On Fri, 23 Jan 2026 10:20:37 GMT, Tobias Hartmann <[email protected]> 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 Thanks for taking a look at this Marc! > A question nevertheless: I see that now we have & markWord::hash_mask, so I > take that this might change the hash (if result.get_int() highest bit is 1, > so negative) to make it fit on 31 bits? Yes, we only have 31 bits in the mark word reserved for the hash code: https://github.com/openjdk/valhalla/blob/258bd5840bb7b029b9877155c53b6b1122b0e9dc/src/hotspot/share/oops/markWord.hpp#L52-L55 Existing code for identity objects already does the same truncation. ------------- PR Comment: https://git.openjdk.org/valhalla/pull/1954#issuecomment-3790043174
