Add support for SystemVerilog-style `define to Verilog frontend

This patch should support things like

  `define foo(a, b = 3, c)   a+b+c

  `foo(1, ,2)

which will evaluate to 1+3+2. It also spots mistakes like

  `foo(1)

(the 3rd argument doesn't have a default value, so a call site is
required to set it).

Most of the patch is a simple parser for the format in preproc.cc, but
I've also taken the opportunity to wrap up the "name -> definition"
map in a type, rather than use multiple std::map's.

Since this type needs to be visible to code that touches defines, I've
pulled it (and the frontend_verilog_preproc declaration) out into a
new file at frontends/verilog/preproc.h and included that where
necessary.

Finally, the patch adds a few tests in tests/various to check that we
are parsing everything correctly.

4 files changed, 578 insertions, 149 deletions

diff --git a/frontends/verilog/preproc.cc b/frontends/verilog/preproc.cc
index 161253a99..7905ea598 100644
--- a/frontends/verilog/preproc.cc
+++ b/frontends/verilog/preproc.cc
@@ -32,8 +32,10 @@
  *
  */
 
+#include "preproc.h"
 #include "verilog_frontend.h"
 #include "kernel/log.h"
+#include <assert.h>
 #include <stdarg.h>
 #include <stdio.h>
 #include <string.h>
@@ -199,6 +201,175 @@ static std::string next_token(bool pass_newline = false)
 	return token;
 }
 
+struct macro_arg_t
+{
+	macro_arg_t(const std::string &name_, const char *default_value_)
+		: name(name_),
+		  has_default(default_value_ != nullptr),
+		  default_value(default_value_ ? default_value_ : "")
+	{}
+
+	std::string name;
+	bool        has_default;
+	std::string default_value;
+};
+
+static bool all_white(const std::string &str)
+{
+	for (char c : str)
+		if (!isspace(c))
+			return false;
+	return true;
+}
+
+struct arg_map_t
+{
+	arg_map_t()
+	{}
+
+	void add_arg(const std::string &name, const char *default_value)
+	{
+		if (find(name)) {
+			log_error("Duplicate macro arguments with name `%s'.\n", name.c_str());
+		}
+
+		name_to_pos[name] = args.size();
+		args.push_back(macro_arg_t(name, default_value));
+	}
+
+	// Find an argument by name; return nullptr if it doesn't exist. If pos is not null, write
+	// the argument's position to it on success.
+	const macro_arg_t *find(const std::string &name, int *pos = nullptr) const
+	{
+		auto it = name_to_pos.find(name);
+		if (it == name_to_pos.end())
+			return nullptr;
+
+		if (pos) *pos = it->second;
+		return &args[it->second];
+	}
+
+	// Construct the name for the local macro definition we use for the given argument
+	// (something like macro_foobar_arg2). This doesn't include the leading backtick.
+	static std::string str_token(const std::string &macro_name, int pos)
+	{
+		return stringf("macro_%s_arg%d", macro_name.c_str(), pos);
+	}
+
+	// Return definitions for the macro arguments (so that substituting in the macro body and
+	// then performing macro expansion will do argument substitution properly).
+	std::vector<std::pair<std::string, std::string>>
+	get_vals(const std::string &macro_name, const std::vector<std::string> &arg_vals) const
+	{
+		std::vector<std::pair<std::string, std::string>> ret;
+		for (int i = 0; i < GetSize(args); ++ i) {
+			// The SystemVerilog rules are:
+			//
+			//   - If the call site specifies an argument and it's not whitespace, use
+			//     it.
+			//
+			//   - Otherwise, if the argument has a default value, use it.
+			//
+			//   - Otherwise, if the call site specified whitespace, use that.
+			//
+			//   - Otherwise, error.
+			const std::string *dflt = nullptr;
+			if (args[i].has_default)
+				dflt = &args[i].default_value;
+
+			const std::string *given = nullptr;
+			if (i < GetSize(arg_vals))
+				given = &arg_vals[i];
+
+			const std::string *val = nullptr;
+			if (given && (! (dflt && all_white(*given))))
+				val = given;
+			else if (dflt)
+				val = dflt;
+			else if (given)
+				val = given;
+			else
+				log_error("Cannot expand macro `%s by giving only %d argument%s "
+				          "(argument %d has no default).\n",
+				          macro_name.c_str(), GetSize(arg_vals),
+				          (GetSize(arg_vals) == 1 ? "" : "s"), i + 1);
+
+			assert(val);
+			ret.push_back(std::make_pair(str_token(macro_name, i), * val));
+		}
+		return ret;
+	}
+
+
+	std::vector<macro_arg_t>   args;
+	std::map<std::string, int> name_to_pos;
+};
+
+struct define_body_t
+{
+	define_body_t(const std::string &body, const arg_map_t *args = nullptr)
+	  : body(body),
+	    has_args(args != nullptr),
+	    args(args ? *args : arg_map_t())
+	{}
+
+	std::string body;
+	bool        has_args;
+	arg_map_t   args;
+};
+
+define_map_t::define_map_t()
+{
+	add("YOSYS", "1");
+	add(formal_mode ? "FORMAL" : "SYNTHESIS", "1");
+}
+
+// We must define this destructor here (rather than relying on the default), because we need to
+// define it somewhere we've got a complete definition of define_body_t.
+define_map_t::~define_map_t()
+{}
+
+void
+define_map_t::add(const std::string &name, const std::string &txt, const arg_map_t *args)
+{
+	defines[name] = std::unique_ptr<define_body_t>(new define_body_t(txt, args));
+}
+
+void define_map_t::merge(const define_map_t &map)
+{
+	for (const auto &pr : map.defines) {
+		// These contortions are so that we take a copy of each definition body in
+		// map.defines.
+		defines[pr.first] = std::unique_ptr<define_body_t>(new define_body_t(*pr.second));
+	}
+}
+
+const define_body_t *define_map_t::find(const std::string &name) const
+{
+	auto it = defines.find(name);
+	return (it == defines.end()) ? nullptr : it->second.get();
+}
+
+void define_map_t::erase(const std::string &name)
+{
+	defines.erase(name);
+}
+
+void define_map_t::clear()
+{
+	defines.clear();
+}
+
+void define_map_t::log() const
+{
+	for (auto &it : defines) {
+		const std::string &name = it.first;
+		const define_body_t &body = *it.second;
+		Yosys::log("`define %s%s %s\n",
+		           name.c_str(), body.has_args ? "()" : "", body.body.c_str());
+	}
+}
+
 static void input_file(std::istream &f, std::string filename)
 {
 	char buffer[513];
@@ -215,11 +386,59 @@ static void input_file(std::istream &f, std::string filename)
 	input_buffer.insert(it, "\n`file_pop\n");
 }
 
+// Read tokens to get one argument (either a macro argument at a callsite or a default argument in a
+// macro definition). Writes the argument to dest. Returns true if we finished with ')' (the end of
+// the argument list); false if we finished with ','.
+static bool read_argument(std::string &dest)
+{
+	std::vector<char> openers;
+	for (;;) {
+		skip_spaces();
+		std::string tok = next_token(true);
+		if (tok == ")") {
+			if (openers.empty())
+				return true;
+			if (openers.back() != '(')
+				log_error("Mismatched brackets in macro argument: %c and %c.\n",
+				          openers.back(), tok[0]);
+
+			openers.pop_back();
+			dest += tok;
+			continue;
+		}
+		if (tok == "]") {
+			char opener = openers.empty() ? '(' : openers.back();
+			if (opener != '[')
+				log_error("Mismatched brackets in macro argument: %c and %c.\n",
+				          opener, tok[0]);
+
+			openers.pop_back();
+			dest += tok;
+			continue;
+		}
+		if (tok == "}") {
+			char opener = openers.empty() ? '(' : openers.back();
+			if (opener != '{')
+				log_error("Mismatched brackets in macro argument: %c and %c.\n",
+				          opener, tok[0]);
+
+			openers.pop_back();
+			dest += tok;
+			continue;
+		}
+
+		if (tok == "," && openers.empty()) {
+			return false;
+		}
+
+		if (tok == "(" || tok == "[" || tok == "{")
+			openers.push_back(tok[0]);
 
-static bool try_expand_macro(std::set<std::string> &defines_with_args,
-			     std::map<std::string, std::string> &defines_map,
-			     std::string &tok
-				    )
+		dest += tok;
+	}
+}
+
+static bool try_expand_macro(define_map_t &defines, std::string &tok)
 {
 	if (tok == "`\"") {
 		std::string literal("\"");
@@ -229,54 +448,272 @@ static bool try_expand_macro(std::set<std::string> &defines_with_args,
 			if (ntok == "`\"") {
 				insert_input(literal+"\"");
 				return true;
-			} else if (!try_expand_macro(defines_with_args, defines_map, ntok)) {
+			} else if (!try_expand_macro(defines, ntok)) {
 					literal += ntok;
 			}
 		}
 		return false; // error - unmatched `"
-	} else if (tok.size() > 1 && tok[0] == '`' && defines_map.count(tok.substr(1)) > 0) {
-			std::string name = tok.substr(1);
-			// printf("expand: >>%s<< -> >>%s<<\n", name.c_str(), defines_map[name].c_str());
-			std::string skipped_spaces = skip_spaces();
-			tok = next_token(false);
-			if (tok == "(" && defines_with_args.count(name) > 0) {
-				int level = 1;
-				std::vector<std::string> args;
-				args.push_back(std::string());
-				while (1)
-				{
-					skip_spaces();
-					tok = next_token(true);
-					if (tok == ")" || tok == "}" || tok == "]")
-						level--;
-					if (level == 0)
-						break;
-					if (level == 1 && tok == ",")
-						args.push_back(std::string());
-					else
-						args.back() += tok;
-					if (tok == "(" || tok == "{" || tok == "[")
-						level++;
-				}
-				for (int i = 0; i < GetSize(args); i++)
-					defines_map[stringf("macro_%s_arg%d", name.c_str(), i+1)] = args[i];
-			} else {
-				insert_input(tok);
-				insert_input(skipped_spaces);
-			}
-			insert_input(defines_map[name]);
-			return true;
-	} else if (tok == "``") {
+	}
+
+	if (tok == "``") {
 		// Swallow `` in macro expansion
 		return true;
-	} else return false;
+	}
+
+	if (tok.size() <= 1 || tok[0] != '`')
+		return false;
+
+	// This token looks like a macro name (`foo).
+	std::string macro_name = tok.substr(1);
+	const define_body_t *body = defines.find(tok.substr(1));
+
+	if (! body) {
+		// Apparently not a name we know.
+		return false;
+	}
+
+	std::string name = tok.substr(1);
+	std::string skipped_spaces = skip_spaces();
+	tok = next_token(false);
+	if (tok == "(" && body->has_args) {
+		std::vector<std::string> args;
+		bool done = false;
+		while (!done) {
+			std::string arg;
+			done = read_argument(arg);
+			args.push_back(arg);
+		}
+		for (const auto &pr : body->args.get_vals(name, args)) {
+			defines.add(pr.first, pr.second);
+		}
+	} else {
+		insert_input(tok);
+		insert_input(skipped_spaces);
+	}
+	insert_input(body->body);
+	return true;
 }
 
-std::string frontend_verilog_preproc(std::istream &f, std::string filename, const std::map<std::string, std::string> &pre_defines_map,
-		dict<std::string, std::pair<std::string, bool>> &global_defines_cache, const std::list<std::string> &include_dirs)
+// Read the arguments for a `define preprocessor directive with formal arguments. This is called
+// just after reading the token containing "(". Returns the number of newlines to emit afterwards to
+// keep line numbers in sync, together with the map from argument name to data (pos and default
+// value).
+static std::pair<int, arg_map_t>
+read_define_args()
 {
-	std::set<std::string> defines_with_args;
-	std::map<std::string, std::string> defines_map(pre_defines_map);
+	// Each argument looks like one of the following:
+	//
+	//     identifier
+	//     identifier = default_text
+	//     identifier =
+	//
+	// The first example is an argument with no default value. The second is an argument whose
+	// default value is default_text. The third is an argument with default value the empty
+	// string.
+
+	int newline_count = 0;
+	arg_map_t args;
+
+	// FSM state.
+	//
+	//   0: At start of identifier
+	//   1: After identifier (stored in arg_name)
+	//   2: After closing paren
+	int state = 0;
+
+	std::string arg_name, default_val;
+
+	skip_spaces();
+	for (;;) {
+		if (state == 2)
+			// We've read the closing paren.
+			break;
+
+		std::string tok = next_token();
+
+		// Cope with escaped EOLs
+		if (tok == "\\") {
+			char ch = next_char();
+			if (ch == '\n') {
+				// Eat the \, the \n and any trailing space and keep going.
+				skip_spaces();
+				continue;
+			} else {
+				// There aren't any other situations where a backslash makes sense.
+				log_error("Backslash in macro arguments (not at end of line).\n");
+			}
+		}
+
+		switch (state) {
+		case 0:
+			// At start of argument. If the token is ')', we've presumably just seen
+			// something like "`define foo() ...". Set state to 2 to finish. Otherwise,
+			// the token should be a valid simple identifier, but we'll allow anything
+			// here.
+			if (tok == ")") {
+				state = 2;
+			} else {
+				arg_name = tok;
+				state = 1;
+			}
+			skip_spaces();
+			break;
+
+		case 1:
+			// After argument. The token should either be an equals sign or a comma or
+			// closing paren.
+			if (tok == "=") {
+				std::string default_val;
+				//Read an argument into default_val and set state to 2 if we're at
+				// the end; 0 if we hit a comma.
+				state = read_argument(default_val) ? 2 : 0;
+				args.add_arg(arg_name, default_val.c_str());
+				skip_spaces();
+				break;
+			}
+			if (tok == ",") {
+				// Take the identifier as an argument with no default value.
+				args.add_arg(arg_name, nullptr);
+				state = 0;
+				skip_spaces();
+				break;
+			}
+			if (tok == ")") {
+				// As with comma, but set state to 2 (end of args)
+				args.add_arg(arg_name, nullptr);
+				state = 2;
+				skip_spaces();
+				break;
+			}
+			log_error("Trailing contents after identifier in macro argument `%s': "
+				  "expected '=', ',' or ')'.\n",
+				  arg_name.c_str());
+
+		default:
+			// The only FSM states are 0-2 and we dealt with 2 at the start of the loop.
+			__builtin_unreachable();
+		}
+	}
+
+	return std::make_pair(newline_count, args);
+}
+
+// Read a `define preprocessor directive. This is called just after reading the token containing
+// "`define".
+static void
+read_define(const std::string &filename,
+            define_map_t      &defines_map,
+            define_map_t      &global_defines_cache)
+{
+	std::string name, value;
+	arg_map_t args;
+
+	skip_spaces();
+	name = next_token(true);
+
+	bool here_doc_mode = false;
+	int newline_count = 0;
+
+	// The FSM state starts at 0. If it sees space (or enters here_doc_mode), it assumes this is
+	// a macro without formal arguments and jumps to state 1.
+	//
+	// In state 0, if it sees an opening parenthesis, it assumes this is a macro with formal
+	// arguments. It reads the arguments with read_define_args() and then jumps to state 2.
+	//
+	// In states 1 or 2, the FSM reads tokens to the end of line (or end of here_doc): this is
+	// the body of the macro definition.
+	int state = 0;
+
+	if (skip_spaces() != "")
+		state = 1;
+
+	for (;;) {
+		std::string tok = next_token();
+		if (tok.empty())
+			break;
+
+		// printf("define-tok: >>%s<<\n", tok != "\n" ? tok.c_str() : "NEWLINE");
+
+		if (tok == "\"\"\"") {
+			here_doc_mode = !here_doc_mode;
+			continue;
+		}
+
+		if (state == 0 && tok == "(") {
+			auto pr = read_define_args();
+			newline_count += pr.first;
+			args = pr.second;
+
+			state = 2;
+			continue;
+		}
+
+		// This token isn't an opening parenthesis immediately following the macro name, so
+		// it's presumably at or after the start of the macro body. If state isn't already 2
+		// (which would mean we'd parsed an argument list), set it to 1.
+		if (state == 0) {
+			state = 1;
+		}
+
+		if (tok == "\n") {
+			if (here_doc_mode) {
+				value += " ";
+				newline_count++;
+			} else {
+				return_char('\n');
+				break;
+			}
+			continue;
+		}
+
+		if (tok == "\\") {
+			char ch = next_char();
+			if (ch == '\n') {
+				value += " ";
+				newline_count++;
+			} else {
+				value += std::string("\\");
+				return_char(ch);
+			}
+			continue;
+		}
+
+		// Is this token the name of a macro argument? If so, replace it with a magic symbol
+		// that we'll replace with the argument value.
+		int arg_pos;
+		if (args.find(tok, &arg_pos)) {
+			value += '`' + args.str_token(name, arg_pos);
+			continue;
+		}
+
+		// This token is nothing special. Insert it verbatim into the macro body.
+		value += tok;
+	}
+
+	// Append some newlines so that we don't mess up line counts in error messages.
+	while (newline_count-- > 0)
+		return_char('\n');
+
+	if (strchr("abcdefghijklmnopqrstuvwxyz_ABCDEFGHIJKLMNOPQRSTUVWXYZ$0123456789", name[0])) {
+		// printf("define: >>%s<< -> >>%s<<\n", name.c_str(), value.c_str());
+		defines_map.add(name, value, (state == 2) ? &args : nullptr);
+		global_defines_cache.add(name, value, (state == 2) ? &args : nullptr);
+	} else {
+		log_file_error(filename, 0, "Invalid name for macro definition: >>%s<<.\n", name.c_str());
+	}
+}
+
+std::string
+frontend_verilog_preproc(std::istream                 &f,
+                         std::string                   filename,
+                         const define_map_t           &pre_defines,
+                         define_map_t                 &global_defines_cache,
+                         const std::list<std::string> &include_dirs)
+{
+	define_map_t defines;
+	defines.merge(pre_defines);
+	defines.merge(global_defines_cache);
+
 	std::vector<std::string> filename_stack;
 	int ifdef_fail_level = 0;
 	bool in_elseif = false;
@@ -287,18 +724,6 @@ std::string frontend_verilog_preproc(std::istream &f, std::string filename, cons
 
 	input_file(f, filename);
 
-	defines_map["YOSYS"] = "1";
-	defines_map[formal_mode ? "FORMAL" : "SYNTHESIS"] = "1";
-
-	for (auto &it : pre_defines_map)
-		defines_map[it.first] = it.second;
-
-	for (auto &it : global_defines_cache) {
-		if (it.second.second)
-			defines_with_args.insert(it.first);
-		defines_map[it.first] = it.second.first;
-	}
-
 	while (!input_buffer.empty())
 	{
 		std::string tok = next_token();
@@ -325,7 +750,7 @@ std::string frontend_verilog_preproc(std::istream &f, std::string filename, cons
 			std::string name = next_token(true);
 			if (ifdef_fail_level == 0)
 				ifdef_fail_level = 1, in_elseif = true;
-			else if (ifdef_fail_level == 1 && defines_map.count(name) != 0)
+			else if (ifdef_fail_level == 1 && defines.find(name))
 				ifdef_fail_level = 0, in_elseif = true;
 			continue;
 		}
@@ -333,7 +758,7 @@ std::string frontend_verilog_preproc(std::istream &f, std::string filename, cons
 		if (tok == "`ifdef") {
 			skip_spaces();
 			std::string name = next_token(true);
-			if (ifdef_fail_level > 0 || defines_map.count(name) == 0)
+			if (ifdef_fail_level > 0 || !defines.find(name))
 				ifdef_fail_level++;
 			continue;
 		}
@@ -341,7 +766,7 @@ std::string frontend_verilog_preproc(std::istream &f, std::string filename, cons
 		if (tok == "`ifndef") {
 			skip_spaces();
 			std::string name = next_token(true);
-			if (ifdef_fail_level > 0 || defines_map.count(name) != 0)
+			if (ifdef_fail_level > 0 || defines.find(name))
 				ifdef_fail_level++;
 			continue;
 		}
@@ -355,7 +780,7 @@ std::string frontend_verilog_preproc(std::istream &f, std::string filename, cons
 		if (tok == "`include") {
 			skip_spaces();
 			std::string fn = next_token(true);
-			while (try_expand_macro(defines_with_args, defines_map, fn)) {
+			while (try_expand_macro(defines, fn)) {
 				fn = next_token();
 			}
 			while (1) {
@@ -433,74 +858,7 @@ std::string frontend_verilog_preproc(std::istream &f, std::string filename, cons
 		}
 
 		if (tok == "`define") {
-			std::string name, value;
-			std::map<std::string, int> args;
-			skip_spaces();
-			name = next_token(true);
-			bool here_doc_mode = false;
-			int newline_count = 0;
-			int state = 0;
-			if (skip_spaces() != "")
-				state = 3;
-			while (!tok.empty()) {
-				tok = next_token();
-				if (tok == "\"\"\"") {
-					here_doc_mode = !here_doc_mode;
-					continue;
-				}
-				if (state == 0 && tok == "(") {
-					state = 1;
-					skip_spaces();
-				} else
-				if (state == 1) {
-					if (tok == ")")
-						state = 2;
-					else if (tok != ",") {
-						int arg_idx = args.size()+1;
-						args[tok] = arg_idx;
-					}
-					skip_spaces();
-				} else {
-					if (state != 2)
-						state = 3;
-					if (tok == "\n") {
-						if (here_doc_mode) {
-							value += " ";
-							newline_count++;
-						} else {
-							return_char('\n');
-							break;
-						}
-					} else
-					if (tok == "\\") {
-						char ch = next_char();
-						if (ch == '\n') {
-							value += " ";
-							newline_count++;
-						} else {
-							value += std::string("\\");
-							return_char(ch);
-						}
-					} else
-					if (args.count(tok) > 0)
-						value += stringf("`macro_%s_arg%d", name.c_str(), args.at(tok));
-					else
-						value += tok;
-				}
-			}
-			while (newline_count-- > 0)
-				return_char('\n');
-			if (strchr("abcdefghijklmnopqrstuvwxyz_ABCDEFGHIJKLMNOPQRSTUVWXYZ$0123456789", name[0])) {
-				// printf("define: >>%s<< -> >>%s<<\n", name.c_str(), value.c_str());
-				defines_map[name] = value;
-				if (state == 2)
-					defines_with_args.insert(name);
-				else
-					defines_with_args.erase(name);
-				global_defines_cache[name] = std::pair<std::string, bool>(value, state == 2);
-			} else {
-				log_file_error(filename, 0, "Invalid name for macro definition: >>%s<<.\n", name.c_str());
-			}
+			read_define(filename, defines, global_defines_cache);
 			continue;
 		}
 
@@ -509,8 +867,7 @@ std::string frontend_verilog_preproc(std::istream &f, std::string filename, cons
 			skip_spaces();
 			name = next_token(true);
 			// printf("undef: >>%s<<\n", name.c_str());
-			defines_map.erase(name);
-			defines_with_args.erase(name);
+			defines.erase(name);
 			global_defines_cache.erase(name);
 			continue;
 		}
@@ -525,13 +882,12 @@ std::string frontend_verilog_preproc(std::istream &f, std::string filename, cons
 		}
 
 		if (tok == "`resetall") {
-			defines_map.clear();
-			defines_with_args.clear();
+			defines.clear();
 			global_defines_cache.clear();
 			continue;
 		}
 
-		if (try_expand_macro(defines_with_args, defines_map, tok))
+		if (try_expand_macro(defines, tok))
 			continue;
 
 		output_code.push_back(tok);
diff --git a/frontends/verilog/preproc.h b/frontends/verilog/preproc.h
new file mode 100644
index 000000000..673d633c0
--- /dev/null
+++ b/frontends/verilog/preproc.h
@@ -0,0 +1,77 @@
+/*
+ *  yosys -- Yosys Open SYnthesis Suite
+ *
+ *  Copyright (C) 2012  Clifford Wolf <clifford@clifford.at>
+ *
+ *  Permission to use, copy, modify, and/or distribute this software for any
+ *  purpose with or without fee is hereby granted, provided that the above
+ *  copyright notice and this permission notice appear in all copies.
+ *
+ *  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ *  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ *  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ *  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ *  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ *  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ *  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *
+ *  ---
+ *
+ *  The Verilog preprocessor.
+ *
+ */
+#ifndef VERILOG_PREPROC_H
+#define VERILOG_PREPROC_H
+
+#include "kernel/yosys.h"
+
+#include <iosfwd>
+#include <list>
+#include <memory>
+#include <string>
+
+YOSYS_NAMESPACE_BEGIN
+
+struct define_body_t;
+struct arg_map_t;
+
+struct define_map_t
+{
+	define_map_t();
+	~ define_map_t();
+
+	// Add a definition, overwriting any existing definition for name.
+	void add(const std::string &name, const std::string &txt, const arg_map_t *args = nullptr);
+
+	// Merge in another map of definitions (which take precedence
+	// over anything currently defined).
+	void merge(const define_map_t &map);
+
+	// Find a definition by name. If no match, returns null.
+	const define_body_t *find(const std::string &name) const;
+
+	// Erase a definition by name (no effect if not defined).
+	void erase(const std::string &name);
+
+	// Clear any existing definitions
+	void clear();
+
+	// Print a list of definitions, using the log function
+	void log() const;
+
+	std::map<std::string, std::unique_ptr<define_body_t>> defines;
+};
+
+
+struct define_map_t;
+
+std::string
+frontend_verilog_preproc(std::istream                 &f,
+                         std::string                   filename,
+                         const define_map_t           &pre_defines,
+                         define_map_t                 &global_defines_cache,
+                         const std::list<std::string> &include_dirs);
+
+YOSYS_NAMESPACE_END
+
+#endif
diff --git a/frontends/verilog/verilog_frontend.cc b/frontends/verilog/verilog_frontend.cc
index f2c1c227f..a2334e654 100644
--- a/frontends/verilog/verilog_frontend.cc
+++ b/frontends/verilog/verilog_frontend.cc
@@ -27,6 +27,7 @@
  */
 
 #include "verilog_frontend.h"
+#include "preproc.h"
 #include "kernel/yosys.h"
 #include "libs/sha1/sha1.h"
 #include <stdarg.h>
@@ -253,7 +254,8 @@ struct VerilogFrontend : public Frontend {
 		bool flag_defer = false;
 		bool flag_noblackbox = false;
 		bool flag_nowb = false;
-		std::map<std::string, std::string> defines_map;
+		define_map_t defines_map;
+
 		std::list<std::string> include_dirs;
 		std::list<std::string> attributes;
 
@@ -369,7 +371,7 @@ struct VerilogFrontend : public Frontend {
 			}
 			if (arg == "-lib") {
 				lib_mode = true;
-				defines_map["BLACKBOX"] = string();
+				defines_map.add("BLACKBOX", "");
 				continue;
 			}
 			if (arg == "-nowb") {
@@ -421,7 +423,7 @@ struct VerilogFrontend : public Frontend {
 					value = name.substr(equal+1);
 					name = name.substr(0, equal);
 				}
-				defines_map[name] = value;
+				defines_map.add(name, value);
 				continue;
 			}
 			if (arg.compare(0, 2, "-D") == 0) {
@@ -430,7 +432,7 @@ struct VerilogFrontend : public Frontend {
 				std::string value;
 				if (equal != std::string::npos)
 					value = arg.substr(equal+1);
-				defines_map[name] = value;
+				defines_map.add(name, value);
 				continue;
 			}
 			if (arg == "-I" && argidx+1 < args.size()) {
@@ -460,7 +462,7 @@ struct VerilogFrontend : public Frontend {
 		std::string code_after_preproc;
 
 		if (!flag_nopp) {
-			code_after_preproc = frontend_verilog_preproc(*f, filename, defines_map, design->verilog_defines, include_dirs);
+			code_after_preproc = frontend_verilog_preproc(*f, filename, defines_map, *design->verilog_defines, include_dirs);
 			if (flag_ppdump)
 				log("-- Verilog code after preprocessor --\n%s-- END OF DUMP --\n", code_after_preproc.c_str());
 			lexin = new std::istringstream(code_after_preproc);
@@ -592,7 +594,7 @@ struct VerilogDefines : public Pass {
 					value = name.substr(equal+1);
 					name = name.substr(0, equal);
 				}
-				design->verilog_defines[name] = std::pair<std::string, bool>(value, false);
+				design->verilog_defines->add(name, value);
 				continue;
 			}
 			if (arg.compare(0, 2, "-D") == 0) {
@@ -601,27 +603,25 @@ struct VerilogDefines : public Pass {
 				std::string value;
 				if (equal != std::string::npos)
 					value = arg.substr(equal+1);
-				design->verilog_defines[name] = std::pair<std::string, bool>(value, false);
+				design->verilog_defines->add(name, value);
 				continue;
 			}
 			if (arg == "-U" && argidx+1 < args.size()) {
 				std::string name = args[++argidx];
-				design->verilog_defines.erase(name);
+				design->verilog_defines->erase(name);
 				continue;
 			}
 			if (arg.compare(0, 2, "-U") == 0) {
 				std::string name = arg.substr(2);
-				design->verilog_defines.erase(name);
+				design->verilog_defines->erase(name);
 				continue;
 			}
 			if (arg == "-reset") {
-				design->verilog_defines.clear();
+				design->verilog_defines->clear();
 				continue;
 			}
 			if (arg == "-list") {
-				for (auto &it : design->verilog_defines) {
-					log("`define %s%s %s\n", it.first.c_str(), it.second.second ? "()" : "", it.second.first.c_str());
-				}
+				design->verilog_defines->log();
 				continue;
 			}
 			break;
diff --git a/frontends/verilog/verilog_frontend.h b/frontends/verilog/verilog_frontend.h
index 73ea51e6c..636a4ceca 100644
--- a/frontends/verilog/verilog_frontend.h
+++ b/frontends/verilog/verilog_frontend.h
@@ -85,10 +85,6 @@ namespace VERILOG_FRONTEND
 	extern std::istream *lexin;
 }
 
-// the pre-processor
-std::string frontend_verilog_preproc(std::istream &f, std::string filename, const std::map<std::string, std::string> &pre_defines_map,
-		dict<std::string, std::pair<std::string, bool>> &global_defines_cache, const std::list<std::string> &include_dirs);
-
 YOSYS_NAMESPACE_END
 
 // the usual bison/flex stuff