diff --git a/gc.c b/gc.c
index e492082d..93401c33 100644
--- a/gc.c
+++ b/gc.c
@@ -1863,28 +1863,29 @@ void gc_sum_pending_writes(gc_thread_data * thd, int locked)
   }
 }
 
-/**
- * @brief Determine if object lives on the thread's stack
- * @param thd Mutator's thread data
- * @param obj Object to inspect
- * @return True if `obj` is on the mutator's stack, false otherwise
- */
-int gc_is_stack_obj(gc_thread_data * thd, object obj)
-{
-  char tmp;
-  object low_limit = &tmp;
-  object high_limit = thd->stack_start;
-  return (stack_overflow(low_limit, obj) && stack_overflow(obj, high_limit));
-}
+// /**
+//  * @brief Determine if object lives on the thread's stack
+//  * @param thd Mutator's thread data
+//  * @param obj Object to inspect
+//  * @return True if `obj` is on the mutator's stack, false otherwise
+//  */
+// int gc_is_stack_obj(gc_thread_data * thd, object obj)
+// {
+//   char tmp;
+//   object low_limit = &tmp;
+//   object high_limit = thd->stack_start;
+//   return (stack_overflow(low_limit, obj) && stack_overflow(obj, high_limit));
+// }
 
 /**
  * @brief Helper function for `gc_mut_update`
  */
 static void mark_stack_or_heap_obj(gc_thread_data * thd, object obj, int locked)
 {
+  char tmp;
   if (!is_object_type(obj) || type_of(obj) == boolean_tag) {
     return;
-  } else if (gc_is_stack_obj(thd, obj)) {
+  } else if (gc_is_stack_obj(&tmp, thd, obj)) {
     // Set object to be marked after moved to heap by next GC.
     // This avoids having to recursively examine the stack now, 
     // which we have to do anyway during minor GC.
@@ -2937,7 +2938,7 @@ void gc_mutator_thread_runnable(gc_thread_data * thd, object result, object mayb
     // Check if obj was copied while we slept
     if (maybe_copied && 
         is_object_type(maybe_copied) && 
-        gc_is_stack_obj(thd, maybe_copied) &&
+        gc_is_stack_obj(&stack_limit, thd, maybe_copied) &&
         type_of(maybe_copied) == forward_tag) {
       gc_recopy_obj(maybe_copied, thd);
     }
diff --git a/include/cyclone/types.h b/include/cyclone/types.h
index 8a3a5156..75f25b2d 100644
--- a/include/cyclone/types.h
+++ b/include/cyclone/types.h
@@ -395,7 +395,16 @@ void gc_thread_data_init(gc_thread_data * thd, int mut_num, char *stack_base,
                          long stack_size);
 void gc_thread_data_free(gc_thread_data * thd);
 // Prototypes for mutator/collector:
-int gc_is_stack_obj(gc_thread_data * thd, object obj);
+/**
+ * @brief Determine if object lives on the thread's stack
+ * @param low_limit Temporary object at the current "end" of the stack
+ * @param thd Mutator's thread data
+ * @param obj Object to inspect
+ * @return True if `obj` is on the mutator's stack, false otherwise
+ */
+#define gc_is_stack_obj(low_limit, thd, obj) \
+ (stack_overflow(((object)low_limit), ((object)obj)) && \
+  stack_overflow(((object)obj), ((object)((gc_thread_data *)thd)->stack_start)))
 void gc_mut_update(gc_thread_data * thd, object old_obj, object value);
 void gc_mut_cooperate(gc_thread_data * thd, int buf_len);
 void gc_mark_gray(gc_thread_data * thd, object obj);
diff --git a/libs/cyclone/concurrent.sld b/libs/cyclone/concurrent.sld
index 913f8a5c..13837836 100644
--- a/libs/cyclone/concurrent.sld
+++ b/libs/cyclone/concurrent.sld
@@ -101,7 +101,7 @@
     atomic atm;
     atomic_type tmp;
     Cyc_verify_immutable(data, obj); // TODO: verify obj is not on local stack???
-    if (gc_is_stack_obj(data, obj)){
+    if (gc_is_stack_obj(&tmp, data, obj)){
       Cyc_rt_raise2(data, \"Atom cannot contain a thread-local object\", obj);
     }
     tmp.hdr.mark = gc_color_red;
@@ -157,9 +157,10 @@
 (define-c compare-and-set!
   "(void *data, int argc, closure _, object k, object obj, object oldval, object newval)"
   " atomic a;
+    char tmp;
     Cyc_check_atomic(data, obj);
     Cyc_verify_immutable(data, newval);
-    if (gc_is_stack_obj(data, obj)){
+    if (gc_is_stack_obj(&tmp, data, obj)){
       Cyc_rt_raise2(data, \"Atom cannot contain a thread-local object\", obj);
     }
     a = (atomic) obj;
diff --git a/runtime.c b/runtime.c
index 911ebdff..462309fd 100644
--- a/runtime.c
+++ b/runtime.c
@@ -487,13 +487,14 @@ void Cyc_set_globals_changed(gc_thread_data *thd)
 void add_mutation(void *data, object var, int index, object value)
 {
   gc_thread_data *thd = (gc_thread_data *) data;
+  char tmp;
 
   // No need to track for minor GC purposes unless we are mutating
   // a heap variable to point to a stack var.
   //
   // If var is on stack we'll get it anyway in minor GC,
   // and if value is on heap we don't care (no chance of heap pointing to nursery)
-  if (!gc_is_stack_obj(data, var) && gc_is_stack_obj(data, value)) {
+  if (!gc_is_stack_obj(&tmp, data, var) && gc_is_stack_obj(&tmp, data, value)) {
     thd->mutations = vpbuffer_add(thd->mutations, 
                                   &(thd->mutation_buflen), 
                                   thd->mutation_count, 
@@ -5777,7 +5778,7 @@ void Cyc_make_shared_object(void *data, object k, object obj)
   object buf[1];
   int tmp, *heap_grown = &tmp;
   if (!is_object_type(obj) || // Immediates do not have to be moved
-      !gc_is_stack_obj(data, obj)) { // Not thread-local, assume already on heap
+      !gc_is_stack_obj(&tmp, data, obj)) { // Not thread-local, assume already on heap
     return_closcall1(data, k, obj);
   }