On Mon, 14 Apr 2025 14:47:59 GMT, Per Minborg <pminb...@openjdk.org> wrote:
> This PR proposes to add the `@Stable` annotation to `j.l.String.hash` and
> `j.l.String.hashIsZero`. This means the VM can trust these fields to never
> change which enables constant folding optimizations.
>
> This PR is tested in tier1, tier2, tier3, and tier4 which all pass.
As recommended by @iwanowww, an approach is to write 2 to `hashIsZero` if we
calculated a non-zero hash, so we can constant-fold zero hashes like:
var isZero = hashIsZero;
if (isZero == 1) return 0;
if (isZero == 2) {
int h = hash;
if (h != 0) return h;
}
return computeHash(); // and set hash, hashIsZero fields
I tried something like that at
https://github.com/liachmodded/jdk/commit/f265710ce7d2d02ee51d6429b5603ecc0590c383,
but did not submit because it end up still having a regression compared to
baseline in remi's benchmark, with the resulting throughput/average cost
between the other constant cases and the non-folding string hash cases.
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PR Comment: https://git.openjdk.org/jdk/pull/24625#issuecomment-2802485318
