diff --git a/src/hb-face-private.hh b/src/hb-face-private.hh
index 43e7b1c..eb0e850 100644
--- a/src/hb-face-private.hh
+++ b/src/hb-face-private.hh
@@ -54,6 +54,13 @@
   mutable unsigned int upem;		/* Units-per-EM. */
   mutable unsigned int num_glyphs;	/* Number of glyphs. */
 
+  enum dirty_t {
+    NOTHING	= 0x0000,
+    INDEX	= 0x0001,
+    UPEM	= 0x0002,
+    NUM_GLYPHS	= 0x0004,
+  } dirty;
+
   struct hb_shaper_data_t shaper_data;	/* Various shaper data. */
 
   /* Various non-shaping data. */
@@ -99,6 +106,8 @@
   HB_INTERNAL void load_num_glyphs (void) const;
 };
 
+HB_MARK_AS_FLAG_T (hb_face_t::dirty_t);
+
 extern HB_INTERNAL const hb_face_t _hb_face_nil;
 
 #define HB_SHAPER_DATA_CREATE_FUNC_EXTRA_ARGS
diff --git a/src/hb-face.cc b/src/hb-face.cc
index 1ba9707..1800c99 100644
--- a/src/hb-face.cc
+++ b/src/hb-face.cc
@@ -51,6 +51,8 @@
   1000, /* upem */
   0,    /* num_glyphs */
 
+  hb_face_t::NOTHING, /* dirty */
+
   {
 #define HB_SHAPER_IMPLEMENT(shaper) HB_SHAPER_DATA_INVALID,
 #include "hb-shaper-list.hh"
@@ -171,7 +173,7 @@
 				    closure,
 				    (hb_destroy_func_t) _hb_face_for_data_closure_destroy);
 
-  hb_face_set_index (face, index);
+  face->index = index;
 
   return face;
 }
@@ -365,6 +367,11 @@
   if (face->immutable)
     return;
 
+  if (face->index == index)
+    return;
+
+  face->dirty |= face->INDEX;
+
   face->index = index;
 }
 
@@ -400,6 +407,11 @@
   if (face->immutable)
     return;
 
+  if (face->upem == upem)
+    return;
+
+  face->dirty |= face->UPEM;
+
   face->upem = upem;
 }
 
@@ -444,6 +456,11 @@
   if (face->immutable)
     return;
 
+  if (face->num_glyphs == glyph_count)
+    return;
+
+  face->dirty |= face->NUM_GLYPHS;
+
   face->num_glyphs = glyph_count;
 }
 
diff --git a/src/hb-font.cc b/src/hb-font.cc
index cc0e6c3..a08766f 100644
--- a/src/hb-font.cc
+++ b/src/hb-font.cc
@@ -1196,7 +1196,7 @@
     NULL, /* user_data */
     NULL, /* destroy */
 
-    hb_font_t::NOTHING, /* dirty_bits */
+    hb_font_t::NOTHING, /* dirty */
 
     {
 #define HB_SHAPER_IMPLEMENT(shaper) HB_SHAPER_DATA_INVALID,