diff --git a/passes/hierarchy/hierarchy.cc b/passes/hierarchy/hierarchy.cc
index 08b14dca0..dceda1624 100644
--- a/passes/hierarchy/hierarchy.cc
+++ b/passes/hierarchy/hierarchy.cc
@@ -684,24 +684,30 @@ void hierarchy_clean(RTLIL::Design *design, RTLIL::Module *top, bool purge_lib)
 
 bool set_keep_print(std::map<RTLIL::Module*, bool> &cache, RTLIL::Module *mod)
 {
-	if (cache.count(mod) == 0)
-		for (auto c : mod->cells()) {
-			RTLIL::Module *m = mod->design->module(c->type);
-			if ((m != nullptr && set_keep_print(cache, m)) || c->type == ID($print))
-				return cache[mod] = true;
-		}
-	return cache[mod];
+	auto it = cache.find(mod);
+	if (it != cache.end())
+		return it->second;
+	cache[mod] = false;
+	for (auto c : mod->cells()) {
+		RTLIL::Module *m = mod->design->module(c->type);
+		if ((m != nullptr && set_keep_print(cache, m)) || c->type == ID($print))
+			return cache[mod] = true;
+	}
+	return false;
 }
 
 bool set_keep_assert(std::map<RTLIL::Module*, bool> &cache, RTLIL::Module *mod)
 {
-	if (cache.count(mod) == 0)
-		for (auto c : mod->cells()) {
-			RTLIL::Module *m = mod->design->module(c->type);
-			if ((m != nullptr && set_keep_assert(cache, m)) || c->type.in(ID($check), ID($assert), ID($assume), ID($live), ID($fair), ID($cover)))
-				return cache[mod] = true;
-		}
-	return cache[mod];
+	auto it = cache.find(mod);
+	if (it != cache.end())
+		return it->second;
+	cache[mod] = false;
+	for (auto c : mod->cells()) {
+		RTLIL::Module *m = mod->design->module(c->type);
+		if ((m != nullptr && set_keep_assert(cache, m)) || c->type.in(ID($check), ID($assert), ID($assume), ID($live), ID($fair), ID($cover)))
+			return cache[mod] = true;
+	}
+	return false;
 }
 
 int find_top_mod_score(Design *design, Module *module, dict<Module*, int> &db)