diff --git a/libcpp/ChangeLog b/libcpp/ChangeLog
index c72804f2cb0..50fa2535888 100644
--- a/libcpp/ChangeLog
+++ b/libcpp/ChangeLog
@@ -1,3 +1,8 @@
+2018-08-07  Nathan Sidwell  <nathan@acm.org>
+
+	* line-map.c: (linemap_init): Set default allocator here.
+	(new_linemap): Rather than here.  Refactor allocation logic.
+
 2018-07-20  David Malcolm  <dmalcolm@redhat.com>
 
 	* include/line-map.h (rich_location::set_range): Remove redundant
diff --git a/libcpp/line-map.c b/libcpp/line-map.c
index a1a765f14fd..a5dc485e79e 100644
--- a/libcpp/line-map.c
+++ b/libcpp/line-map.c
@@ -346,6 +346,8 @@ linemap_init (struct line_maps *set,
 #else
   new (set) line_maps();
 #endif
+  /* Set default reallocator (used for initial alloc too).  */
+  set->reallocator = xrealloc;
   set->highest_location = RESERVED_LOCATION_COUNT - 1;
   set->highest_line = RESERVED_LOCATION_COUNT - 1;
   set->location_adhoc_data_map.htab =
@@ -376,82 +378,60 @@ linemap_check_files_exited (struct line_maps *set)
 static struct line_map *
 new_linemap (struct line_maps *set,  source_location start_location)
 {
-  struct line_map *result;
-  bool macro_map_p = start_location >= LINE_MAP_MAX_LOCATION;
+  bool macro_p = start_location >= LINE_MAP_MAX_LOCATION;
+  unsigned num_maps_allocated = LINEMAPS_ALLOCATED (set, macro_p);
+  unsigned num_maps_used = LINEMAPS_USED (set, macro_p);
 
-  if (LINEMAPS_USED (set, macro_map_p) == LINEMAPS_ALLOCATED (set, macro_map_p))
+  if (num_maps_used == num_maps_allocated)
     {
-      /* We ran out of allocated line maps. Let's allocate more.  */
-      size_t alloc_size;
+      /* We need more space!  */
+      if (!num_maps_allocated)
+	num_maps_allocated = 128;
+      num_maps_allocated *= 2;
 
-      /* Cast away extern "C" from the type of xrealloc.  */
-      line_map_realloc reallocator = (set->reallocator
-				      ? set->reallocator
-				      : (line_map_realloc) xrealloc);
-      line_map_round_alloc_size_func round_alloc_size =
-	set->round_alloc_size;
-
-      size_t map_size = (macro_map_p
-			 ? sizeof (line_map_macro)
-			 : sizeof (line_map_ordinary));
+      size_t size_of_a_map;
+      void *buffer;
+      if (macro_p)
+	{
+	  size_of_a_map = sizeof (line_map_macro);
+	  buffer = set->info_macro.maps;
+	}
+      else
+	{
+	  size_of_a_map = sizeof (line_map_ordinary);
+	  buffer = set->info_ordinary.maps;
+	}
 
       /* We are going to execute some dance to try to reduce the
 	 overhead of the memory allocator, in case we are using the
 	 ggc-page.c one.
 	 
 	 The actual size of memory we are going to get back from the
-	 allocator is the smallest power of 2 that is greater than the
-	 size we requested.  So let's consider that size then.  */
-
-      alloc_size =
-	(2 * LINEMAPS_ALLOCATED (set, macro_map_p) +  256)
-	* map_size;
-
-      /* Get the actual size of memory that is going to be allocated
-	 by the allocator.  */
-      alloc_size = round_alloc_size (alloc_size);
+	 allocator may well be larger than what we ask for.  Use this
+	 hook to find what that size is.  */
+      size_t alloc_size
+	= set->round_alloc_size (num_maps_allocated * size_of_a_map);
 
       /* Now alloc_size contains the exact memory size we would get if
 	 we have asked for the initial alloc_size amount of memory.
-	 Let's get back to the number of macro map that amounts
-	 to.  */
-      LINEMAPS_ALLOCATED (set, macro_map_p) =
-	alloc_size / map_size;
+	 Let's get back to the number of map that amounts to.  */
+      unsigned num_maps = alloc_size / size_of_a_map;
+      buffer = set->reallocator (buffer, num_maps * size_of_a_map);
+      memset ((char *)buffer + num_maps_used * size_of_a_map, 0,
+	      (num_maps - num_maps_used) * size_of_a_map);
+      if (macro_p)
+	set->info_macro.maps = (line_map_macro *)buffer;
+      else
+	set->info_ordinary.maps = (line_map_ordinary *)buffer;
+      LINEMAPS_ALLOCATED (set, macro_p) = num_maps;
+    }
 
-      /* And now let's really do the re-allocation.  */
-      if (macro_map_p)
-	{
-	  set->info_macro.maps
-	    = (line_map_macro *) (*reallocator) (set->info_macro.maps,
-						 (LINEMAPS_ALLOCATED (set, macro_map_p)
-						  * map_size));
-	  result = &set->info_macro.maps[LINEMAPS_USED (set, macro_map_p)];
-	}
-      else
-	{
-	  set->info_ordinary.maps =
-	    (line_map_ordinary *) (*reallocator) (set->info_ordinary.maps,
-						  (LINEMAPS_ALLOCATED (set, macro_map_p)
-						   * map_size));
-	  result = &set->info_ordinary.maps[LINEMAPS_USED (set, macro_map_p)];
-	}
-      memset (result, 0,
-	      ((LINEMAPS_ALLOCATED (set, macro_map_p)
-		- LINEMAPS_USED (set, macro_map_p))
-	       * map_size));
-    }
-  else
-    {
-      if (macro_map_p)
-	result = &set->info_macro.maps[LINEMAPS_USED (set, macro_map_p)];
-      else
-	result = &set->info_ordinary.maps[LINEMAPS_USED (set, macro_map_p)];
-    }
+  line_map *result = (macro_p ? (line_map *)&set->info_macro.maps[num_maps_used]
+		      : (line_map *)&set->info_ordinary.maps[num_maps_used]);
+  LINEMAPS_USED (set, macro_p)++;
 
   result->start_location = start_location;
 
-  LINEMAPS_USED (set, macro_map_p)++;
-
   return result;
 }