diff --git a/kernel/celltypes.h b/kernel/celltypes.h
index 11640c25f..f08a695e9 100644
--- a/kernel/celltypes.h
+++ b/kernel/celltypes.h
@@ -303,24 +303,24 @@ struct CellTypes
 		cell_types.clear();
 	}
 
-	bool cell_known(RTLIL::IdString type) const
+	bool cell_known(const RTLIL::IdString &type) const
 	{
 		return cell_types.count(type) != 0;
 	}
 
-	bool cell_output(RTLIL::IdString type, RTLIL::IdString port) const
+	bool cell_output(const RTLIL::IdString &type, const RTLIL::IdString &port) const
 	{
 		auto it = cell_types.find(type);
 		return it != cell_types.end() && it->second.outputs.count(port) != 0;
 	}
 
-	bool cell_input(RTLIL::IdString type, RTLIL::IdString port) const
+	bool cell_input(const RTLIL::IdString &type, const RTLIL::IdString &port) const
 	{
 		auto it = cell_types.find(type);
 		return it != cell_types.end() && it->second.inputs.count(port) != 0;
 	}
 
-	bool cell_evaluable(RTLIL::IdString type) const
+	bool cell_evaluable(const RTLIL::IdString &type) const
 	{
 		auto it = cell_types.find(type);
 		return it != cell_types.end() && it->second.is_evaluable;
diff --git a/kernel/hashlib.h b/kernel/hashlib.h
index 9c53e6687..39530baea 100644
--- a/kernel/hashlib.h
+++ b/kernel/hashlib.h
@@ -188,6 +188,12 @@ template<typename P, typename Q> struct hash_ops<std::pair<P, Q>> {
 		return h;
 	}
 	HASH_TOP_LOOP_FST (const std::pair<P, Q> &a) HASH_TOP_LOOP_SND
+	[[nodiscard]] static inline Hasher hash(const P &p, const Q &q) {
+		Hasher h;
+		h = hash_ops<P>::hash_into(p, h);
+		h = hash_ops<Q>::hash_into(q, h);
+		return h;
+	}
 };
 
 template<typename... T> struct hash_ops<std::tuple<T...>> {
diff --git a/kernel/rtlil.h b/kernel/rtlil.h
index e0de79ea9..c81a0c00a 100644
--- a/kernel/rtlil.h
+++ b/kernel/rtlil.h
@@ -411,7 +411,7 @@ struct RTLIL::IdString
 	// often one needs to check if a given IdString is part of a list (for example a list
 	// of cell types). the following functions helps with that.
 	template<typename... Args>
-	bool in(Args... args) const {
+	bool in(const Args &... args) const {
 		return (... || in(args));
 	}
 
@@ -430,10 +430,10 @@ namespace hashlib {
 		static inline bool cmp(const RTLIL::IdString &a, const RTLIL::IdString &b) {
 			return a == b;
 		}
-		[[nodiscard]] static inline Hasher hash(const RTLIL::IdString id) {
+		[[nodiscard]] static inline Hasher hash(const RTLIL::IdString &id) {
 			return id.hash_top();
 		}
-		[[nodiscard]] static inline Hasher hash_into(const RTLIL::IdString id, Hasher h) {
+		[[nodiscard]] static inline Hasher hash_into(const RTLIL::IdString &id, Hasher h) {
 			return id.hash_into(h);
 		}
 	};
diff --git a/kernel/yosys_common.h b/kernel/yosys_common.h
index bc92e7869..943aa4f05 100644
--- a/kernel/yosys_common.h
+++ b/kernel/yosys_common.h
@@ -285,7 +285,7 @@ RTLIL::IdString new_id_suffix(std::string file, int line, std::string func, std:
 //
 //  sed -i.orig -r 's/"\\\\([a-zA-Z0-9_]+)"/ID(\1)/g; s/"(\$[a-zA-Z0-9_]+)"/ID(\1)/g;' <filename>
 //
-#define ID(_id) ([]() { const char *p = "\\" #_id, *q = p[1] == '$' ? p+1 : p; \
+#define ID(_id) ([]() -> const RTLIL::IdString & { const char *p = "\\" #_id, *q = p[1] == '$' ? p+1 : p; \
         static const YOSYS_NAMESPACE_PREFIX RTLIL::IdString id(q); return id; })()
 namespace ID = RTLIL::ID;
 
diff --git a/passes/opt/opt_merge.cc b/passes/opt/opt_merge.cc
index ba8168e74..6c81ee241 100644
--- a/passes/opt/opt_merge.cc
+++ b/passes/opt/opt_merge.cc
@@ -107,7 +107,7 @@ struct OptMergeWorker
 			for (const auto& [port, sig] : cell->connections()) {
 				if (cell->output(port))
 					continue;
-				comm.eat(hash_ops<std::pair<IdString, SigSpec>>::hash({port, assign_map(sig)}));
+				comm.eat(hash_ops<std::pair<IdString, SigSpec>>::hash(port, assign_map(sig)));
 			}
 			h = comm.hash_into(h);
 			if (RTLIL::builtin_ff_cell_types().count(cell->type))