Xilinx bitstream AutoCRC algorithm

Hi

is the Xilinx FPGA bitstream AutoCRC documented somewhere? XAPP151 only covers normal CRC what is not used for Virtex2/Pro and S3 FPGAs. I guess the CRC itself is calculated with same CRC code, but there must be something different as all my attempts to calc the the proper AutoCRC have been failing so far.

any help appreciated :) Antti