General Description: Timing-driven packing causes a Virtex-E design to fail with the following error:
"ERROR:Place:871 - Some of the slices in the carry chain containing the slice xxx have LOC constraints that make it impossible to place the chain legally. Please either fix or remove the constraints."
This error occurs because the packer is performing an unrelated merge that combines LOC'd registers and a part of a carry chain, which makes it impossible for the placer to successfully align the carry chain.
An environment variable exists that makes the packer more conservative regarding the packing of carry chains and other structures requiring alignment:
Workstations: setenv XIL_MAP_PROTECTSHAPES 1
PCs: set XIL_MAP_PROTECTSHAPES=1
The next major software release after 4.2i will contain a fix that addresses the issue of LOC'd registers being merged with carry chain components under the above-mentioned environment variable control.