[lworld] RFR: 8367073: [lworld] Conform to JEP 450 encoding of mark word

[Frederic Parain]

On Fri, 12 Sep 2025 09:43:41 GMT, Paul Hübner <phubner at openjdk.org> wrote:

> Hi all, 
> 
> This PR refactors the mark word to use the order of bits as specified in [JEP 450](https://openjdk.org/jeps/450). 
> 
> Specifically, there are three portions to this change:
> - Moving the age bits lower to their intended position.
> - Fixing a miscompilation in the native `hashCode` intrinsic that got unearthed as a result.
> - Updating the documentation to be more inline with the mainline docs and minimal Valhalla additions.
> 
> Feedback is welcome, especially for the doc changes. If there are any ideas of other places in which similar miscompilations could occur, that would be valuable to know as well.

src/hotspot/share/oops/markWord.hpp line 428:

> 426: 
> 427:  private:
> 428:   inline uintptr_t write_bits(uintptr_t input, uintptr_t val, uint start, uint end) const {

Where is this method used?

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1578#discussion_r2394561107