[lworld] RFR: 8376171: [lworld] Enable storing the hash code in the mark word of value objects

[Paul Hübner]

On Fri, 23 Jan 2026 10:20:37 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

> Since [JDK-8247299](https://bugs.openjdk.org/browse/JDK-8247299), value objects no longer use `markWord::always_locked_pattern` (see description of [JDK-8247298](https://bugs.openjdk.org/browse/JDK-8247298) for more details). Therefore, the limitations around storing the hash code in the mark word that are mentioned in [JDK-8244975](https://bugs.openjdk.org/browse/JDK-8244975) no longer exist. Let's enable this optimization.
> 
> Running `System.identityHashCode(obj)` in a loop and passing various primitive boxes leads to a **17-18x speedup** on my machine. I'm also seeing a **7% improvement** in the score of the SPECjvm2008 serial benchmark but we are still a bit behind baseline without `--enable-preview`. More improvements to come.
> 
> It's worth noting that value objects will lose their buffer object when stored in a flat container or sometimes when being scalarized ([JDK-8372268](https://bugs.openjdk.org/browse/JDK-8372268) will improve this). The hash then needs to be re-computed, which is expensive.
> 
> I'm working on ways to further improve this, see [JDK-8252185](https://bugs.openjdk.org/browse/JDK-8252185).
> 
> Thanks,
> Tobias

Thank you for doing this, and nice that we're not wasting the bits anymore :-). 

I have left some smaller nit-ish comments, one open question I have myself is whether it's safe to `set_mark` when running with a concurrent GC. If anyone has any insights, it'd be good to hear.

src/hotspot/share/prims/jvm.cpp line 780:

> 778:     // Check if mark word contains hash code already
> 779:     intptr_t hash = obj->mark().hash();
> 780:     if (hash != 0) {

We have a `has_no_hash` which might be better instead. We actually use a constant no hash value in the markWord (albeit it is zero right now).

src/hotspot/share/prims/jvm.cpp line 799:

> 797:       }
> 798:     }
> 799:     hash = result.get_jint() & markWord::hash_mask;

I don't think it's a good idea to do mask ops outside of the markWord, because it gets difficult to find them and maintain them. Can we introduce a helper function inside the markWord to do this for us?

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PR Review: https://git.openjdk.org/valhalla/pull/1954#pullrequestreview-3698196680
PR Review Comment: https://git.openjdk.org/valhalla/pull/1954#discussion_r2721785985
PR Review Comment: https://git.openjdk.org/valhalla/pull/1954#discussion_r2721752945