diff --git a/common/hash_table.h b/common/hash_table.h
new file mode 100644
index 00000000..759580da
--- /dev/null
+++ b/common/hash_table.h
@@ -0,0 +1,48 @@
+/*
+ *  nextpnr -- Next Generation Place and Route
+ *
+ *  Copyright (C) 2021  Symbiflow Authors
+ *
+ *  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.
+ *
+ */
+
+#ifndef HASH_TABLE_H
+#define HASH_TABLE_H
+
+#if defined(NPNR_DISABLE_THREADS)
+#include <unordered_map>
+#include <unordered_set>
+#else
+#include <absl/container/flat_hash_map.h>
+#include <absl/container/flat_hash_set.h>
+#endif
+
+#include "nextpnr_namespaces.h"
+
+NEXTPNR_NAMESPACE_BEGIN
+
+namespace HashTables {
+#if defined(NPNR_DISABLE_THREADS)
+template <typename Key, typename Value> using HashMap = std::unordered_map<Key, Value>;
+template <typename Value> using HashSet = std::unordered_set<Value>;
+#else
+template <typename Key, typename Value> using HashMap = absl::flat_hash_map<Key, Value>;
+template <typename Value> using HashSet = absl::flat_hash_set<Value>;
+#endif
+
+}; // namespace HashTables
+
+NEXTPNR_NAMESPACE_END
+
+#endif /* HASH_TABLE_H */
diff --git a/common/router2.cc b/common/router2.cc
index 27f8ec0d..002a05a3 100644
--- a/common/router2.cc
+++ b/common/router2.cc
@@ -27,15 +27,15 @@
  */
 
 #include "router2.h"
-#if !defined(NPNR_DISABLE_THREADS)
-#include <absl/container/flat_hash_map.h>
-#endif
+
 #include <algorithm>
 #include <boost/container/flat_map.hpp>
 #include <chrono>
 #include <deque>
 #include <fstream>
 #include <queue>
+
+#include "hash_table.h"
 #include "log.h"
 #include "nextpnr.h"
 #include "router1.h"
@@ -193,11 +193,7 @@ struct Router2
         }
     }
 
-#if defined(NPNR_DISABLE_THREADS)
-    std::unordered_map<WireId, int> wire_to_idx;
-#else
-    absl::flat_hash_map<WireId, int> wire_to_idx;
-#endif
+    HashTables::HashMap<WireId, int> wire_to_idx;
     std::vector<PerWireData> flat_wires;
 
     PerWireData &wire_data(WireId w) { return flat_wires[wire_to_idx.at(w)]; }