Do not iterate over heap blocks in GC_get_memory_use

GC_get_memory_use() return the total memory use (in bytes) by all
allocated blocks, but this is equal to GC_get_heap_size() minus
GC_get_free_bytes().

* include/gc/gc.h (GC_get_memory_use): Refine comment.
* misc.c (block_add_size): Remove static function definition.
* misc.c (GC_get_memory_use): Add assertion that
GC_heapsize>=GC_large_free_bytes; set bytes to
GC_heapsize-GC_large_free_bytes instead of calling
GC_apply_to_all_blocks(block_add_size).

include/gc/gc.h

@@ -919,8 +919,9 @@ GC_API size_t GC_CALL GC_get_prof_stats(struct GC_prof_stats_s *,
 /* typically to avoid data race on multiprocessors.                     */
 GC_API size_t GC_CALL GC_get_size_map_at(int i);
 
-/* Count total memory use (in bytes) by all allocated blocks.  Acquires */
-/* the allocator lock in the reader mode.                               */
+/* Return the total memory use (in bytes) by all allocated blocks.      */
+/* The result is equal to GC_get_heap_size() - GC_get_free_bytes().     */
+/* Acquires the allocator lock in the reader mode.                      */
 GC_API GC_word GC_CALL GC_get_memory_use(void);
 
 /* Disable garbage collection.  Even GC_gcollect calls will be          */

misc.c

@@ -2483,22 +2483,13 @@ GC_API void * GC_CALL GC_do_blocking(GC_fn_type fn, void * client_data)
   }
 #endif /* !NO_DEBUGGING */
 
-static void GC_CALLBACK block_add_size(struct hblk *h, void *pbytes)
-{
-  const hdr *hhdr = HDR(h);
-
-  *(word *)pbytes += (hhdr -> hb_sz + HBLKSIZE-1) & ~(HBLKSIZE-1);
-# if defined(CPPCHECK)
-    GC_noop1_ptr(h);
-# endif
-}
-
 GC_API GC_word GC_CALL GC_get_memory_use(void)
 {
-  word bytes = 0;
+  word bytes;
 
   READER_LOCK();
-  GC_apply_to_all_blocks(block_add_size, &bytes);
+  GC_ASSERT(GC_heapsize >= GC_large_free_bytes);
+  bytes = GC_heapsize - GC_large_free_bytes;
   READER_UNLOCK();
   return bytes;
 }