Assignment to a Record with zero-width fields generates invalid Verilog

[nmigen-issue-migration]

Issue by jfng
Friday Jan 24, 2020 at 16:39 GMT
Originally opened as m-labs/nmigen#312

---

Repro:

from nmigen import *
from nmigen.back import rtlil, verilog


class Top(Elaboratable):
    def elaborate(self, platform):
        m = Module()

        rec = Record([
            ("a", 1),
            ("b", 0),
        ])

        m.d.comb += rec.eq(1)

        return m


if __name__ == "__main__":
    top = Top()
    print(verilog.convert(top))
    # print(rtlil.convert(top))

Verilog output:

/* Generated by Yosys 0.9+1706 (git sha1 2bda51ac, clang 9.0.0 -fPIC -Os) */

(* generator = "nMigen" *)
(* top =  1  *)
(* \nmigen.hierarchy  = "top" *)
module top();
  (* src = "repro.py:9" *)
  wire rec__a;
  (* src = "repro.py:9" *)
  wire [-1:0] rec__b;
  assign "" = "";
  assign rec__a = 1'h1;
endmodule

RTLIL output:

attribute \top 1
attribute \nmigen.hierarchy "top"
module \top
  attribute \src "repro.py:9"
  wire width 1 \rec__a
  attribute \src "repro.py:9"
  wire width 0 \rec__b
  wire width 1 $verilog_initial_trigger
  process $group_0
    assign \rec__a 1'0
    assign \rec__b 0'0
    assign { \rec__b \rec__a } 1'1
    assign $verilog_initial_trigger $verilog_initial_trigger
    sync init
      update $verilog_initial_trigger 1'0
  end
  connect \rec__b 0'0
end

[nmigen-issue-migration]

Comment by whitequark
Friday Jan 24, 2020 at 16:49 GMT

---

Hm, arguably this is a bug in Yosys, but I think we can just not generate assignments to zero width lvalues.

[jfng]

A variant of this bug can still occur accross module boundaries.

Repro:

from nmigen import *
from nmigen.back import rtlil, verilog


class Top(Elaboratable):
    def elaborate(self, platform):
        m = Module()

        r1 = Record([("addr", 0), ("data", 8)])
        r2 = Record([("addr", 0), ("data", 8)])
        r3 = Record([("addr", 0), ("data", 8)])

        m.submodules.a = a = Module()
        m.submodules.b = b = Module()

        a.d.comb += r1.eq(0)
        m.d.comb += r2.eq(r1)
        b.d.comb += r3.eq(r2)

        return m


print(verilog.convert(Top()))
# print(rtlil.convert(Top()))

This time Yosys chokes with a parser error:

Traceback (most recent call last):
  File "repro.py", line 23, in <module>
    print(verilog.convert(Top()))
  File "/home/jf/src/nmigen/nmigen/back/verilog.py", line 78, in convert
    return _convert_rtlil_text(rtlil_text, strip_internal_attrs=strip_internal_attrs)
  File "/home/jf/src/nmigen/nmigen/back/verilog.py", line 66, in _convert_rtlil_text
    raise YosysError(error.strip())
nmigen.back.verilog.YosysError: ERROR: Parser error in line 37: syntax error

RTLIL output:

attribute \generator "nMigen"
attribute \nmigen.hierarchy "top.a"
module \a
  attribute \src "repro.py:9"
  wire width 8 output 1 \r1__data
  wire width 1 $verilog_initial_trigger
  process $group_0
    assign { } 0'0
    assign \r1__data 8'00000000
    assign { \r1__data { } } 8'00000000
    assign $verilog_initial_trigger $verilog_initial_trigger
    sync init
      update $verilog_initial_trigger 1'0
  end
end
attribute \generator "nMigen"
attribute \nmigen.hierarchy "top.b"
module \b
  attribute \src "repro.py:10"
  wire width 8 input 1 \r2__data
  attribute \src "repro.py:11"
  wire width 8 \r3__data
  process $group_0
    assign { } 0'0
    assign \r3__data 8'00000000
    assign { \r3__data { } } { \r2__data { } }
    sync init
  end
end
attribute \generator "nMigen"
attribute \top 1
attribute \nmigen.hierarchy "top"
module \top
  attribute \src "repro.py:9"
  wire width 8 \a_r1__data
  cell \a \a
    connect { } { }
    connect \r1__data \a_r1__data
  end
  attribute \src "repro.py:10"
  wire width 8 \b_r2__data
  cell \b \b
    connect { } { }
    connect \r2__data \b_r2__data
  end
  process $group_0
    assign { } 0'0
    assign \b_r2__data 8'00000000
    assign { \b_r2__data { } } { \a_r1__data { } }
    sync init
  end
end

[whitequark]

@jfng That's a slightly different bug with a different cause--could you open another issue to track that one?