RFR: 8374145: Remove legacy locking remnants from markWord

[Axel Boldt-Christmas]

On Fri, 19 Dec 2025 14:05:13 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> There's a bunch of unused code in markWord that used to be used by the removed legacy locking. I propose that we remove it.
> 
> There's a Shenandoah change that should be checked closer. My thinking is:
>  `is_being_inflated`: always returns false
>  `has_displaced_mark_helper`: Only returns true if both these are true:
>    * `!UseCompactObjectTable` - there's an early return for this earlier in the function.
>    * `lockbits == monitor_value` - checked in the if-statement above

Looks good. Just one thought.

src/hotspot/share/gc/shenandoah/shenandoahHeap.inline.hpp line 349:

> 347:   } else if (w.is_being_inflated() || w.has_displaced_mark_helper()) {
> 348:     // Informs caller that we aren't able to determine the age
> 349:     return markWord::max_age + 1; // sentinel

Not sure if we want an `!w.has_displaced_mark_helper()` assert. Even if we already check  for (and handled) the exact conditions (in the current implementation) for `has_displaced_mark_helper()`.

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Marked as reviewed by aboldtch (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/28927#pullrequestreview-3598758617
PR Review Comment: https://git.openjdk.org/jdk/pull/28927#discussion_r2635220791