`resource-sharing`: Sharing generates invalid verilog

[rachitnigam]

The unrolled MVT benchmarks generates invalid verilog:

%Warning-UNOPTFLAT: /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:1076:18: Signal unoptimizable: Feedback to clock or circular logic: 'main.add9_right'
 1076 |     logic [31:0] add9_right;
      |                  ^~~~~~~~~~
                    ... Use "/* verilator lint_off UNOPTFLAT */" and lint_on around source to disable this message.
                    /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:1076:18:      Example path: main.add9_right
                    /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:750:14:      Example path: ASSIGNW
                    /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:1077:18:      Example path: main.add9_out
                    /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:4700:24:      Example path: ASSIGNW
                    /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:1067:18:      Example path: main.add10_right
                    /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:750:14:      Example path: ASSIGNW
                    /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:1068:18:      Example path: main.add10_out
                    /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:4706:24:      Example path: ASSIGNW
                    /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:1070:18:      Example path: main.add11_right
                    /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:750:14:      Example path: ASSIGNW
                    /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:1071:18:      Example path: main.add11_out
                    /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:4712:24:      Example path: ASSIGNW
                    /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:1073:18:      Example path: main.add12_right
                    /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:750:14:      Example path: ASSIGNW
                    /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:1074:18:      Example path: main.add12_out
                    /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:4718:23:      Example path: ASSIGNW
                    /var/folders/sk/f158_qn147vc0jhjsfdcswq40000gn/T/tmpweji9c61:1076:18:      Example path: main.add9_right
%Error: Exiting due to 1 warning(s)

[rachitnigam]

Looks like the resource-sharing pass causes this problem. Running the benchmark with -d resource-sharing generates valid verilog and generates the right values.

[rachitnigam]

Minimal test case:

import "primitives/core.futil";

component main(@go go: 1, @clk clk: 1, @reset reset: 1) -> (@done done: 1) {
  cells {
    add10 = std_add(32);
    add7 = std_add(32);
    add8 = std_add(32);
    add9 = std_add(32);

    r1 = std_reg(32);
    r2 = std_reg(32);
  }
  wires {
    group upd33 {
      add7.left = 32'd0;
      add7.right = 32'd0;
      add8.left = 32'd0;
      add8.right = add7.out;
      r1.write_en = 1'd1;
      r1.in = add8.out;
      upd33[done] = r1.done ? 1'd1;
    }
    group upd34 {
      add9.left = 32'd0;
      add9.right = 32'd0;
      add10.left = 32'd0;
      add10.right = add9.out;
      r2.write_en = 1'd1;
      r2.in = add10.out;
      upd34[done] = r2.done ? 1'd1;
    }
  }

  control {
    seq {
      upd33;
      upd34;
    }
  }
}

[rachitnigam]

https://github.com/verilator/verilator/issues/3146 could be the potential problem

[rachitnigam]

Argh, resource sharing might be a fundamentally broken pass. It might only make sense to share sequential components.

[rachitnigam]

@sampsyo @EclecticGriffin and I discussed this problem and we agree that the generated code is "valid" in that the combinational loop is stable. However, we need to do some digging before we can decide what the right steps are here.

Two things to do:

1. See what vivado does when you give it this program
2. See if the verilator can simulate this problem when the UNOPTFLAT warning is disabled.