On Fri, 5 Sep 2025 11:42:42 GMT, Marc Chevalier <mcheval...@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 This pull request has now been integrated. Changeset: b53b0491 Author: Marc Chevalier <mcheval...@openjdk.org> Committer: Tobias Hartmann <thartm...@openjdk.org> URL: https://git.openjdk.org/valhalla/commit/b53b0491ebafdfc8a3b7f20989b1f49c52e69755 Stats: 101 lines in 2 files changed: 91 ins; 0 del; 10 mod 8365978: [lworld] C2: assert(vk->maybe_flat_in_array()) when using compareAndSetFlatValue with -XX:-UseArrayFlattening Reviewed-by: thartmann ------------- PR: https://git.openjdk.org/valhalla/pull/1549
