diff --git a/kernel/hashlib.h b/kernel/hashlib.h
index be759fdf0a4..9cf43da6ca5 100644
--- a/kernel/hashlib.h
+++ b/kernel/hashlib.h
@@ -90,6 +90,12 @@ template<> struct hash_ops<uint32_t> : hash_int_ops
 		return a;
 	}
 };
+template<> struct hash_ops<uint64_t> : hash_int_ops
+{
+	static inline unsigned int hash(uint64_t a) {
+		return mkhash((unsigned int)(a), (unsigned int)(a >> 32));
+	}
+};
 
 template<> struct hash_ops<std::string> {
 	static inline bool cmp(const std::string &a, const std::string &b) {
diff --git a/kernel/rtlil.h b/kernel/rtlil.h
index 3dff48b4f79..9c318eac93c 100644
--- a/kernel/rtlil.h
+++ b/kernel/rtlil.h
@@ -308,10 +308,14 @@ namespace RTLIL
 		bool operator!=(const char *rhs) const { return strcmp(c_str(), rhs) != 0; }
 
 		char operator[](size_t i) const {
-			const char *p = c_str();
+                        const char *p = c_str();
+#ifndef NDEBUG
 			for (; i != 0; i--, p++)
 				log_assert(*p != 0);
 			return *p;
+#else
+			return *(p + i);
+#endif
 		}
 
 		std::string substr(size_t pos = 0, size_t len = std::string::npos) const {
diff --git a/passes/opt/opt_merge.cc b/passes/opt/opt_merge.cc
index e9d98cd4304..248ba80913b 100644
--- a/passes/opt/opt_merge.cc
+++ b/passes/opt/opt_merge.cc
@@ -41,7 +41,6 @@ struct OptMergeWorker
 
 	CellTypes ct;
 	int total_count;
-	SHA1 checksum;
 
 	static void sort_pmux_conn(dict<RTLIL::IdString, RTLIL::SigSpec> &conn)
 	{
@@ -78,7 +77,7 @@ struct OptMergeWorker
 		return str;
 	}
 
-	std::string hash_cell_parameters_and_connections(const RTLIL::Cell *cell)
+        uint64_t hash_cell_parameters_and_connections(const RTLIL::Cell *cell)
 	{
 		vector<string> hash_conn_strings;
 		std::string hash_string = cell->type.str() + "\n";
@@ -149,8 +148,7 @@ struct OptMergeWorker
 		for (auto it : hash_conn_strings)
 			hash_string += it;
 
-		checksum.update(hash_string);
-		return checksum.final();
+		return std::hash<std::string>{}(hash_string);
 	}
 
 	bool compare_cell_parameters_and_connections(const RTLIL::Cell *cell1, const RTLIL::Cell *cell2)
@@ -268,13 +266,13 @@ struct OptMergeWorker
 			}
 
 			did_something = false;
-			dict<std::string, RTLIL::Cell*> sharemap;
+                        dict<uint64_t, RTLIL::Cell*> sharemap;
 			for (auto cell : cells)
 			{
 				if ((!mode_share_all && !ct.cell_known(cell->type)) || !cell->known())
 					continue;
 
-				auto hash = hash_cell_parameters_and_connections(cell);
+				uint64_t hash = hash_cell_parameters_and_connections(cell);
 				auto r = sharemap.insert(std::make_pair(hash, cell));
 				if (!r.second) {
 					if (compare_cell_parameters_and_connections(cell, r.first->second)) {
diff --git a/passes/sat/recover_names.cc b/passes/sat/recover_names.cc
index 2d7e7f01cfa..4c30a363202 100644
--- a/passes/sat/recover_names.cc
+++ b/passes/sat/recover_names.cc
@@ -29,13 +29,6 @@
 
 USING_YOSYS_NAMESPACE
 
-template<> struct hashlib::hash_ops<uint64_t> : hashlib::hash_int_ops
-{
-	static inline unsigned int hash(uint64_t a) {
-		return mkhash((unsigned int)(a), (unsigned int)(a >> 32));
-	}
-};
-
 PRIVATE_NAMESPACE_BEGIN
 
 // xorshift128 params
@@ -453,7 +446,7 @@ struct RecoverNamesWorker {
     pool<IdString> comb_whiteboxes, buffer_types;
 
     // class -> (gold, (gate, inverted))
-    dict<equiv_cls_t, std::pair<pool<IdBit>, dict<IdBit, bool>>> cls2bits; 
+    dict<equiv_cls_t, std::pair<pool<IdBit>, dict<IdBit, bool>>> cls2bits;
 
     void analyse_boxes()
     {