When you apply NDGBuild option -insert_keep_hierarchy on the top-level design with SPI-4.2 core, following MAP error occurs:
"ERROR:Pack:679 - Unable to obey design constraints (LOC=SLICE_X26Y125) which require the combination of the following symbols into a single SLICEcomponent:
FLOP symbol "pl4_snk_top0/pl4_snk_io0/DynamicAlign.pbd/dc/dk.3.q"
(Output Signal = pl4_snk_top0/pl4_snk_io0/DynamicAlign.pbd/dc/q_1_5)
FLOP symbol "pl4_snk_top0/pl4_snk_io0/DynamicAlign.pbd/dc/d4.4.q"
(Output Signal = pl4_snk_top0/pl4_snk_io0/DynamicAlign.pbd/dc/q_2_0)
Symbols have different XGROUP parameters. Please correct the design constraints accordingly."
This is an ISE tool issue, and the problem is fixed in ISE 6.2i Service Pack 2.