write_verilog: dump zero width constants correctly.

Before this commit, zero width constants were dumped as "" (empty
string). Unfortunately, 1364-2005 5.2.3.3 indicates that an empty
string is equivalent to "\0", and is 8 bits wide, so that's wrong.

After this commit, a replication operation with a count of zero is
used instead, which is explicitly permitted per 1364-2005 5.1.14,
and is defined to have size zero. (Its operand has to have a non-zero
size for it to be legal, though.)

Fixes #948 (again).

diff --git a/backends/verilog/verilog_backend.cc b/backends/verilog/verilog_backend.cc
index a020d82b633b76bf8dca01601963e54ee5a49565..48404b34c14584eb84dc627a10bc6d82c86205d3 100644 (file)
--- a/backends/verilog/verilog_backend.cc
+++ b/backends/verilog/verilog_backend.cc
@@ -189,7 +189,8 @@ void dump_const(std::ostream &f, const RTLIL::Const &data, int width = -1, int o
        if (width < 0)
                width = data.bits.size() - offset;
        if (width == 0) {
-               f << "\"\"";
+               // See IEEE 1364-2005 Clause 5.1.14.
+               f << "{0{1'b0}}";
                return;
        }
        if (nostr)