We did, and we've been talking to the developers about a potential future collaboration. Our goals are really complementary; hiphip is about getting your code into the inner loop of Java bytecode (not just a set of canned operations), whereas core.matrix is about abstractions for a fixed set of operations across different matrix types. There may eventually be overlap, if core.matrix gets into compiling expressions into new operation types, which sounds like something they're interested in.
core.matrix developer here :-) we're definitely looking at expression compilation. Also hiphip could be very useful for writing fast core.matrix implementations for the standard API. So definitely good room for collaboration.
