Mapping to flip-flops with an enable in dfflibmap

[povik]

Feature Description

dfflibmap is responsible for mapping to technology flip-flops as described in a Liberty file. This ticket is to add support for mapping to flip-flop types with an enable, which there's no support for as of yet.

[yliu-hashed]

I had a similar need. What I did is first use difflegalize to reduce all ffs into to a compatable subset of dffs, and use techmap to map them manually.

# script.ys
dfflegalize -cell $_DFF_P_ 0 -cell $_DFFE_PP_ 0
techmap -map techmap_dff.v

// techmap_dff.v
module \$_DFF_P_ (input D, C, output Q);
MY_DFF _TECHMAP_REPLACE_ (.C(C), .D(D), .Q(Q));
endmodule

module \$_DFFE_PP_ (input D, C, E, output Q);
MY_DFFE _TECHMAP_REPLACE_ (.C(C), .D(D), .E(E), .Q(Q));
endmodule

This requires a separate techmap_dff.v with all your flops. I guess you can create a techmap verilog file for your library, or find some way to extract and generate the techmap file.

I also took a look at the code of dfflibmap a while back. It looked quite hard-coded. Matching is done directly on Liberty AST. Matching flops with enable is too difficult for me to code up, but I think it can be done, since there's clearly a working Liberty frontend driven by read_liberty.

[povik]

Yes, I have written a similar file for sky130hd to estimate the impact of inferring/not inferring the flip-flops. Let me share it on the off chance it's useful to someone.

(* techmap_celltype="$_DFFE_PP_" *)
module dffe_rule(D, C, E, Q);
    input D, C, E;
    output Q;
    sky130_fd_sc_hd__edfxtp_1 _TECHMAP_REPLACE_
        (.CLK(C), .DE(E), .D(D), .Q(Q));
endmodule

It might need a bit of code to write generic detection but the information in the Liberty file is there:

        ff ("IQ","IQ_N") {
            clocked_on : "CLK";
            next_state : "(D&DE) | (IQ&!DE)";
        }
        pin ("Q") {
            function : "IQ";
        }

[povik]

Duplicate of #4652