=== modified file 'src/alloc.c'
--- src/alloc.c	2013-12-09 08:23:01 +0000
+++ src/alloc.c	2013-12-11 05:38:41 +0000
@@ -5299,17 +5299,22 @@
 
 #ifdef HAVE_WINDOW_SYSTEM
 
+/* Rather arbitrary but willing to fix Bug#15876.  */
+
+#define FONT_CACHE_THRESHOLD 4096
+
 /* Remove unmarked font-spec and font-entity objects from ENTRY, which is
    (DRIVER-TYPE NUM-FRAMES FONT-CACHE-DATA ...), and return changed entry.  */
 
 static Lisp_Object
-compact_font_cache_entry (Lisp_Object entry)
+compact_font_cache_entry (struct terminal *t, Lisp_Object entry)
 {
   Lisp_Object tail, *prev = &entry;
 
   for (tail = entry; CONSP (tail); tail = XCDR (tail))
     {
       bool drop = 0;
+      ptrdiff_t size = 0;
       Lisp_Object obj = XCAR (tail);
 
       /* Consider OBJ if it is (font-spec . [font-entity font-entity ...]).  */
@@ -5317,8 +5322,9 @@
 	  && !VECTOR_MARKED_P (XFONT_SPEC (XCAR (obj)))
 	  && VECTORP (XCDR (obj)))
 	{
-	  ptrdiff_t i, size = ASIZE (XCDR (obj)) & ~ARRAY_MARK_FLAG;
+	  ptrdiff_t i;
 
+	  size = ASIZE (XCDR (obj)) & ~ARRAY_MARK_FLAG;
 	  /* If font-spec is not marked, most likely all font-entities
 	     are not marked too.  But we must be sure that nothing is
 	     marked within OBJ before we really drop it.  */
@@ -5330,7 +5336,11 @@
 	    drop = 1;
 	}
       if (drop)
-	*prev = XCDR (tail);
+	{
+	  *prev = XCDR (tail);
+	  /* Count font-spec and vector of font-entities.  */
+	  FONT_CACHE_SIZE (t) -= (size + 1);
+	}
       else
 	prev = xcdr_addr (tail);
     }
@@ -5351,10 +5361,20 @@
 
       if (CONSP (cache))
 	{
-	  Lisp_Object entry;
+	  eassert (FONT_CACHE_SIZE (t) >= 0);
+	  if (FONT_CACHE_SIZE (t) > FONT_CACHE_THRESHOLD)
+	    {
+	      Lisp_Object entry;
 
-	  for (entry = XCDR (cache); CONSP (entry); entry = XCDR (entry))
-	    XSETCAR (entry, compact_font_cache_entry (XCAR (entry)));
+	      fprintf (stderr, "GC%ld: font cache compaction %ld -> ",
+		       gcs_done, FONT_CACHE_SIZE (t));
+	      for (entry = XCDR (cache); CONSP (entry); entry = XCDR (entry))
+		XSETCAR (entry, compact_font_cache_entry (t, XCAR (entry)));
+	      fprintf (stderr, "%ld\n", FONT_CACHE_SIZE (t));
+	    }
+	  else
+	    fprintf (stderr, "GC%ld: font cache too small (%d <= %d)\n",
+		     gcs_done, FONT_CACHE_SIZE (t), FONT_CACHE_THRESHOLD);
 	}
       mark_object (cache);
     }

=== modified file 'src/font.c'
--- src/font.c	2013-12-10 03:36:36 +0000
+++ src/font.c	2013-12-11 05:35:25 +0000
@@ -2740,6 +2740,8 @@
 	    copy = copy_font_spec (scratch_font_spec);
 	    ASET (copy, FONT_TYPE_INDEX, driver_list->driver->type);
 	    XSETCDR (cache, Fcons (Fcons (copy, val), XCDR (cache)));
+	    /* Count font-spec and a vector of font-entities.  */
+	    FONT_CACHE_SIZE (f->terminal) += ASIZE (val) + 1;
 	  }
 	if (ASIZE (val) > 0
 	    && (need_filtering
@@ -2793,6 +2795,8 @@
 	    copy = copy_font_spec (work);
 	    ASET (copy, FONT_TYPE_INDEX, driver_list->driver->type);
 	    XSETCDR (cache, Fcons (Fcons (copy, entity), XCDR (cache)));
+	    /* Count font-spec and entity.  */
+	    FONT_CACHE_SIZE (f->terminal) += 2;
 	  }
 	if (! NILP (entity))
 	  break;

=== modified file 'src/nsterm.h'
--- src/nsterm.h	2013-12-07 16:48:12 +0000
+++ src/nsterm.h	2013-12-11 05:35:25 +0000
@@ -560,6 +560,9 @@
   /* This is a cons cell of the form (NAME . FONT-LIST-CACHE).  */
   Lisp_Object name_list_element;
 
+  /* Amount of font-entities and font-specs objects in the cache above.  */
+  ptrdiff_t font_cache_size;
+
   /* The number of fonts loaded. */
   int n_fonts;
 

=== modified file 'src/termhooks.h'
--- src/termhooks.h	2013-10-18 12:57:44 +0000
+++ src/termhooks.h	2013-12-11 05:35:25 +0000
@@ -628,12 +628,15 @@
 #if defined (HAVE_X_WINDOWS)
 #define TERMINAL_FONT_CACHE(t)						\
   (t->type == output_x_window ? t->display_info.x->name_list_element : Qnil)
+#define FONT_CACHE_SIZE(t) (t)->display_info.x->font_cache_size
 #elif defined (HAVE_NTGUI)
 #define TERMINAL_FONT_CACHE(t)						\
   (t->type == output_w32 ? t->display_info.w32->name_list_element : Qnil)
+#define FONT_CACHE_SIZE(t) (t)->display_info.w32->font_cache_size
 #elif defined (HAVE_NS)
 #define TERMINAL_FONT_CACHE(t)						\
   (t->type == output_ns ? t->display_info.ns->name_list_element : Qnil)
+#define FONT_CACHE_SIZE(t) (t)->display_info.ns->font_cache_size
 #endif
 
 extern struct terminal *get_terminal (Lisp_Object terminal, bool);

=== modified file 'src/w32term.h'
--- src/w32term.h	2013-12-02 13:35:53 +0000
+++ src/w32term.h	2013-12-11 05:35:25 +0000
@@ -74,6 +74,9 @@
   /* This is a cons cell of the form (NAME . FONT-LIST-CACHE).  */
   Lisp_Object name_list_element;
 
+  /* Amount of font-entities and font-specs objects in the cache above.  */
+  ptrdiff_t font_cache_size;
+
   /* Number of frames that are on this display.  */
   int reference_count;
 

=== modified file 'src/xterm.h'
--- src/xterm.h	2013-12-07 23:04:10 +0000
+++ src/xterm.h	2013-12-11 05:35:25 +0000
@@ -140,6 +140,9 @@
   /* This is a cons cell of the form (NAME . FONT-LIST-CACHE).  */
   Lisp_Object name_list_element;
 
+  /* Amount of font-entities and font-specs objects in the cache above.  */
+  ptrdiff_t font_cache_size;
+
   /* Number of frames that are on this display.  */
   int reference_count;