From http://cr.openjdk.java.net/~briangoetz/valhalla/sov/02-object-model.html:

System::identityHashCode. The main use of identityHashCode is in the implementation of data structures such as IdentityHashMap, as a filter in front of ==. We can totalize identityHashCode in the same way we totalize equality – deriving a hash from the hash of all the fields.

Currently, we barf if it can be determined statically that the operand to the identityHashCode call is an inline instance

The actual derivation of the hash may be handled by a separate ticket against the relevant JRE project.