[lworld] RFR: 8365978: [lworld] C2: assert(vk->maybe_flat_in_array()) when using compareAndSetFlatValue with -XX:-UseArrayFlattening [v3]

[Marc Chevalier]

On Mon, 8 Sep 2025 07:17:11 GMT, Marc Chevalier <mchevalier at openjdk.org> wrote:

>> `Unsafe::compareAndSetFlatValue` calls `Unsafe::compareAndSetFlatValueAsBytes` which then calls `Unsafe::putFlatValue` on a flat array created by `Unsafe::newSpecialArray`.
>> 
>> https://github.com/openjdk/valhalla/blob/858be30119bd5bc37a69d16042523f53bea71a36/src/java.base/share/classes/jdk/internal/misc/Unsafe.java#L2870-L2875
>> 
>> `putFlatValue` can be intrinsified in
>> 
>> https://github.com/openjdk/valhalla/blob/858be30119bd5bc37a69d16042523f53bea71a36/src/hotspot/share/opto/library_call.cpp#L2727
>> 
>> that calls `cast_to_flat_array`
>> 
>> https://github.com/openjdk/valhalla/blob/858be30119bd5bc37a69d16042523f53bea71a36/src/hotspot/share/opto/library_call.cpp#L2831
>> 
>> which fails the assert
>> 
>> https://github.com/openjdk/valhalla/blob/858be30119bd5bc37a69d16042523f53bea71a36/src/hotspot/share/opto/graphKit.cpp#L1875-L1876
>> 
>> because of
>> 
>> https://github.com/openjdk/valhalla/blob/858be30119bd5bc37a69d16042523f53bea71a36/src/hotspot/share/oops/inlineKlass.cpp#L287-L290
>> 
>> Of course, if array flattening is disabled, one can't have flat arrays. And indeed, `Unsafe::newSpecialArray` will raise if called. So at runtime, the call to `Unsafe::compareAndSetFlatValue` should simply raise. But when compiled the assert is hit, crashing the VM, because it cannot tell that the code is dead, but can check that if it's not the array is not flat. That doesn't seem reasonable to me. I propose to insert a trap instead. Even in the case where this code wouldn't be dead (like if `Unsafe::newSpecialArray` is just undefined behavior instead of throwing), crashing the compiler doesn't seem like a good option.
>> 
>> The situation is actually more surprising than it seems: using `Unsafe::compareAndSetFlatValue` on a flat field with `-XX:-UseArrayFlattening` sounds reasonable, but doesn't actually work since the implementation will (attempt to) create flat arrays under the hood. It is not clear to me whether it's actually desirable, as `Unsafe::compareAndSetFlatValue` introduce some coupling of field and array flattening, but on the other hand, it's an unsafe API, so it's not that crazy to require more constrains to use.
>> 
>> Thanks,
>> Marc
>
> Marc Chevalier has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add +UseFieldFlattening

Thanks @TobiHartmann

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PR Comment: https://git.openjdk.org/valhalla/pull/1549#issuecomment-3264953054