[backport from gcc-4.7/trunk r183213 <mikpe@it.uu.se>]

gcc/

2012-01-16  Jason Merrill  <jason@redhat.com>

	PR c++/14179
	* vec.c (vec_gc_o_reserve_1): Use ggc_round_alloc_size.

--- gcc-4.6.2/gcc/vec.c.~1~	2011-01-03 21:52:22.000000000 +0100
+++ gcc-4.6.2/gcc/vec.c	2012-01-18 16:15:32.000000000 +0100
@@ -229,6 +229,7 @@ vec_gc_o_reserve_1 (void *vec, int reser
 {
   struct vec_prefix *pfx = (struct vec_prefix *) vec;
   unsigned alloc = calculate_allocation (pfx, reserve, exact);
+  size_t size;
 
   if (!alloc)
     {
@@ -237,7 +238,17 @@ vec_gc_o_reserve_1 (void *vec, int reser
       return NULL;
     }
 
-  vec = ggc_realloc_stat (vec, vec_offset + alloc * elt_size PASS_MEM_STAT);
+  /* Calculate the amount of space we want.  */
+  size = vec_offset + alloc * elt_size;
+  /* Ask the allocator how much space it will really give us.  */
+  size = ggc_round_alloc_size (size);
+  /* Adjust the number of slots accordingly.  */
+  alloc = (size - vec_offset) / elt_size;
+  /* And finally, recalculate the amount of space we ask for.  */
+  size = vec_offset + alloc * elt_size;
+
+  vec = ggc_realloc_stat (vec, size PASS_MEM_STAT);
+
   ((struct vec_prefix *)vec)->alloc = alloc;
   if (!pfx)
     ((struct vec_prefix *)vec)->num = 0;