diff --git a/gc-api.h b/gc-api.h
index 4779cbd2c..6c072bc31 100644
--- a/gc-api.h
+++ b/gc-api.h
@@ -1,26 +1,62 @@
-#ifndef GC_TYPES_H_
-#define GC_TYPES_H_
+#ifndef GC_API_H_
+#define GC_API_H_
+
+#include <stdint.h>
+
+#ifndef GC_DEBUG
+#define GC_DEBUG 0
+#endif
+
+#define GC_UNLIKELY(e) __builtin_expect(e, 0)
+#define GC_LIKELY(e) __builtin_expect(e, 1)
+
+#if GC_DEBUG
+#define GC_ASSERT(x) do { if (GC_UNLIKELY(!(x))) __builtin_trap(); } while (0)
+#else
+#define GC_ASSERT(x) do { } while (0)
+#endif
+
+struct gc_ref {
+  uintptr_t value;
+};
+
+static inline struct gc_ref gc_ref(uintptr_t value) {
+  return (struct gc_ref){value};
+}
+static inline uintptr_t gc_ref_value(struct gc_ref ref) {
+  return ref.value;
+}
+
+static inline struct gc_ref gc_ref_null(void) {
+  return gc_ref(0);
+}
+static inline int gc_ref_is_heap_object(struct gc_ref ref) {
+  return ref.value != 0;
+}
+static inline struct gc_ref gc_ref_from_heap_object_or_null(void *obj) {
+  return gc_ref((uintptr_t) obj);
+}
+static inline struct gc_ref gc_ref_from_heap_object(void *obj) {
+  GC_ASSERT(obj);
+  return gc_ref_from_heap_object_or_null(obj);
+}
+static inline void* gc_ref_heap_object(struct gc_ref ref) {
+  GC_ASSERT(gc_ref_is_heap_object(ref));
+  return (void *) gc_ref_value(ref);
+}
 
 struct gc_edge {
-  union {
-    void *addr;
-    void **loc;
-  };
+  struct gc_ref *dst;
 };
 
 static inline struct gc_edge gc_edge(void* addr) {
-  struct gc_edge edge;
-  edge.addr = addr;
-  return edge;
+  return (struct gc_edge){addr};
 }
-static inline struct gc_edge object_field(void* addr) {
-  return gc_edge(addr);
+static struct gc_ref gc_edge_ref(struct gc_edge edge) {
+  return *edge.dst;
 }
-static inline void* dereference_edge(struct gc_edge edge) {
-  return *edge.loc;
-}
-static inline void update_edge(struct gc_edge edge, void *value) {
-  *edge.loc = value;
+static inline void gc_edge_update(struct gc_edge edge, struct gc_ref ref) {
+  *edge.dst = ref;
 }
 
-#endif // GC_TYPES_H_
+#endif // GC_API_H_
diff --git a/mt-gcbench.c b/mt-gcbench.c
index 1819655fb..4d1c92255 100644
--- a/mt-gcbench.c
+++ b/mt-gcbench.c
@@ -88,8 +88,8 @@ static inline void
 visit_node_fields(Node *node,
                   void (*visit)(struct gc_edge edge, void *visit_data),
                   void *visit_data) {
-  visit(object_field(&node->left), visit_data);
-  visit(object_field(&node->right), visit_data);
+  visit(gc_edge(&node->left), visit_data);
+  visit(gc_edge(&node->right), visit_data);
 }
 static inline void
 visit_double_array_fields(DoubleArray *obj,
diff --git a/parallel-tracer.h b/parallel-tracer.h
index 02f641352..92c0a86d4 100644
--- a/parallel-tracer.h
+++ b/parallel-tracer.h
@@ -471,7 +471,8 @@ tracer_visit(struct gc_edge edge, void *trace_data) {
   if (trace_edge(trace->heap, edge)) {
     if (local_trace_queue_full(&trace->local))
       tracer_share(trace);
-    local_trace_queue_push(&trace->local, dereference_edge(edge));
+    local_trace_queue_push(&trace->local,
+                           gc_ref_heap_object(gc_edge_ref(edge)));
   }
 }
 
diff --git a/quads.c b/quads.c
index 9743b88a8..d2cba2bf0 100644
--- a/quads.c
+++ b/quads.c
@@ -18,7 +18,7 @@ visit_quad_fields(Quad *quad,
                   void (*visit)(struct gc_edge edge, void *visit_data),
                   void *visit_data) {
   for (size_t i = 0; i < 4; i++)
-    visit(object_field(&quad->kids[i]), visit_data);
+    visit(gc_edge(&quad->kids[i]), visit_data);
 }
 typedef HANDLE_TO(Quad) QuadHandle;
 
diff --git a/semi.h b/semi.h
index e2769fe7f..7cd776398 100644
--- a/semi.h
+++ b/semi.h
@@ -144,7 +144,7 @@ static void* forward(struct semi_space *space, void *obj) {
 
 static void visit_semi_space(struct heap *heap, struct semi_space *space,
                              struct gc_edge edge, void *obj) {
-  update_edge(edge, forward(space, obj));
+  gc_edge_update(edge, gc_ref_from_heap_object(forward(space, obj)));
 }
 
 static void visit_large_object_space(struct heap *heap,
@@ -160,10 +160,11 @@ static int semi_space_contains(struct semi_space *space, void *obj) {
 
 static void visit(struct gc_edge edge, void *visit_data) {
   struct heap *heap = visit_data;
-  void *obj = dereference_edge(edge);
-  if (obj == NULL)
+  struct gc_ref ref = gc_edge_ref(edge);
+  if (!gc_ref_is_heap_object(ref))
     return;
-  else if (semi_space_contains(heap_semi_space(heap), obj))
+  void *obj = gc_ref_heap_object(ref);
+  if (semi_space_contains(heap_semi_space(heap), obj))
     visit_semi_space(heap, heap_semi_space(heap), edge, obj);
   else if (large_object_space_contains(heap_large_object_space(heap), obj))
     visit_large_object_space(heap, heap_large_object_space(heap), obj);
diff --git a/serial-tracer.h b/serial-tracer.h
index 81664ddf3..3376d9608 100644
--- a/serial-tracer.h
+++ b/serial-tracer.h
@@ -156,7 +156,8 @@ static inline void
 tracer_visit(struct gc_edge edge, void *trace_data) {
   struct heap *heap = trace_data;
   if (trace_edge(heap, edge))
-    tracer_enqueue_root(heap_tracer(heap), dereference_edge(edge));
+    tracer_enqueue_root(heap_tracer(heap),
+                        gc_ref_heap_object(gc_edge_ref(edge)));
 }
 static inline void
 tracer_trace(struct heap *heap) {
diff --git a/whippet.h b/whippet.h
index f8232bf44..93936aaae 100644
--- a/whippet.h
+++ b/whippet.h
@@ -423,8 +423,8 @@ static size_t mark_space_live_object_granules(uint8_t *metadata) {
 }
 
 static inline int mark_space_mark_object(struct mark_space *space,
-                                         struct gc_edge edge) {
-  struct gcobj *obj = dereference_edge(edge);
+                                         struct gc_ref ref) {
+  struct gcobj *obj = gc_ref_heap_object(ref);
   uint8_t *loc = object_metadata_byte(obj);
   uint8_t byte = *loc;
   if (byte & space->marked_mask)
@@ -567,8 +567,9 @@ static struct gcobj *evacuation_allocate(struct mark_space *space,
 }
 
 static inline int mark_space_evacuate_or_mark_object(struct mark_space *space,
-                                                     struct gc_edge edge) {
-  struct gcobj *obj = dereference_edge(edge);
+                                                     struct gc_edge edge,
+                                                     struct gc_ref old_ref) {
+  struct gcobj *obj = gc_ref_heap_object(old_ref);
   uint8_t *metadata = object_metadata_byte(obj);
   uint8_t byte = *metadata;
   if (byte & space->marked_mask)
@@ -588,8 +589,7 @@ static inline int mark_space_evacuate_or_mark_object(struct mark_space *space,
       // The object has been evacuated already.  Update the edge;
       // whoever forwarded the object will make sure it's eventually
       // traced.
-      struct gcobj *forwarded = (struct gcobj*) header_word;
-      update_edge(edge, forwarded);
+      gc_edge_update(edge, gc_ref(header_word));
       return 0;
     }
     // Otherwise try to claim it for evacuation.
@@ -613,7 +613,7 @@ static inline int mark_space_evacuate_or_mark_object(struct mark_space *space,
                object_granules * GRANULE_SIZE - sizeof(header_word));
         uint8_t *new_metadata = object_metadata_byte(new_obj);
         memcpy(new_metadata + 1, metadata + 1, object_granules - 1);
-        update_edge(edge, new_obj);
+        gc_edge_update(edge, gc_ref_from_heap_object(new_obj));
         obj = new_obj;
         metadata = new_metadata;
         // Fall through to set mark bits.
@@ -634,10 +634,8 @@ static inline int mark_space_evacuate_or_mark_object(struct mark_space *space,
           break;
         yield_for_spin(spin_count);
       }
-      if ((header_word & gcobj_not_forwarded_bit) == 0) {
-        struct gcobj *forwarded = (struct gcobj*) header_word;
-        update_edge(edge, forwarded);
-      }
+      if ((header_word & gcobj_not_forwarded_bit) == 0)
+        gc_edge_update(edge, gc_ref(header_word));
       // Either way, the other party is responsible for adding the
       // object to the mark queue.
       return 0;
@@ -661,13 +659,15 @@ static inline int large_object_space_mark_object(struct large_object_space *spac
 }
 
 static inline int trace_edge(struct heap *heap, struct gc_edge edge) {
-  struct gcobj *obj = dereference_edge(edge);
-  if (!obj)
+  struct gc_ref ref = gc_edge_ref(edge);
+  if (!gc_ref_is_heap_object(ref))
     return 0;
-  else if (LIKELY(mark_space_contains(heap_mark_space(heap), obj))) {
+  struct gcobj *obj = gc_ref_heap_object(ref);
+  if (LIKELY(mark_space_contains(heap_mark_space(heap), obj))) {
     if (heap_mark_space(heap)->evacuating)
-      return mark_space_evacuate_or_mark_object(heap_mark_space(heap), edge);
-    return mark_space_mark_object(heap_mark_space(heap), edge);
+      return mark_space_evacuate_or_mark_object(heap_mark_space(heap), edge,
+                                                ref);
+    return mark_space_mark_object(heap_mark_space(heap), ref);
   }
   else if (large_object_space_contains(heap_large_object_space(heap), obj))
     return large_object_space_mark_object(heap_large_object_space(heap),
@@ -955,7 +955,8 @@ static void mark_stopping_mutator_roots(struct mutator *mut) {
   for (struct handle *h = mut->roots; h; h = h->next) {
     struct gc_edge root = gc_edge(&h->v);
     if (trace_edge(heap, root))
-      mutator_mark_buf_push(local_roots, dereference_edge(root));
+      mutator_mark_buf_push(local_roots,
+                            gc_ref_heap_object(gc_edge_ref(root)));
   }
 }
 
@@ -965,7 +966,8 @@ static void mark_mutator_roots_with_lock(struct mutator *mut) {
   for (struct handle *h = mut->roots; h; h = h->next) {
     struct gc_edge root = gc_edge(&h->v);
     if (trace_edge(heap, root))
-      tracer_enqueue_root(&heap->tracer, dereference_edge(root));
+      tracer_enqueue_root(&heap->tracer,
+                          gc_ref_heap_object(gc_edge_ref(root)));
   }
 }
 
@@ -1018,7 +1020,8 @@ static void trace_global_roots(struct heap *heap) {
   for (struct handle *h = heap->global_roots; h; h = h->next) {
     struct gc_edge edge = gc_edge(&h->v);
     if (trace_edge(heap, edge))
-      tracer_enqueue_root(&heap->tracer, dereference_edge(edge));
+      tracer_enqueue_root(&heap->tracer,
+                          gc_ref_heap_object(gc_edge_ref(edge)));
   }
 }