cxxrtl: simplify generated edge detection logic.

This commit changes the way edge detectors are represented in
generated code from a variable that is set in commit() and reset in
eval() to a function that considers .curr and .next of the clock
wire. Behavior remains the same. Besides being simpler to generate
and providing more opportunities for optimization, this commit paves
way for unbuffering module inputs.

1 files changed, 29 insertions, 56 deletions

diff --git a/backends/cxxrtl/cxxrtl.cc b/backends/cxxrtl/cxxrtl.cc
index 8f2bf6836..89b059504 100644
--- a/backends/cxxrtl/cxxrtl.cc
+++ b/backends/cxxrtl/cxxrtl.cc
@@ -915,7 +915,7 @@ struct CxxrtlWorker {
 				RTLIL::SigBit clk_bit = cell->getPort(ID::CLK)[0];
 				clk_bit = sigmaps[clk_bit.wire->module](clk_bit);
 				f << indent << "if (" << (cell->getParam(ID::CLK_POLARITY).as_bool() ? "posedge_" : "negedge_")
-				            << mangle(clk_bit) << ") {\n";
+				            << mangle(clk_bit) << "()) {\n";
 				inc_indent();
 					if (cell->type == ID($dffe)) {
 						f << indent << "if (";
@@ -992,7 +992,7 @@ struct CxxrtlWorker {
 				RTLIL::SigBit clk_bit = cell->getPort(ID::CLK)[0];
 				clk_bit = sigmaps[clk_bit.wire->module](clk_bit);
 				f << indent << "if (" << (cell->getParam(ID::CLK_POLARITY).as_bool() ? "posedge_" : "negedge_")
-				            << mangle(clk_bit) << ") {\n";
+				            << mangle(clk_bit) << "()) {\n";
 				inc_indent();
 			}
 			RTLIL::Memory *memory = cell->module->memories[cell->getParam(ID::MEMID).decode_string()];
@@ -1217,16 +1217,16 @@ struct CxxrtlWorker {
 			switch (sync->type) {
 				case RTLIL::STp:
 					log_assert(sync_bit.wire != nullptr);
-					events.insert("posedge_" + mangle(sync_bit));
+					events.insert("posedge_" + mangle(sync_bit) + "()");
 					break;
 				case RTLIL::STn:
 					log_assert(sync_bit.wire != nullptr);
-					events.insert("negedge_" + mangle(sync_bit));
+					events.insert("negedge_" + mangle(sync_bit) + "()");
 					break;
 				case RTLIL::STe:
 					log_assert(sync_bit.wire != nullptr);
-					events.insert("posedge_" + mangle(sync_bit));
-					events.insert("negedge_" + mangle(sync_bit));
+					events.insert("posedge_" + mangle(sync_bit) + "()");
+					events.insert("negedge_" + mangle(sync_bit) + "()");
 					break;
 
 				case RTLIL::STa:
@@ -1290,10 +1290,23 @@ struct CxxrtlWorker {
 			if (sync_wires[wire]) {
 				for (auto sync_type : sync_types) {
 					if (sync_type.first.wire == wire) {
-						if (sync_type.second != RTLIL::STn)
-							f << indent << "bool posedge_" << mangle(sync_type.first) << " = false;\n";
-						if (sync_type.second != RTLIL::STp)
-							f << indent << "bool negedge_" << mangle(sync_type.first) << " = false;\n";
+						if (sync_type.second != RTLIL::STn) {
+							f << indent << "bool posedge_" << mangle(sync_type.first) << "() const {\n";
+							inc_indent();
+								f << indent << "return ";
+								f << "!" << mangle(sync_type.first.wire) << ".curr.slice<" << sync_type.first.offset << ">().val() && ";
+								f <<        mangle(sync_type.first.wire) << ".next.slice<" << sync_type.first.offset << ">().val();\n";
+							dec_indent();
+							f << indent << "}\n";
+						} else {
+							f << indent << "bool negedge_" << mangle(sync_type.first) << "() const {\n";
+							inc_indent();
+								f << indent << "return ";
+								f <<        mangle(sync_type.first.wire) << ".curr.slice<" << sync_type.first.offset << ">().val() && ";
+								f << "!" << mangle(sync_type.first.wire) << ".next.slice<" << sync_type.first.offset << ">().val();\n";
+							dec_indent();
+							f << indent << "}\n";
+						}
 					}
 				}
 			}
@@ -1365,14 +1378,6 @@ struct CxxrtlWorker {
 					}
 				}
 			}
-			for (auto sync_type : sync_types) {
-				if (sync_type.first.wire->module == module) {
-					if (sync_type.second != RTLIL::STn)
-						f << indent << "posedge_" << mangle(sync_type.first) << " = false;\n";
-					if (sync_type.second != RTLIL::STp)
-						f << indent << "negedge_" << mangle(sync_type.first) << " = false;\n";
-				}
-			}
 		dec_indent();
 	}
 
@@ -1383,39 +1388,8 @@ struct CxxrtlWorker {
 			for (auto wire : module->wires()) {
 				if (elided_wires.count(wire) || localized_wires.count(wire))
 					continue;
-				if (sync_wires[wire]) {
-					std::string wire_prev = mangle(wire) + "_prev";
-					std::string wire_curr = mangle(wire) + ".curr";
-					std::string wire_edge = mangle(wire) + "_edge";
-					f << indent << "value<" << wire->width << "> " << wire_prev << " = " << wire_curr << ";\n";
-					f << indent << "if (" << mangle(wire) << ".commit()) {\n";
-					inc_indent();
-						f << indent << "value<" << wire->width << "> " << wire_edge << " = "
-						            << wire_prev << ".bit_xor(" << wire_curr << ");\n";
-						for (auto sync_type : sync_types) {
-							if (sync_type.first.wire != wire)
-								continue;
-							if (sync_type.second != RTLIL::STn) {
-								f << indent << "if (" << wire_edge << ".slice<" << sync_type.first.offset << ">().val() && "
-								            << wire_curr << ".slice<" << sync_type.first.offset << ">().val())\n";
-								inc_indent();
-									f << indent << "posedge_" << mangle(sync_type.first) << " = true;\n";
-								dec_indent();
-							}
-							if (sync_type.second != RTLIL::STp) {
-								f << indent << "if (" << wire_edge << ".slice<" << sync_type.first.offset << ">().val() && "
-								            << "!" << wire_curr << ".slice<" << sync_type.first.offset << ">().val())\n";
-								inc_indent();
-									f << indent << "negedge_" << mangle(sync_type.first) << " = true;\n";
-								dec_indent();
-							}
-							f << indent << "changed = true;\n";
-						}
-					dec_indent();
-					f << indent << "}\n";
-				} else if (!module->get_bool_attribute(ID(cxxrtl.blackbox)) || wire->port_id != 0) {
+				if (!module->get_bool_attribute(ID(cxxrtl.blackbox)) || wire->port_id != 0)
 					f << indent << "changed |= " << mangle(wire) << ".commit();\n";
-				}
 			}
 			if (!module->get_bool_attribute(ID(cxxrtl.blackbox))) {
 				for (auto memory : module->memories) {
@@ -2005,7 +1979,7 @@ struct CxxrtlBackend : public Backend {
 		log("\n");
 		log("    struct bb_p_debug : public module {\n");
 		log("      wire<1> p_clk;\n");
-		log("      bool posedge_p_clk = false;\n");
+		log("      bool posedge_p_clk() const { /* ... */ }\n");
 		log("      wire<1> p_en;\n");
 		log("      wire<8> p_data;\n");
 		log("\n");
@@ -2023,7 +1997,7 @@ struct CxxrtlBackend : public Backend {
 		log("\n");
 		log("    struct stderr_debug : public bb_p_debug {\n");
 		log("      void eval() override {\n");
-		log("        if (posedge_p_clk && p_en.curr)\n");
+		log("        if (posedge_p_clk() && p_en.curr)\n");
 		log("          fprintf(stderr, \"debug: %%02x\\n\", p_data.curr.data[0]);\n");
 		log("        bb_p_debug::eval();\n");
 		log("      }\n");
@@ -2086,10 +2060,9 @@ struct CxxrtlBackend : public Backend {
 		log("\n");
 		log("    cxxrtl.edge\n");
 		log("        only valid on inputs of black boxes. must be one of \"p\", \"n\", \"a\".\n");
-		log("        if specified on signal `clk`, the generated code includes boolean fields\n");
-		log("        `posedge_p_clk` (if \"p\"), `negedge_p_clk` (if \"n\"), or both (if \"a\"),\n");
-		log("        as well as edge detection logic, simplifying implementation of clocked\n");
-		log("        black boxes.\n");
+		log("        if specified on signal `clk`, the generated code includes edge detectors\n");
+		log("        `posedge_p_clk()` (if \"p\"), `negedge_p_clk()` (if \"n\"), or both (if\n");
+		log("        \"a\"), simplifying implementation of clocked black boxes.\n");
 		log("\n");
 		log("    cxxrtl.template\n");
 		log("        only valid on black boxes. must contain a space separated sequence of\n");