Revert changes to TryMark(), as they break conservative stack scanning.

src/julia_gc.c

@@ -408,13 +408,37 @@ static inline int JMark(void * obj)
 static void TryMark(void * p)
 {
     jl_value_t * p2 = jl_gc_internal_obj_base_ptr(p);
-    if (p2) {
+    if (!p2) {
+        // It is possible for p to point past the end of
+        // the object, so we subtract one word from the
+        // address. This is safe, as the object is preceded
+        // by a larger header.
+        p2 = treap_find(bigvals, (char *)p - 1);
+        if (p2) {
+            // It is possible for types to not be valid objects.
+            // Objects with such types are not normally made visible
+            // to the mark loop, so we need to avoid marking them
+            // during conservative stack scanning.
+            // While jl_gc_internal_obj_base_ptr(p) already eliminates this
+            // case, it can still happen for bigval_t objects, so
+            // we run an explicit check that the type is a valid
+            // object for these.
+            p2 = (jl_value_t *)((char *)p2 + bigval_startoffset);
+            jl_taggedvalue_t * hdr = jl_astaggedvalue(p2);
+            if (hdr->type != jl_gc_internal_obj_base_ptr(hdr->type))
+                return;
+        }
+    } else {
         if (jl_typeis(p2, datatype_mptr))
-            MarkCache[MARK_HASH((UInt)p2)] = (Bag)p2;
+            MarkCache[MARK_HASH((UInt)p2)] = (Bag) p2;
+    }
+    if (p2) {
         JMark(p2);
     }
 }
 
+
+
 static void TryMarkRange(void * start, void * end)
 {
     if (lt_ptr(end, start)) {