xilinx_srl - Xilinx shift register extraction

yosys> help xilinx_srl
xilinx_srl [options] [selection]
This pass converts chains of built-in flops (bit-level: $_DFF_[NP]_, $_DFFE_*
and word-level: $dff, $dffe) as well as Xilinx flops (FDRE, FDRE_1) into a
$__XILINX_SHREG cell. Chains must be of the same cell type, clock, clock
polarity, enable, and enable polarity (where relevant).
Flops with resets cannot be mapped to Xilinx devices and will not be inferred.
-minlen N
min length of shift register (default = 3)
infer fixed-length shift registers.
infer variable-length shift registers (i.e. fixed-length shifts where
each element also fans-out to a $shiftx cell).