Convert RMoved to a doubly linked list

This commit converts RMoved slots to a doubly linked list. I want to
convert this to a doubly linked list because the read barrier (currently
in development) must remove nodes from the moved list sometimes.
Removing nodes from the list is much easier if the list is doubly
linked.  In addition, we can reuse the list manipulation routines.

gc.c

@@ -530,7 +530,10 @@ typedef struct gc_profile_record {
 struct RMoved {
     VALUE flags;
     VALUE destination;
-    VALUE next;
+    union {
+        struct list_node node;
+        struct list_head head;
+    } as;
 };
 
 #if defined(_MSC_VER) || defined(__CYGWIN__)
@@ -7614,7 +7617,7 @@ gc_is_moveable_obj(rb_objspace_t *objspace, VALUE obj)
 }
 
 static VALUE
-gc_move(rb_objspace_t *objspace, VALUE scan, VALUE free, VALUE moved_list)
+gc_move(rb_objspace_t *objspace, VALUE scan, VALUE free, struct RMoved * moved_list)
 {
     int marked;
     int wb_unprotected;
@@ -7692,7 +7695,7 @@ gc_move(rb_objspace_t *objspace, VALUE scan, VALUE free, VALUE moved_list)
     /* Assign forwarding address */
     src->as.moved.flags = T_MOVED;
     src->as.moved.destination = (VALUE)dest;
-    src->as.moved.next = moved_list;
+    list_add(&moved_list->as.head, &src->as.moved.as.node);
     GC_ASSERT(BUILTIN_TYPE((VALUE)dest) != T_NONE);
 
     return (VALUE)src;
@@ -7822,15 +7825,13 @@ allocate_page_list(rb_objspace_t *objspace, page_compare_func_t *comparator)
     return page_list;
 }
 
-static VALUE
-gc_compact_heap(rb_objspace_t *objspace, page_compare_func_t *comparator)
+static void
+gc_compact_heap(rb_objspace_t *objspace, page_compare_func_t *comparator, struct RMoved * moved_list)
 {
     struct heap_cursor free_cursor;
     struct heap_cursor scan_cursor;
     struct heap_page **page_list;
-    VALUE moved_list;
 
-    moved_list = Qfalse;
     memset(objspace->rcompactor.considered_count_table, 0, T_MASK * sizeof(size_t));
     memset(objspace->rcompactor.moved_count_table, 0, T_MASK * sizeof(size_t));
 
@@ -7891,7 +7892,7 @@ gc_compact_heap(rb_objspace_t *objspace, page_compare_func_t *comparator)
             GC_ASSERT(BUILTIN_TYPE((VALUE)scan_cursor.slot) != T_NONE);
             GC_ASSERT(BUILTIN_TYPE((VALUE)scan_cursor.slot) != T_MOVED);
 
-            moved_list = gc_move(objspace, (VALUE)scan_cursor.slot, (VALUE)free_cursor.slot, moved_list);
+            gc_move(objspace, (VALUE)scan_cursor.slot, (VALUE)free_cursor.slot, moved_list);
 
             GC_ASSERT(BUILTIN_TYPE((VALUE)free_cursor.slot) != T_MOVED);
             GC_ASSERT(BUILTIN_TYPE((VALUE)free_cursor.slot) != T_NONE);
@@ -7902,8 +7903,6 @@ gc_compact_heap(rb_objspace_t *objspace, page_compare_func_t *comparator)
         }
     }
     free(page_list);
-
-    return moved_list;
 }
 
 static void
@@ -8614,22 +8613,20 @@ gc_check_references_for_moved(rb_objspace_t *objspace)
 }
 
 static void
-gc_unlink_moved_list(rb_objspace_t *objspace, VALUE moved_list_head)
+gc_unlink_moved_list(rb_objspace_t *objspace, struct RMoved * moved_list_head)
 {
     /* For each moved slot */
-    while (moved_list_head) {
-        VALUE next_moved;
+    struct RMoved * obj = NULL, *next;
+    list_for_each_safe(&RMOVED(moved_list_head)->as.head, obj, next, as.node) {
+        if (obj != moved_list_head) {
             struct heap_page *page;
 
-        page = GET_HEAP_PAGE(moved_list_head);
-        next_moved = RMOVED(moved_list_head)->next;
+            page = GET_HEAP_PAGE((VALUE)obj);
 
             /* clear the memory for that moved slot */
-        RMOVED(moved_list_head)->flags = 0;
-        RMOVED(moved_list_head)->destination = 0;
-        RMOVED(moved_list_head)->next = 0;
+            memset(obj, 0, sizeof(VALUE));
             page->free_slots++;
-        heap_page_add_freeobj(objspace, page, moved_list_head);
+            heap_page_add_freeobj(objspace, page, (VALUE)obj);
 
             if (page->free_slots == page->total_slots && heap_pages_freeable_pages > 0) {
                 heap_pages_freeable_pages--;
@@ -8637,7 +8634,7 @@ gc_unlink_moved_list(rb_objspace_t *objspace, VALUE moved_list_head)
                 heap_add_page(objspace, heap_tomb, page);
             }
             objspace->profile.total_freed_objects++;
-        moved_list_head = next_moved;
+        }
     }
 }
 
@@ -8659,14 +8656,16 @@ gc_compact_after_gc(rb_objspace_t *objspace, int use_toward_empty, int use_doubl
         heap_add_pages(objspace, heap_eden, heap_allocated_pages);
     }
 
-    VALUE moved_list_head;
+    struct RMoved moved_list_head;
+    list_head_init(&moved_list_head.as.head);
+
     VALUE disabled = rb_objspace_gc_disable(objspace);
 
     if (use_toward_empty) {
-        moved_list_head = gc_compact_heap(objspace, compare_free_slots);
+        gc_compact_heap(objspace, compare_free_slots, &moved_list_head);
     }
     else {
-        moved_list_head = gc_compact_heap(objspace, compare_pinned);
+        gc_compact_heap(objspace, compare_pinned, &moved_list_head);
     }
     heap_eden->freelist = NULL;
 
@@ -8681,7 +8680,7 @@ gc_compact_after_gc(rb_objspace_t *objspace, int use_toward_empty, int use_doubl
     heap_eden->free_pages = NULL;
     heap_eden->using_page = NULL;
 
-    gc_unlink_moved_list(objspace, moved_list_head);
+    gc_unlink_moved_list(objspace, &moved_list_head);
 
     /* Add any eden pages with free slots back to the free pages list */
     struct heap_page *page = NULL;