ice40_dsp - iCE40: map multipliers

yosys> help ice40_dsp
ice40_dsp [options] [selection]
Map multipliers ($mul/SB_MAC16) and multiply-accumulate ($mul/SB_MAC16 + $add)
cells into iCE40 DSP resources.
Currently, only the 16x16 multiply mode is supported and not the 2 x 8x8 mode.

Pack input registers (A, B, {C,D}; with optional hold), pipeline registers
({F,J,K,G}, H), output registers (O -- full 32-bits or lower 16-bits only; with
optional hold), and post-adder into the SB_MAC16 resource.

Multiply-accumulate operations using the post-adder with feedback on the {C,D}
input will be folded into the DSP. In this scenario only, resetting the
the accumulator to an arbitrary value can be inferred to use the {C,D} input.