Is there any way to get default parameter value for verilog module with Yosys

Question

I am trying to get default parameter values for verilog modules using Yosys command shell. Is there any way to do it? In addition, is parsing a `write_ilang' command output file a good idea or its format can change dramatically in the near future?

Attempts were made to find default values in ilang, json, table dumps and even using chparam -list command, but they did not bring any result.

Consider this example (file param_test.v):

module stub();
  parameter PUBLIC_PARAM = 1;
  parameter HIDDEN_PARAM = 2;
endmodule

module testbench();
  stub no_param_stub ();
  stub #(.PUBLIC_PARAM(1)) one_param_stub ();
endmodule

I follow these steps:

1. load source code read -sv param_test.v
2. elaborate design hierarchy -top testbench
3. try to get parameter values.

Output from write_ilang command:

# Generated by Yosys 0.8+634 (git sha1 ac2fc3a, clang 3.8.0-2ubuntu4 -fPIC -Os)
autoidx 1
attribute \blackbox 1
attribute \src "param_test.v:1"
module $paramod\stub\PUBLIC_PARAM=1
  parameter \HIDDEN_PARAM
  parameter \PUBLIC_PARAM
end
attribute \blackbox 1
attribute \src "param_test.v:1"
module \stub
  parameter \HIDDEN_PARAM
  parameter \PUBLIC_PARAM
end
attribute \top 1
attribute \src "param_test.v:6"
module \testbench
  attribute \module_not_derived 1
  attribute \src "param_test.v:8"
  cell \stub \no_param_stub
  end
  attribute \module_not_derived 1
  attribute \src "param_test.v:10"
  cell $paramod\stub\PUBLIC_PARAM=1 \one_param_stub
  end
end

Output from write_json command does not even contain information about the HIDDEN_PARAM parameter:

{
  "creator": "Yosys 0.8+634 (git sha1 ac2fc3a, clang 3.8.0-2ubuntu4 -fPIC -Os)",
  "modules": {
    "$paramod\\stub\\PUBLIC_PARAM=1": {
      "attributes": {
        "blackbox": 1,
        "src": "param_test.v:1"
      },
      "ports": {
      },
      "cells": {
      },
      "netnames": {
      }
    },
    "stub": {
      "attributes": {
        "blackbox": 1,
        "src": "param_test.v:1"
      },
      "ports": {
      },
      "cells": {
      },
      "netnames": {
      }
    },
    "testbench": {
      "attributes": {
        "top": 1,
        "src": "param_test.v:6"
      },
      "ports": {
      },
      "cells": {
        "no_param_stub": {
          "hide_name": 0,
          "type": "stub",
          "parameters": {
          },
          "attributes": {
            "module_not_derived": 1,
            "src": "param_test.v:8"
          },
          "port_directions": {
          },
          "connections": {
          }
        },
        "one_param_stub": {
          "hide_name": 0,
          "type": "$paramod\\stub\\PUBLIC_PARAM=1",
          "parameters": {
          },
          "attributes": {
            "module_not_derived": 1,
            "src": "param_test.v:10"
          },
          "port_directions": {
          },
          "connections": {
          }
        }
      },
      "netnames": {
      }
    }
  }
}

Answer 1

Yosys recently gained the functionality to preserve parameters through elaboration (whereas they are usually thrown away). Add -pwires to read_verilog and parameters will be converted into wires driven by the parameter's default value with the parameter attribute set to 1.

In terms of stability, there are several other tools (such as nMigen) that create or read RTLIL ilang, so it is unlikely to have significant breaking changes now.