Pass $display et al. in initial blocks on to sim

backends/cxxrtl/cxxrtl_backend.cc

@@ -1291,20 +1291,29 @@ struct CxxrtlWorker {
 			log_assert(!for_debug);
 
 			// Sync $print cells are grouped into PRINT_SYNC nodes in the FlowGraph.
-			log_assert(!cell->getParam(ID::TRG_ENABLE).as_bool());
+			log_assert(!cell->getParam(ID::TRG_ENABLE).as_bool() || (cell->getParam(ID::TRG_ENABLE).as_bool() && cell->getParam(ID::TRG_WIDTH).as_int() == 0));
 
-			f << indent << "auto " << mangle(cell) << "_curr = ";
-			dump_sigspec_rhs(cell->getPort(ID::EN));
-			f << ".concat(";
-			dump_sigspec_rhs(cell->getPort(ID::ARGS));
-			f << ").val();\n";
+			if (!cell->getParam(ID::TRG_ENABLE).as_bool()) { // async $print cell
+				f << indent << "auto " << mangle(cell) << "_curr = ";
+				dump_sigspec_rhs(cell->getPort(ID::EN));
+				f << ".concat(";
+				dump_sigspec_rhs(cell->getPort(ID::ARGS));
+				f << ").val();\n";
 
-			f << indent << "if (" << mangle(cell) << " != " << mangle(cell) << "_curr) {\n";
-			inc_indent();
-				dump_print(cell);
-				f << indent << mangle(cell) << " = " << mangle(cell) << "_curr;\n";
-			dec_indent();
-			f << indent << "}\n";
+				f << indent << "if (" << mangle(cell) << " != " << mangle(cell) << "_curr) {\n";
+				inc_indent();
+					dump_print(cell);
+					f << indent << mangle(cell) << " = " << mangle(cell) << "_curr;\n";
+				dec_indent();
+				f << indent << "}\n";
+			} else { // initial $print cell
+				f << indent << "if (!" << mangle(cell) << ") {\n";
+				inc_indent();
+					dump_print(cell);
+					f << indent << mangle(cell) << " = value<1>{1u};\n";
+				dec_indent();
+				f << indent << "}\n";
+			}
 		// Flip-flops
 		} else if (is_ff_cell(cell->type)) {
 			log_assert(!for_debug);
@@ -2002,6 +2011,11 @@ struct CxxrtlWorker {
 				}
 			}
 			for (auto cell : module->cells()) {
+				// Certain $print cells have additional state, which must be reset as well.
+				if (cell->type == ID($print) && !cell->getParam(ID::TRG_ENABLE).as_bool())
+					f << indent << mangle(cell) << " = value<" << (1 + cell->getParam(ID::ARGS_WIDTH).as_int()) << ">();\n";
+				if (cell->type == ID($print) && cell->getParam(ID::TRG_ENABLE).as_bool() && cell->getParam(ID::TRG_WIDTH).as_int() == 0)
+					f << indent << mangle(cell) << " = value<1>();\n";
 				if (is_internal_cell(cell->type))
 					continue;
 				f << indent << mangle(cell);
@@ -2430,11 +2444,11 @@ struct CxxrtlWorker {
 					f << "\n";
 				bool has_cells = false;
 				for (auto cell : module->cells()) {
-					if (cell->type == ID($print) && !cell->getParam(ID::TRG_ENABLE).as_bool()) {
-						// comb $print cell -- store the last EN/ARGS values to know when they change.
-						dump_attrs(cell);
+					// Certain $print cells have additional state, which requires storage.
+					if (cell->type == ID($print) && !cell->getParam(ID::TRG_ENABLE).as_bool())
 						f << indent << "value<" << (1 + cell->getParam(ID::ARGS_WIDTH).as_int()) << "> " << mangle(cell) << ";\n";
-					}
+					if (cell->type == ID($print) && cell->getParam(ID::TRG_ENABLE).as_bool() && cell->getParam(ID::TRG_WIDTH).as_int() == 0)
+						f << indent << "value<1> " << mangle(cell) << ";\n";
 					if (is_internal_cell(cell->type))
 						continue;
 					dump_attrs(cell);
@@ -2964,8 +2978,9 @@ struct CxxrtlWorker {
 			for (auto node : node_order)
 				if (live_nodes[node]) {
 					if (node->type == FlowGraph::Node::Type::CELL_EVAL &&
-					    node->cell->type == ID($print) &&
-					    node->cell->getParam(ID::TRG_ENABLE).as_bool())
+							node->cell->type == ID($print) &&
+							node->cell->getParam(ID::TRG_ENABLE).as_bool() &&
+							node->cell->getParam(ID::TRG_WIDTH).as_int() != 0)
 						sync_print_cells[make_pair(node->cell->getPort(ID::TRG), node->cell->getParam(ID::TRG_POLARITY))].push_back(node->cell);
 					else
 						schedule[module].push_back(*node);

backends/verilog/verilog_backend.cc

@@ -1830,7 +1830,8 @@ void dump_cell(std::ostream &f, std::string indent, RTLIL::Cell *cell)
 			if (it != cell->parameters.begin())
 				f << stringf(",");
 			f << stringf("\n%s  .%s(", indent.c_str(), id(it->first).c_str());
-			dump_const(f, it->second);
+			if (it->second.size() > 0)
+				dump_const(f, it->second);
 			f << stringf(")");
 		}
 		f << stringf("\n%s" ")", indent.c_str());
@@ -1895,17 +1896,21 @@ void dump_cell(std::ostream &f, std::string indent, RTLIL::Cell *cell)
 
 void dump_sync_print(std::ostream &f, std::string indent, const RTLIL::SigSpec &trg, const RTLIL::Const &polarity, std::vector<const RTLIL::Cell*> &cells)
 {
-	f << stringf("%s" "always @(", indent.c_str());
-	for (int i = 0; i < trg.size(); i++) {
-		if (i != 0)
-			f << " or ";
-		if (polarity[i])
-			f << "posedge ";
-		else
-			f << "negedge ";
-		dump_sigspec(f, trg[i]);
+	if (trg.size() == 0) {
+		f << stringf("%s" "initial begin\n", indent.c_str());
+	} else {
+		f << stringf("%s" "always @(", indent.c_str());
+		for (int i = 0; i < trg.size(); i++) {
+			if (i != 0)
+				f << " or ";
+			if (polarity[i])
+				f << "posedge ";
+			else
+				f << "negedge ";
+			dump_sigspec(f, trg[i]);
+		}
+		f << ") begin\n";
 	}
-	f << ") begin\n";
 
 	std::sort(cells.begin(), cells.end(), [](const RTLIL::Cell *a, const RTLIL::Cell *b) {
 		return a->getParam(ID::PRIORITY).as_int() > b->getParam(ID::PRIORITY).as_int();

docs/source/CHAPTER_CellLib.rst

@@ -120,7 +120,7 @@ All binary RTL cells have two input ports ``\A`` and ``\B`` and one output port
 	:verilog:`Y = A >>> B`  $sshr         :verilog:`Y = A  - B`   $sub
 	:verilog:`Y = A && B`   $logic_and    :verilog:`Y = A  * B`   $mul
 	:verilog:`Y = A || B`   $logic_or     :verilog:`Y = A  / B`   $div
-	:verilog:`Y = A === B`  $eqx          :verilog:`Y = A  % B`   $mod 
+	:verilog:`Y = A === B`  $eqx          :verilog:`Y = A  % B`   $mod
 	:verilog:`Y = A !== B`  $nex          ``N/A``                 $divfloor
 	:verilog:`Y = A ** B`   $pow          ``N/A``                 $modfoor
 	======================= ============= ======================= =========
@@ -661,6 +661,8 @@ Ports:
 
 ``\TRG``
 	The signals that control when this ``$print`` cell is triggered.
+	If the width of this port is zero and ``\TRG_ENABLE`` is true, the cell is
+	triggered during initial evaluation (time zero) only.
 
 ``\EN``
 	Enable signal for the whole cell.

frontends/ast/ast.cc

@@ -45,7 +45,7 @@ namespace AST {
 
 // instantiate global variables (private API)
 namespace AST_INTERNAL {
-	bool flag_dump_ast1, flag_dump_ast2, flag_no_dump_ptr, flag_dump_vlog1, flag_dump_vlog2, flag_dump_rtlil, flag_nolatches, flag_nomeminit;
+	bool flag_nodisplay, flag_dump_ast1, flag_dump_ast2, flag_no_dump_ptr, flag_dump_vlog1, flag_dump_vlog2, flag_dump_rtlil, flag_nolatches, flag_nomeminit;
 	bool flag_nomem2reg, flag_mem2reg, flag_noblackbox, flag_lib, flag_nowb, flag_noopt, flag_icells, flag_pwires, flag_autowire;
 	AstNode *current_ast, *current_ast_mod;
 	std::map<std::string, AstNode*> current_scope;
@@ -1320,11 +1320,12 @@ static void rename_in_package_stmts(AstNode *pkg)
 }
 
 // create AstModule instances for all modules in the AST tree and add them to 'design'
-void AST::process(RTLIL::Design *design, AstNode *ast, bool dump_ast1, bool dump_ast2, bool no_dump_ptr, bool dump_vlog1, bool dump_vlog2, bool dump_rtlil,
+void AST::process(RTLIL::Design *design, AstNode *ast, bool nodisplay, bool dump_ast1, bool dump_ast2, bool no_dump_ptr, bool dump_vlog1, bool dump_vlog2, bool dump_rtlil,
 		bool nolatches, bool nomeminit, bool nomem2reg, bool mem2reg, bool noblackbox, bool lib, bool nowb, bool noopt, bool icells, bool pwires, bool nooverwrite, bool overwrite, bool defer, bool autowire)
 {
 	current_ast = ast;
 	current_ast_mod = nullptr;
+	flag_nodisplay = nodisplay;
 	flag_dump_ast1 = dump_ast1;
 	flag_dump_ast2 = dump_ast2;
 	flag_no_dump_ptr = no_dump_ptr;

frontends/ast/ast.h

@@ -287,7 +287,7 @@ namespace AST
 		bool is_simple_const_expr();
 
 		// helper for parsing format strings
-		Fmt processFormat(int stage, bool sformat_like, int default_base = 10, size_t first_arg_at = 0);
+		Fmt processFormat(int stage, bool sformat_like, int default_base = 10, size_t first_arg_at = 0, bool may_fail = false);
 
 		bool is_recursive_function() const;
 		std::pair<AstNode*, AstNode*> get_tern_choice();
@@ -376,7 +376,7 @@ namespace AST
 	};
 
 	// process an AST tree (ast must point to an AST_DESIGN node) and generate RTLIL code
-	void process(RTLIL::Design *design, AstNode *ast, bool dump_ast1, bool dump_ast2, bool no_dump_ptr, bool dump_vlog1, bool dump_vlog2, bool dump_rtlil, bool nolatches, bool nomeminit,
+	void process(RTLIL::Design *design, AstNode *ast, bool nodisplay, bool dump_ast1, bool dump_ast2, bool no_dump_ptr, bool dump_vlog1, bool dump_vlog2, bool dump_rtlil, bool nolatches, bool nomeminit,
 			bool nomem2reg, bool mem2reg, bool noblackbox, bool lib, bool nowb, bool noopt, bool icells, bool pwires, bool nooverwrite, bool overwrite, bool defer, bool autowire);
 
 	// parametric modules are supported directly by the AST library
@@ -432,7 +432,7 @@ namespace AST
 namespace AST_INTERNAL
 {
 	// internal state variables
-	extern bool flag_dump_ast1, flag_dump_ast2, flag_no_dump_ptr, flag_dump_rtlil, flag_nolatches, flag_nomeminit;
+	extern bool flag_nodisplay, flag_dump_ast1, flag_dump_ast2, flag_no_dump_ptr, flag_dump_rtlil, flag_nolatches, flag_nomeminit;
 	extern bool flag_nomem2reg, flag_mem2reg, flag_lib, flag_noopt, flag_icells, flag_pwires, flag_autowire;
 	extern AST::AstNode *current_ast, *current_ast_mod;
 	extern std::map<std::string, AST::AstNode*> current_scope;

frontends/ast/genrtlil.cc

@@ -718,7 +718,7 @@ struct AST_INTERNAL::ProcessGenerator
 					}
 				}
 				cell->parameters[ID::TRG_WIDTH] = triggers.size();
-				cell->parameters[ID::TRG_ENABLE] = !triggers.empty();
+				cell->parameters[ID::TRG_ENABLE] = (always->type == AST_INITIAL) || !triggers.empty();
 				cell->parameters[ID::TRG_POLARITY] = polarity;
 				cell->parameters[ID::PRIORITY] = --last_print_priority;
 				cell->setPort(ID::TRG, triggers);

frontends/ast/simplify.cc

@@ -145,7 +145,7 @@ void AstNode::fixup_hierarchy_flags(bool force_descend)
 
 // Process a format string and arguments for $display, $write, $sprintf, etc
 
-Fmt AstNode::processFormat(int stage, bool sformat_like, int default_base, size_t first_arg_at) {
+Fmt AstNode::processFormat(int stage, bool sformat_like, int default_base, size_t first_arg_at, bool may_fail) {
 	std::vector<VerilogFmtArg> args;
 	for (size_t index = first_arg_at; index < children.size(); index++) {
 		AstNode *node_arg = children[index];
@@ -169,6 +169,9 @@ Fmt AstNode::processFormat(int stage, bool sformat_like, int default_base, size_
 			arg.type = VerilogFmtArg::INTEGER;
 			arg.sig = node_arg->bitsAsConst();
 			arg.signed_ = node_arg->is_signed;
+		} else if (may_fail) {
+			log_file_info(filename, location.first_line, "Skipping system task `%s' with non-constant argument at position %zu.\n", str.c_str(), index + 1);
+			return Fmt();
 		} else {
 			log_file_error(filename, location.first_line, "Failed to evaluate system task `%s' with non-constant argument at position %zu.\n", str.c_str(), index + 1);
 		}
@@ -1055,30 +1058,31 @@ bool AstNode::simplify(bool const_fold, int stage, int width_hint, bool sign_hin
 	{
 		if (!current_always) {
 			log_file_warning(filename, location.first_line, "System task `%s' outside initial or always block is unsupported.\n", str.c_str());
-		} else if (current_always->type == AST_INITIAL) {
-			int default_base = 10;
-			if (str.back() == 'b')
-				default_base = 2;
-			else if (str.back() == 'o')
-				default_base = 8;
-			else if (str.back() == 'h')
-				default_base = 16;
-
-			// when $display()/$write() functions are used in an initial block, print them during synthesis
-			Fmt fmt = processFormat(stage, /*sformat_like=*/false, default_base);
-			if (str.substr(0, 8) == "$display")
-				fmt.append_string("\n");
-			log("%s", fmt.render().c_str());
+			delete_children();
+			str = std::string();
 		} else {
-			// when $display()/$write() functions are used in an always block, simplify the expressions and
-			// convert them to a special cell later in genrtlil
+			// simplify the expressions and convert them to a special cell later in genrtlil
 			for (auto node : children)
 				while (node->simplify(true, stage, -1, false)) {}
+
+			if (current_always->type == AST_INITIAL && !flag_nodisplay && stage == 2) {
+				int default_base = 10;
+				if (str.back() == 'b')
+					default_base = 2;
+				else if (str.back() == 'o')
+					default_base = 8;
+				else if (str.back() == 'h')
+					default_base = 16;
+
+				// when $display()/$write() functions are used in an initial block, print them during synthesis
+				Fmt fmt = processFormat(stage, /*sformat_like=*/false, default_base, /*first_arg_at=*/0, /*may_fail=*/true);
+				if (str.substr(0, 8) == "$display")
+					fmt.append_string("\n");
+				log("%s", fmt.render().c_str());
+			}
+
 			return false;
 		}
-
-		delete_children();
-		str = std::string();
 	}
 
 	// activate const folding if this is anything that must be evaluated statically (ranges, parameters, attributes, etc.)

frontends/verilog/verilog_frontend.cc

@@ -100,6 +100,10 @@ struct VerilogFrontend : public Frontend {
 		log("    -assert-assumes\n");
 		log("        treat all assume() statements like assert() statements\n");
 		log("\n");
+		log("    -nodisplay\n");
+		log("        suppress output from display system tasks ($display et. al).\n");
+		log("        This does not affect the output from a later 'sim' command.\n");
+		log("\n");
 		log("    -debug\n");
 		log("        alias for -dump_ast1 -dump_ast2 -dump_vlog1 -dump_vlog2 -yydebug\n");
 		log("\n");
@@ -235,6 +239,7 @@ struct VerilogFrontend : public Frontend {
 	}
 	void execute(std::istream *&f, std::string filename, std::vector<std::string> args, RTLIL::Design *design) override
 	{
+		bool flag_nodisplay = false;
 		bool flag_dump_ast1 = false;
 		bool flag_dump_ast2 = false;
 		bool flag_no_dump_ptr = false;
@@ -308,6 +313,10 @@ struct VerilogFrontend : public Frontend {
 				assert_assumes_mode = true;
 				continue;
 			}
+			if (arg == "-nodisplay") {
+				flag_nodisplay = true;
+				continue;
+			}
 			if (arg == "-debug") {
 				flag_dump_ast1 = true;
 				flag_dump_ast2 = true;
@@ -510,7 +519,7 @@ struct VerilogFrontend : public Frontend {
 		if (flag_nodpi)
 			error_on_dpi_function(current_ast);
 
-		AST::process(design, current_ast, flag_dump_ast1, flag_dump_ast2, flag_no_dump_ptr, flag_dump_vlog1, flag_dump_vlog2, flag_dump_rtlil, flag_nolatches,
+		AST::process(design, current_ast, flag_nodisplay, flag_dump_ast1, flag_dump_ast2, flag_no_dump_ptr, flag_dump_vlog1, flag_dump_vlog2, flag_dump_rtlil, flag_nolatches,
 				flag_nomeminit, flag_nomem2reg, flag_mem2reg, flag_noblackbox, lib_mode, flag_nowb, flag_noopt, flag_icells, flag_pwires, flag_nooverwrite, flag_overwrite, flag_defer, default_nettype_wire);
 
 

passes/sat/sim.cc

@@ -88,6 +88,17 @@ struct TriggeredAssertion {
 	{ }
 };
 
+struct DisplayOutput {
+	int step;
+	SimInstance *instance;
+	Cell *cell;
+	std::string output;
+
+	DisplayOutput(int step, SimInstance *instance, Cell *cell, std::string output) :
+		step(step), instance(instance), cell(cell), output(output)
+	{ }
+};
+
 struct SimShared
 {
 	bool debug = false;
@@ -110,6 +121,7 @@ struct SimShared
 	int next_output_id = 0;
 	int step = 0;
 	std::vector<TriggeredAssertion> triggered_assertions;
+	std::vector<DisplayOutput> display_output;
 	bool serious_asserts = false;
 	bool initstate = true;
 };
@@ -870,6 +882,7 @@ struct SimInstance
 
 				std::string rendered = print.fmt.render();
 				log("%s", rendered.c_str());
+				shared->display_output.emplace_back(shared->step, this, cell, rendered);
 			}
 
 		update_print:
@@ -2055,6 +2068,20 @@ struct SimWorker : SimShared
 			json.end_object();
 		}
 		json.end_array();
+		json.name("display_output");
+		json.begin_array();
+		for (auto &output : display_output) {
+			json.begin_object();
+			json.entry("step", output.step);
+			json.entry("path", output.instance->witness_full_path(output.cell));
+			auto src = output.cell->get_string_attribute(ID::src);
+			if (!src.empty()) {
+				json.entry("src", src);
+			}
+			json.entry("output", output.output);
+			json.end_object();
+		}
+		json.end_array();
 		json.end_object();
 	}