cxxrtl: escape colon in variable names in VCD writer.

The following VCD file crashes GTKWave's VCD loader:

    $var wire 1 ! x:1 $end
    $enddefinitions $end

In practice, a colon can be a part of a variable name that is
translated from a Verilog function, something like:

    update$func$.../hdl/hazard3_csr.v:350$2534.$result

1 files changed, 14 insertions, 1 deletions

diff --git a/backends/cxxrtl/cxxrtl_vcd.h b/backends/cxxrtl/cxxrtl_vcd.h
index 3f40a8d12..b76922bbd 100644
--- a/backends/cxxrtl/cxxrtl_vcd.h
+++ b/backends/cxxrtl/cxxrtl_vcd.h
@@ -69,12 +69,25 @@ class vcd_writer {
 		} while (ident != 0);
 	}
 
+	void emit_name(const std::string &name) {
+		for (char c : name) {
+			if (c == ':') {
+				// Due to a bug, GTKWave cannot parse a colon in the variable name, causing the VCD file
+				// to be unreadable. It cannot be escaped either, so replace it with the sideways colon.
+				buffer += "..";
+			} else {
+				buffer += c;
+			}
+		}
+	}
+
 	void emit_var(const variable &var, const std::string &type, const std::string &name,
 	              size_t lsb_at, bool multipart) {
 		assert(!streaming);
 		buffer += "$var " + type + " " + std::to_string(var.width) + " ";
 		emit_ident(var.ident);
-		buffer += " " + name;
+		buffer += " ";
+		emit_name(name);
 		if (multipart || name.back() == ']' || lsb_at != 0) {
 			if (var.width == 1)
 				buffer += " [" + std::to_string(lsb_at) + "]";