circt-synth

circt-synth [options] [filename]

A logic synthesis tool converting designs expressed in the comb and HW dialects. There are no additional build requirements and can build directly ninja circt-synth. The tool currently only supports combinatorial logic but we are actively working to extend support to sequential designs (get involved).

By default the design is lowered to an and-inverter graph (AIG) represented using the synth dialect using optimisations that target general cell libraries. A --synthesis-strategy option can configure area vs delay optimisation. The tool uses a longest path analysis to guide design decisions and can output the critical path to a file via the --output-longest-path=<filename> option.

Example ¶

// sum.mlir
module {
  hw.module @sum(in %a : i4, in %b : i4, out sum : i4) {
    %0 = comb.add %a, %b : i4
    hw.output %0 : i4
  }
}

Writing circt-synth sum.mlir the addition is lowered to the synth dialect that expresses the design using only and-inverter operations and concat/extract operations:

module {
  hw.module private @sum(in %a : i4, in %b : i4, out sum : i4) {
    %0 = comb.extract %a from 0 : (i4) -> i1
    %1 = comb.extract %a from 1 : (i4) -> i1
    %2 = comb.extract %a from 2 : (i4) -> i1
    %3 = comb.extract %a from 3 : (i4) -> i1
    %4 = comb.extract %b from 0 : (i4) -> i1
    %5 = comb.extract %b from 1 : (i4) -> i1
    %6 = comb.extract %b from 2 : (i4) -> i1
    %7 = comb.extract %b from 3 : (i4) -> i1
    %8 = synth.aig.and_inv not %0, not %4 : i1
    %9 = synth.aig.and_inv %0, %4 : i1
    %10 = synth.aig.and_inv not %8, not %9 : i1
    %11 = synth.aig.and_inv not %1, not %5 : i1
    ...
    %31 = comb.concat %30, %23, %16, %10 : i1, i1, i1, i1
    hw.output %31 : i4
  }
}