> src/hotspot/share/oops/oop.inline.hpp line 100:
> 
>> 98: // This is for parallel gc, which doesn't always have the klass.
>> 99: // markWord::must_be_preserved preserves the original prototype header bits for EnableValhalla,
>> 100: // I don't know why serial gc doesn't work the same.
> 
> We should probably find out why. I can ask around.

Thank you! That would be good.  The prototype_mark() function above or even init_mark shouldn't have to be conditional on EnableValhalla if the markWord is always preserved.  But serial gc is very unhappy with that.

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1655#discussion_r2402141866