gc bugfix: maintain free_tail properly.

* gc.c (more): Only set the free_tail if the
new memory is being added to an empty free_list.
Otherwise free_tail is valid.
(make_obj): If we empty the free_list, then
reset the free_tail pointer, so it doesn't
continue pointing into the object we are returning.
(sweep): Remove defensive code which tries to reset
the free_tail in the empty free_list case,
but assumes that free_tail is correct in the non-empty
case.

1 files changed, 6 insertions, 5 deletions

diff --git a/gc.c b/gc.c
index a06e7d57..ab7eb7f8 100644
--- a/gc.c
+++ b/gc.c
@@ -130,6 +130,9 @@ static void more(void)
   heap_t *heap = coerce(heap_t *, chk_malloc_gc_more(sizeof *heap));
   obj_t *block = heap->block, *end = heap->block + HEAP_SIZE;
 
+  if (free_list == 0)
+    free_tail = &heap->block[0].t.next;
+
   if (end > heap_max_bound)
     heap_max_bound = end;
 
@@ -142,8 +145,6 @@ static void more(void)
     free_list = block++;
   }
 
-  free_tail = &heap->block[0].t.next;
-
   heap->next = heap_list;
   heap_list = heap;
 
@@ -183,6 +184,9 @@ val make_obj(void)
         VALGRIND_MAKE_MEM_DEFINED(free_list, sizeof *free_list);
 #endif
       free_list = free_list->t.next;
+
+      if (free_list == 0)
+        free_tail = &free_list;
 #if HAVE_VALGRIND
       if (opt_vg_debug)
         VALGRIND_MAKE_MEM_UNDEFINED(ret, sizeof *ret);
@@ -542,9 +546,6 @@ static int_ptr_t sweep(void)
   const int vg_dbg = opt_vg_debug;
 #endif
 
-  if (free_list == 0)
-    free_tail = &free_list;
-
 #if CONFIG_GEN_GC
   if (!full_gc) {
     int i;