2 files changed, 43 insertions, 3 deletions

diff --git a/passes/hierarchy/hierarchy.cc b/passes/hierarchy/hierarchy.cc
index b98afcc1..d9b52c6d 100644
--- a/passes/hierarchy/hierarchy.cc
+++ b/passes/hierarchy/hierarchy.cc
@@ -280,6 +280,10 @@ struct HierarchyPass : public Pass {
 		log("        use the specified top module to built a design hierarchy. modules\n");
 		log("        outside this tree (unused modules) are removed.\n");
 		log("\n");
+		log("        when the -top option is used, the 'top' attribute will be set on the\n");
+		log("        specified top module. otherwise a module with the 'top' attribute set\n");
+		log("        will implicitly be used as top module, if such a module exists.\n");
+		log("\n");
 		log("In -generate mode this pass generates blackbox modules for the given cell\n");
 		log("types (wildcards supported). For this the design is searched for cells that\n");
 		log("match the given types and then the given port declarations are used to\n");
@@ -381,6 +385,11 @@ struct HierarchyPass : public Pass {
 
 		log_push();
 
+		if (top_mod == NULL)
+			for (auto &mod_it : design->modules)
+				if (mod_it.second->get_bool_attribute("\\top"))
+					top_mod = mod_it.second;
+
 		if (top_mod != NULL)
 			hierarchy(design, top_mod);
 
@@ -407,6 +416,14 @@ struct HierarchyPass : public Pass {
 			hierarchy(design, top_mod);
 		}
 
+		if (top_mod != NULL) {
+			for (auto &mod_it : design->modules)
+				if (mod_it.second == top_mod)
+					mod_it.second->attributes["\\top"] = RTLIL::Const(1);
+				else
+					mod_it.second->attributes.erase("\\top");
+		}
+
 		if (!keep_positionals)
 		{
 			std::set<RTLIL::Module*> pos_mods;
diff --git a/passes/techmap/techmap.cc b/passes/techmap/techmap.cc
index e273769d..7e3ba23e 100644
--- a/passes/techmap/techmap.cc
+++ b/passes/techmap/techmap.cc
@@ -500,19 +500,42 @@ struct FlattenPass : public Pass {
 		for (auto &it : design->modules)
 			celltypeMap[it.first].insert(it.first);
 
+		RTLIL::Module *top_mod = NULL;
+		for (auto &mod_it : design->modules)
+			if (mod_it.second->get_bool_attribute("\\top"))
+				top_mod = mod_it.second;
+
 		bool did_something = true;
 		std::set<RTLIL::Cell*> handled_cells;
 		while (did_something) {
 			did_something = false;
-			for (auto &mod_it : design->modules)
-				if (techmap_module(design, mod_it.second, design, handled_cells, celltypeMap, true))
+			if (top_mod != NULL) {
+				if (techmap_module(design, top_mod, design, handled_cells, celltypeMap, true))
 					did_something = true;
+			} else {
+				for (auto &mod_it : design->modules)
+					if (techmap_module(design, mod_it.second, design, handled_cells, celltypeMap, true))
+						did_something = true;
+			}
 		}
 
 		log("No more expansions possible.\n");
+
+		if (top_mod != NULL) {
+			std::map<RTLIL::IdString, RTLIL::Module*> new_modules;
+			for (auto &mod_it : design->modules)
+				if (mod_it.second == top_mod) {
+					new_modules[mod_it.first] = mod_it.second;
+				} else {
+					log("Deleting now unused module %s.\n", RTLIL::id2cstr(mod_it.first));
+					delete mod_it.second;
+				}
+			design->modules.swap(new_modules);
+		}
+
 		techmap_cache.clear();
 		techmap_do_cache.clear();
 		log_pop();
 	}
 } FlattenPass;
- 
+