diff --git a/src/hb-bit-set.hh b/src/hb-bit-set.hh
index d4a227352..8197ebca4 100644
--- a/src/hb-bit-set.hh
+++ b/src/hb-bit-set.hh
@@ -47,6 +47,7 @@ struct hb_bit_set_t
     if (likely (!a.successful || !b.successful))
       return;
     hb_swap (a.population, b.population);
+    hb_swap (a.hash_, b.hash_);
     hb_swap (a.last_page_lookup, b.last_page_lookup);
     hb_swap (a.page_map, b.page_map);
     hb_swap (a.pages, b.pages);
@@ -56,6 +57,7 @@ struct hb_bit_set_t
   {
     successful = true;
     population = 0;
+    hash_ = 0;
     last_page_lookup = 0;
     page_map.init ();
     pages.init ();
@@ -78,6 +80,7 @@ struct hb_bit_set_t
 
   bool successful = true; /* Allocations successful */
   mutable unsigned int population = 0;
+  mutable unsigned int hash_ = 0;
   mutable unsigned int last_page_lookup = 0;
   hb_sorted_vector_t<page_map_t> page_map;
   hb_vector_t<page_t> pages;
@@ -114,7 +117,10 @@ struct hb_bit_set_t
   {
     resize (0);
     if (likely (successful))
+    {
       population = 0;
+      hash_ = 0;
+    }
   }
   bool is_empty () const
   {
@@ -126,16 +132,26 @@ struct hb_bit_set_t
   }
   explicit operator bool () const { return !is_empty (); }
 
+  bool has_hash () const { return hash_ != UINT_MAX; }
   unsigned hash () const
   {
+    if (has_hash ()) return hash_;
+
     unsigned h = 0;
     for (auto &map : page_map)
       h = h * 31 + hb_hash (map.major) + hb_hash (pages[map.index]);
+
+    hash_ = h;
+
     return h;
   }
 
   private:
-  void dirty () { population = UINT_MAX; }
+  void dirty ()
+  {
+    population = UINT_MAX;
+    hash_ = UINT_MAX;
+  }
   public:
 
   void add (hb_codepoint_t g)
@@ -355,6 +371,7 @@ struct hb_bit_set_t
     if (unlikely (!resize (count)))
       return;
     population = other.population;
+    hash_ = other.hash_;
 
     /* TODO switch to vector operator =. */
     hb_memcpy ((void *) pages, (const void *) other.pages, count * pages.item_size);
@@ -364,7 +381,10 @@ struct hb_bit_set_t
   bool is_equal (const hb_bit_set_t &other) const
   {
     if (has_population () && other.has_population () &&
-	get_population () != other.get_population ())
+	population != other.population)
+      return false;
+    if (has_hash () && other.has_hash () &&
+	hash_ != other.hash_)
       return false;
 
     unsigned int na = pages.length;
@@ -392,7 +412,7 @@ struct hb_bit_set_t
   bool is_subset (const hb_bit_set_t &larger_set) const
   {
     if (has_population () && larger_set.has_population () &&
-	get_population () != larger_set.get_population ())
+	population != larger_set.population)
       return false;
 
     uint32_t spi = 0;