This assertion is observed at least on linux-i586/amd64 with both latest jdk6 (1.6.0_10-ea-b09) and jdk7 (1.7.0-ea-b24):
# Internal Error (/BUILD_AREA/jdk7/hotspot/src/share/vm/opto/parse1.cpp:960), pid=24653, tid=1084402016
# Error: assert(_exits.control()->is_top() || !_gvn.type(ret_phi)->empty(),"return value must be well defined")

It occurs only in -Xcomp mode.

Testcase and hs_err file are attached.