xilinx_dsp - Xilinx: pack resources into DSPs

yosys> help xilinx_dsp
xilinx_dsp [options] [selection]
Pack input registers (A2, A1, B2, B1, C, D, AD; with optional enable/reset),
pipeline registers (M; with optional enable/reset), output registers (P; with
optional enable/reset), pre-adder and/or post-adder into Xilinx DSP resources.

Multiply-accumulate operations using the post-adder with feedback on the 'C'
input will be folded into the DSP. In this scenario only, the 'C' input can be
used to override the current accumulation result with a new value, which will
be added to the multiplier result to form the next accumulation result.

Use of the dedicated 'PCOUT' -> 'PCIN' cascade path is detected for 'P' -> 'C'
connections (optionally, where 'P' is right-shifted by 17-bits and used as an
input to the post-adder -- a pattern common for summing partial products to
implement wide multipliers). Limited support also exists for similar cascading
for A and B using '[AB]COUT' -> '[AB]CIN'. Currently, cascade chains are limited
to a maximum length of 20 cells, corresponding to the smallest Xilinx 7 Series

This pass is a no-op if the scratchpad variable 'xilinx_dsp.multonly' is set
to 1.

Experimental feature: addition/subtractions less than 12 or 24 bits with the
'(* use_dsp="simd" *)' attribute attached to the output wire or attached to
the add/subtract operator will cause those operations to be implemented using
the 'SIMD' feature of DSPs.

Experimental feature: the presence of a `$ge' cell attached to the registered
P output implementing the operation "(P >= <power-of-2>)" will be transformed
into using the DSP48E1's pattern detector feature for overflow detection.
-family {xcup|xcu|xc7|xc6v|xc5v|xc4v|xc6s|xc3sda}
select the family to target
default: xc7