frontend: json: parse negative values

[kgugala]

When scc breaks logic loops with $__ABC9_SCC_BREAKER module it adds the following entry to resulting json:

  "attributes": {
    "blackbox": "00000000000000000000000000000001",
    "cells_not_processed": "00000000000000000000000000000001",
    "dynports": "00000000000000000000000000000001",
    "src": "/(...)/share/yosys/abc9_model.v:9.1-11.10"
  },  
  "cells": {}, 
  "netnames": {
    "I": {
      "attributes": {
        "src": "/(...)/share/yosys/abc9_model.v:9.47-9.48"
      },  
      "bits": [
        2,  
        3   
      ],  
      "hide_name": 0,
      "offset": -1, 
      "upto": 1
    }, 

reading the json back to yosys results in:

ERROR: Unexpected character in JSON file: '-'

in the "offset": -1, line

This PR fixes that

[whitequark]

So what happens if the input is something like 123-456?

[kgugala]

So what happens if the input is something like 123-456?

It will break here https://github.com/YosysHQ/yosys/blob/master/frontends/json/jsonparse.cc#L91

[whitequark]

Actually, is this even the right approach? Is there a (* force_downto *) missing somewhere, like in #2058? cc @mwkmwkmwk

[Ravenslofty]

I think we should ping @eddiehung in here too, since this is from the eddie/abc9_scc_fixes2 branch, and it would be a good idea to roll it up into his next round of ABC9 fixes.

[mwkmwkmwk]

Actually, is this even the right approach? Is there a (* force_downto *) missing somewhere, like in #2058? cc @mwkmwkmwk

Right or not, negative offsets are valid in Verilog either way, so this patch should be merged regardless.

[mwkmwkmwk]

As in, currently converting the following to JSON and trying to read it back just fails:

module top(input [3:-3] I);
endmodule

[mwkmwkmwk]

whitespace error.

[eddiehung]

I'm not sure the $__ABC9_SCC_BREAKER module nor abc9 is relevant here. For one, I think other modules can trigger negative indices, plus, that cell should never leak into the outside world as it's an internal cell used between sub-passes.