The documentation says that vec_mergel and vec_mergeh are supported for the vector double and vector long long types when the VSX instruction set is used, unfortunately it was never implemented in the compiler.
-- Summary: vec_mergel and vec_mergeh should support V2DF/V2DI Product: gcc Version: 4.5.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target AssignedTo: meissner at gcc dot gnu dot org ReportedBy: meissner at gcc dot gnu dot org GCC build triplet: powerpc64-unknown-linux-gnu GCC host triplet: powerpc64-unknown-linux-gnu GCC target triplet: powerpc64-unknown-linux-gnu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43154