`dump_wire` had no code path that emits the `signed` keyword for
wires/ports whose RTLIL `is_signed` flag is set. Reading
module top(input signed [9:0] in, output signed [31:0] o);
assign o = in;
endmodule
and writing it back via `write_verilog` produced
input [9:0] in;
output [31:0] o;
losing the declared signedness even though `wire->is_signed` was
tracked correctly in RTLIL throughout the round trip. The IEEE
1364-2001 grammar (Annex A.2.1.2 / A.2.1.3) allows `signed` after the
direction / net-type keyword, which is the dialect `write_verilog`
targets by default — so the fix is to emit ` signed` between the
direction/net-type and the range when `wire->is_signed`.
Closes the half of chipsalliance/synlig#2425 that lives in Yosys: the
SystemVerilog frontend correctly produces a signed wire for `output int`,
but the Verilog backend dropped it on write.
Adds `tests/various/write_verilog_signed_port.ys`, which round-trips a
module with `signed` inputs, outputs, and an internal wire and greps
for the keyword on each declaration — fails without the fix, passes
with it.
13ab5d4a67