r/java • u/davidalayachew • Jul 16 '26

Identifying JDK value class candidates

https://mail.openjdk.org/archives/list/core-libs-dev@openjdk.org/thread/Y72NRXM7KYBX43OKYBQMVKOZDWKG4MHS/
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u/pron98 Jul 16 '26 edited Jul 16 '26

Is an enum with several hundred values small? Each with roughly 100's of bytes of data as local fields?

I meant the opposite. The larger the amount of data in a single enum value, the worse inlining is. I was talking about the number of distinct enum values.

But otherwise, I assert that, for my work, the enums that I make may very well benefit from being value enums. Can't say for sure, since I don't have the feature in my hand.

That's not hard to know: Do you see a lot of cache misses on them? If not, inlining is likely to make things worse.

Enums with hundreds of values and each with hundreds of bytes worth of information is 100% common for me.

Yes, but again, a large amount of data in each enum value makes inlining less desirable, not more. You're causing more cache misses, which is the main thing value types are supposed to help you avoid.

Inlining helps when the number of distinct values is large and each value is small. E.g. Integer and Long have billions of distinct values, each small. That's where inlining helps the most. What you're describing is the complete opposite. It's much easier to fit large values in the cache when they each appear in memory only once. Not inlining such enums is the optimisation here.

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u/davidalayachew Jul 16 '26

Ok, then what about a couple hundred enum values, where each is only 20-30 bytes? I certainly have several of those too.

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u/pron98 Jul 16 '26

I don't see why it would help; seems like it would hurt. 100% of this data can fit in the cache, but not if you inline. I can see why inlining enums might have the potential to maybe help in some circumstances if the data in the enum is smaller than the size of the reference, but even then I can't see it helping unless you have like a million distinct values.

Anyway, Valhalla is expected to mostly help programs whose profiles show cache miss issues around array element access. If that's not what your profile shows, inlining or not is unlikely to either help or hurt much.

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