[Moore] Drop named_constant op in favor of dbg.variable

[fabianschuiki]

Remove the NamedConstantOp and replace its uses with VariableOp from the debug dialect.

The op was originally added to track the value of constant parameters, localparams, and specparams in the IR. In ImportVerilog, such parameters would generate a corresponding named_constant op and all references to the parameter by name would be replaced with the named_constant's result.

This doesn't really work well for parameters defined outside a module, such as in packages or at the root of the Verilog source file. (Modules are isolated from above, preventing the use of named_constants from outside the module.) Therefore expressions would generally fall back to materializing constants directly where they were used.

Since the named constant ops are only there to track a constant value in the IR for the user's debugging convenience, using the debug dialect directly feels a lot more appropriate.

[hailongSun2000]

LGTM 🎉! It's so perfect if we can implement a case like:

parameter P = 32;
module Foo;
  localparam  LP = P;
endmodule

[fabianschuiki]

@hailongSun2000 circt-verilog --parse-only -g produces this:

module {
  %0 = moore.constant 32 : l32
  dbg.variable "P", %0 : !moore.l32
  moore.module @Foo() {
    %1 = moore.constant 32 : l32
    dbg.variable "LP", %1 : !moore.l32
    moore.output
  }
}

😃

[hailongSun2000]

So nice!

[uenoku]

Awesome that Debug works well here!