cxxrtl: always lazily format print messages.

This is mostly useful for collecting coverage for the future `$check`
cell, where, depending on the flavor, formatting a message may not be
wanted even for a failed assertion.

backends/cxxrtl/cxxrtl_backend.cc

@@ -1072,21 +1072,43 @@ struct CxxrtlWorker {
 		dump_sigspec_rhs(cell->getPort(ID::EN));
 		f << " == value<1>{1u}) {\n";
 		inc_indent();
-			f << indent << "auto formatter = [&](struct performer *performer) {\n";
+			dict<std::string, RTLIL::SigSpec> fmt_args;
+			f << indent << "struct : public lazy_fmt {\n";
 			inc_indent();
-				fmt.emit_cxxrtl(f, indent, [this](const RTLIL::SigSpec &sig) { dump_sigspec_rhs(sig); }, "performer");
+				f << indent << "std::string operator() () const override {\n";
+				inc_indent();
+					fmt.emit_cxxrtl(f, indent, [&](const RTLIL::SigSpec &sig) {
+						if (sig.size() == 0)
+							f << "value<0>()";
+						else {
+							std::string arg_name = "arg" + std::to_string(fmt_args.size());
+							fmt_args[arg_name] = sig;
+							f << arg_name;
+						}
+					}, "performer");
+				dec_indent();
+				f << indent << "}\n";
+				f << indent << "struct performer *performer;\n";
+				for (auto arg : fmt_args)
+					f << indent << "value<" << arg.second.size() << "> " << arg.first << ";\n";
 			dec_indent();
-			f << indent << "};\n";
+			f << indent << "} formatter;\n";
+			f << indent << "formatter.performer = performer;\n";
+			for (auto arg : fmt_args) {
+				f << indent << "formatter." << arg.first << " = ";
+				dump_sigspec_rhs(arg.second);
+				f << ";\n";
+			}
 			f << indent << "if (performer) {\n";
 			inc_indent();
 				f << indent << "static const metadata_map attributes = ";
 				dump_metadata_map(cell->attributes);
 				f << ";\n";
-				f << indent << "performer->on_print(formatter(performer), attributes);\n";
+				f << indent << "performer->on_print(formatter, attributes);\n";
 			dec_indent();
 			f << indent << "} else {\n";
 			inc_indent();
-				f << indent << print_output << " << formatter(performer);\n";
+				f << indent << print_output << " << formatter();\n";
 			dec_indent();
 			f << indent << "}\n";
 		dec_indent();

backends/cxxrtl/runtime/cxxrtl/cxxrtl.h

@@ -945,16 +945,25 @@ struct metadata {
 
 typedef std::map<std::string, metadata> metadata_map;
 
+struct performer;
+
+// An object that allows formatting a string lazily.
+struct lazy_fmt {
+	virtual std::string operator() () const = 0;
+};
+
 // An object that can be passed to a `eval()` method in order to act on side effects.
 struct performer {
-	// Called to evaluate a Verilog `$time` expression.
+	// Called by generated formatting code to evaluate a Verilog `$time` expression.
 	virtual int64_t vlog_time() const { return 0; }
 
-	// Called to evaluate a Verilog `$realtime` expression.
+	// Called by generated formatting code to evaluate a Verilog `$realtime` expression.
 	virtual double vlog_realtime() const { return vlog_time(); }
 
 	// Called when a `$print` cell is triggered.
-	virtual void on_print(const std::string &output, const metadata_map &attributes) { std::cout << output; }
+	virtual void on_print(const lazy_fmt &formatter, const metadata_map &attributes) {
+		std::cout << formatter();
+	}
 };
 
 // An object that can be passed to a `commit()` method in order to produce a replay log of every state change in

tests/fmt/always_full_tb.cc

@@ -4,7 +4,7 @@ int main()
 {
 	struct : public performer {
 		int64_t vlog_time() const override { return 1; }
-		void on_print(const std::string &output, const cxxrtl::metadata_map &) override { std::cerr << output; }
+		void on_print(const lazy_fmt &output, const cxxrtl::metadata_map &) override { std::cerr << output(); }
 	} performer;
 
 	cxxrtl_design::p_always__full uut;