cxxrtl: use one delta cycle for immediately converging netlists.

If it is statically known that eval() will converge in one delta
cycle (that is, the second commit() will always return `false`)
because the design contains no feedback or buffered wires, then
there is no need to run the second delta cycle at all.

After this commit, the case where eval() always converges immediately
is detected and the second delta cycle is omitted. As a result,
Minerva SRAM SoC runs ~25% faster.

backends/cxxrtl/cxxrtl.cc

@@ -441,6 +441,7 @@ struct CxxrtlWorker {
 	dict<const RTLIL::Module*, std::vector<FlowGraph::Node>> schedule;
 	pool<const RTLIL::Wire*> localized_wires;
 	dict<const RTLIL::Module*, pool<std::string>> blackbox_specializations;
+	dict<const RTLIL::Module*, bool> eval_converges;
 
 	void inc_indent() {
 		indent += "\t";
@@ -1108,7 +1109,7 @@ struct CxxrtlWorker {
 					dump_sigspec_rhs(conn.second);
 					f << ";\n";
 				}
-			f << indent << mangle(cell) << access << "eval();\n";
+			f << indent << "converged &= " << mangle(cell) << access << "eval();\n";
 			for (auto conn : cell->connections()) {
 				if (conn.second.is_wire()) {
 					RTLIL::Wire *wire = conn.second.as_wire();
@@ -1382,6 +1383,7 @@ struct CxxrtlWorker {
 	void dump_eval_method(RTLIL::Module *module)
 	{
 		inc_indent();
+			f << indent << "bool converged = " << (eval_converges.at(module) ? "true" : "false") << ";\n";
 			if (!module->get_bool_attribute(ID(cxxrtl.blackbox))) {
 				for (auto wire : module->wires())
 					dump_wire(wire, /*is_local_context=*/true);
@@ -1399,6 +1401,7 @@ struct CxxrtlWorker {
 					}
 				}
 			}
+			f << indent << "return converged;\n";
 		dec_indent();
 	}
 
@@ -1474,7 +1477,7 @@ struct CxxrtlWorker {
 						dump_wire(wire, /*is_local_context=*/false);
 				}
 				f << "\n";
-				f << indent << "void eval() override {\n";
+				f << indent << "bool eval() override {\n";
 				dump_eval_method(module);
 				f << indent << "}\n";
 				f << "\n";
@@ -1542,7 +1545,7 @@ struct CxxrtlWorker {
 				}
 				if (has_cells)
 					f << "\n";
-				f << indent << "void eval() override;\n";
+				f << indent << "bool eval() override;\n";
 				f << indent << "bool commit() override;\n";
 			dec_indent();
 			f << indent << "}; // struct " << mangle(module) << "\n";
@@ -1554,7 +1557,7 @@ struct CxxrtlWorker {
 	{
 		if (module->get_bool_attribute(ID(cxxrtl.blackbox)))
 			return;
-		f << indent << "void " << mangle(module) << "::eval() {\n";
+		f << indent << "bool " << mangle(module) << "::eval() {\n";
 		dump_eval_method(module);
 		f << indent << "}\n";
 		f << "\n";
@@ -1687,6 +1690,10 @@ struct CxxrtlWorker {
 						}
 					}
 				}
+
+				// Black boxes converge by default, since their implementations are quite unlikely to require
+				// internal propagation of comb signals.
+				eval_converges[module] = true;
 				continue;
 			}
 
@@ -1872,7 +1879,7 @@ struct CxxrtlWorker {
 			// it is possible that a design with no feedback arcs would end up with doubly buffered wires in such cases
 			// as a wire with multiple drivers where one of them is combinatorial and the other is synchronous. Such designs
 			// also require more than one delta cycle to converge.
-			pool<RTLIL::Wire*> buffered_wires;
+			pool<const RTLIL::Wire*> buffered_wires;
 			for (auto wire : module->wires()) {
 				if (flow.wire_comb_defs[wire].size() > 0 && !elided_wires.count(wire) && !localized_wires[wire]) {
 					if (!feedback_wires[wire])
@@ -1885,6 +1892,8 @@ struct CxxrtlWorker {
 				for (auto wire : buffered_wires)
 					log("  %s\n", wire->name.c_str());
 			}
+
+			eval_converges[module] = feedback_wires.empty() && buffered_wires.empty();
 		}
 		if (has_feedback_arcs || has_buffered_wires) {
 			// Although both non-feedback buffered combinatorial wires and apparent feedback wires may be eliminated
@@ -2015,7 +2024,7 @@ struct CxxrtlBackend : public Backend {
 		log("      value<8> p_i_data;\n");
 		log("      wire<8> p_o_data;\n");
 		log("\n");
-		log("      void eval() override;\n");
+		log("      bool eval() override;\n");
 		log("      bool commit() override;\n");
 		log("\n");
 		log("      static std::unique_ptr<bb_p_debug>\n");
@@ -2028,11 +2037,11 @@ struct CxxrtlBackend : public Backend {
 		log("    namespace cxxrtl_design {\n");
 		log("\n");
 		log("    struct stderr_debug : public bb_p_debug {\n");
-		log("      void eval() override {\n");
+		log("      bool eval() override {\n");
 		log("        if (posedge_p_clk() && p_en)\n");
 		log("          fprintf(stderr, \"debug: %%02x\\n\", p_i_data.data[0]);\n");
 		log("        p_o_data.next = p_i_data;\n");
-		log("        bb_p_debug::eval();\n");
+		log("        return bb_p_debug::eval();\n");
 		log("      }\n");
 		log("    };\n");
 		log("\n");

backends/cxxrtl/cxxrtl.h

@@ -717,15 +717,16 @@ struct module {
 	module(const module &) = delete;
 	module &operator=(const module &) = delete;
 
-	virtual void eval() = 0;
+	virtual bool eval() = 0;
 	virtual bool commit() = 0;
 
 	size_t step() {
 		size_t deltas = 0;
+		bool converged = false;
 		do {
-			eval();
+			converged = eval();
 			deltas++;
-		} while (commit());
+		} while (commit() && !converged);
 		return deltas;
 	}
 };