[lworld] RFR: 8376566: [lworld] Runtime cleanups of markWord hashCode for inline types after JDK-8376171

[Dan Heidinga]

On Wed, 4 Feb 2026 10:03:55 GMT, Paul Hübner <phubner at openjdk.org> wrote:

> Hi all,
> 
> The main ideas of this patch are to highlight and enforce the invariant we enforce when it comes to value objects' identity hash code. 
> 
> The original JBS issues addresses the following points, which have been addressed to various extents:
> 
> 1. Adding assertions to the CAS-setting of the hash in the markWord. This is vital to enforce the invariant and was added.
> 2. Breaking the loop if the CAS results in a conflict. Putting the identity hash in the markWord is an optimization, so one could break out of the loop whenever. With the assertion, there's a good confidence that CAS will eventually succeed, namely once other threads stop poking at the markWord bits. **If there is demand, I can add a fixed upper bound.**
> 3. SSA-ing the hash variable. Done.
> 4. Possibly introducing a markWord::has_hash to improve legibility. I did not do this as it would yield multiple `obj->mark()` calls in the fast path and the current form is (in my opinion) sufficiently legible. 
> 
> Testing: tiers 1-3 on Linux (x64, AArch64), macOS (x64, AArch64), Windows (x64).

src/hotspot/share/prims/jvm.cpp line 798:

> 796:     //    matter when this is called the same identity hash code is expected.
> 797:     // 2. Oops: the above still applies, but the oops' identity hash code must
> 798:     //    be used as the polymorphic hashCode may change due to mutability.

This is a good description for generating the identity hashcode but seems like its in the wrong place... this function doesn't "own" the calculation of the hash and the various cases - they belong elsewhere.

The first line `// The generated identity hash is invariantly immutable.` is the only part of this that applies in this function.

The reason this comment is odd here is that we don't implement 2 cases here.

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PR Review Comment: https://git.openjdk.org/valhalla/pull/2029#discussion_r2795624600