diff --git a/libguile/bytevectors.c b/libguile/bytevectors.c
index 9093f49c3..db24f91b1 100644
--- a/libguile/bytevectors.c
+++ b/libguile/bytevectors.c
@@ -332,10 +332,11 @@ scm_c_shrink_bytevector (SCM bv, size_t c_new_len)
   SCM_BYTEVECTOR_SET_LENGTH (bv, c_new_len);
 
   if (SCM_BYTEVECTOR_CONTIGUOUS_P (bv))
-    new_bv = PTR2SCM (scm_gc_realloc (SCM2PTR (bv),
-				      c_len + SCM_BYTEVECTOR_HEADER_BYTES,
-				      c_new_len + SCM_BYTEVECTOR_HEADER_BYTES,
-				      SCM_GC_BYTEVECTOR));
+    new_bv = SCM_PACK_POINTER
+      (scm_gc_realloc (SCM_HEAP_OBJECT_BASE (bv),
+                       c_len + SCM_BYTEVECTOR_HEADER_BYTES,
+                       c_new_len + SCM_BYTEVECTOR_HEADER_BYTES,
+                       SCM_GC_BYTEVECTOR));
   else
     {
       signed char *c_bv;
diff --git a/libguile/fluids.c b/libguile/fluids.c
index 8e36acde6..4844b7a0c 100644
--- a/libguile/fluids.c
+++ b/libguile/fluids.c
@@ -151,7 +151,7 @@ new_fluid (SCM init)
   SCM_SET_CELL_WORD_0 (fluid, (scm_tc7_fluid | (n << 8)));
 
   GC_GENERAL_REGISTER_DISAPPEARING_LINK (&allocated_fluids[n],
-					 SCM2PTR (fluid));
+					 SCM_HEAP_OBJECT_BASE (fluid));
 
   scm_dynwind_end ();
 
diff --git a/libguile/foreign.c b/libguile/foreign.c
index 47077f7f8..a8e1da052 100644
--- a/libguile/foreign.c
+++ b/libguile/foreign.c
@@ -1,4 +1,4 @@
-/* Copyright (C) 2010, 2011, 2012  Free Software Foundation, Inc.
+/* Copyright (C) 2010, 2011, 2012, 2013  Free Software Foundation, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public License
@@ -157,7 +157,8 @@ scm_from_pointer (void *ptr, scm_t_pointer_finalizer finalizer)
       ret = scm_cell (scm_tc7_pointer, (scm_t_bits) ptr);
 
       if (finalizer)
-        scm_i_set_finalizer (SCM2PTR (ret), pointer_finalizer_trampoline,
+        scm_i_set_finalizer (SCM_HEAP_OBJECT_BASE (ret),
+                             pointer_finalizer_trampoline,
                              finalizer);
     }
 
@@ -311,7 +312,7 @@ SCM_DEFINE (scm_set_pointer_finalizer_x, "set-pointer-finalizer!", 2, 0, 0,
   SCM_VALIDATE_POINTER (1, pointer);
   SCM_VALIDATE_POINTER (2, finalizer);
 
-  scm_i_add_finalizer (SCM2PTR (pointer), pointer_finalizer_trampoline,
+  scm_i_add_finalizer (SCM_HEAP_OBJECT_BASE (pointer), pointer_finalizer_trampoline,
                        SCM_POINTER_VALUE (finalizer));
 
   return SCM_UNSPECIFIED;
diff --git a/libguile/gc.h b/libguile/gc.h
index b1df82df5..d81546d74 100644
--- a/libguile/gc.h
+++ b/libguile/gc.h
@@ -46,12 +46,13 @@ typedef struct scm_t_cell
  * in debug mode.  In particular these macros will even work for free cells,
  * which should never be encountered by user code.  */
 
-#define SCM_GC_CELL_OBJECT(x, n) (((SCM *)SCM2PTR (x)) [n])
-#define SCM_GC_CELL_WORD(x, n)   (SCM_UNPACK (SCM_GC_CELL_OBJECT ((x), (n))))
+#define SCM_GC_CELL_OBJECT(x, n) (SCM_PACK (SCM_HEAP_OBJECT_BASE (x)[n]))
+#define SCM_GC_CELL_WORD(x, n)   (SCM_HEAP_OBJECT_BASE (x)[n])
 
-#define SCM_GC_SET_CELL_OBJECT(x, n, v) ((((SCM *)SCM2PTR (x)) [n]) = (v))
+#define SCM_GC_SET_CELL_OBJECT(x, n, v) \
+  (SCM_HEAP_OBJECT_BASE (x)[n] = SCM_UNPACK (v))
 #define SCM_GC_SET_CELL_WORD(x, n, v)  \
-  (SCM_GC_SET_CELL_OBJECT ((x), (n), SCM_PACK (v)))
+  (SCM_HEAP_OBJECT_BASE (x)[n] = (v))
 
 #define SCM_GC_CELL_TYPE(x) (SCM_GC_CELL_OBJECT ((x), 0))
 
@@ -97,7 +98,8 @@ typedef struct scm_t_cell
 #define SCM_SET_CELL_OBJECT_2(x, v) SCM_SET_CELL_OBJECT ((x), 2, (v))
 #define SCM_SET_CELL_OBJECT_3(x, v) SCM_SET_CELL_OBJECT ((x), 3, (v))
 
-#define SCM_CELL_OBJECT_LOC(x, n) (SCM_VALIDATE_CELL((x), &SCM_GC_CELL_OBJECT ((x), (n))))
+#define SCM_CELL_WORD_LOC(x, n)   (SCM_VALIDATE_CELL((x), &SCM_GC_CELL_WORD ((x), (n))))
+#define SCM_CELL_OBJECT_LOC(x, n) ((SCM *) SCM_CELL_WORD_LOC (x, n))
 #define SCM_CARLOC(x)             (SCM_CELL_OBJECT_LOC ((x), 0))
 #define SCM_CDRLOC(x)             (SCM_CELL_OBJECT_LOC ((x), 1))
 
diff --git a/libguile/guardians.c b/libguile/guardians.c
index 8a0d2961c..4944a32ef 100644
--- a/libguile/guardians.c
+++ b/libguile/guardians.c
@@ -219,6 +219,8 @@ scm_i_guard (SCM guardian, SCM obj)
                                     SCM_EOL);
       finalizer_data = scm_cons (SCM_BOOL_F, guardians_for_obj);
 
+      /* FIXME: should be SCM_HEAP_OBJECT_BASE, but will the finalizer
+         strip the tag bits of pairs or structs?  */
       GC_REGISTER_FINALIZER_NO_ORDER (SCM_UNPACK_POINTER (obj), finalize_guarded,
 				      SCM_UNPACK_POINTER (finalizer_data),
 				      &prev_finalizer, &prev_data);
diff --git a/libguile/macros.c b/libguile/macros.c
index 47b252d85..c9745cdbd 100644
--- a/libguile/macros.c
+++ b/libguile/macros.c
@@ -103,7 +103,7 @@ SCM_DEFINE (scm_make_syntax_transformer, "make-syntax-transformer", 3, 0, 0,
   SCM_VALIDATE_SYMBOL (2, type);
 
   z = scm_words (scm_tc16_macro, 5);
-  SCM_SET_SMOB_DATA_N (z, 1, prim);
+  SCM_SET_SMOB_DATA_N (z, 1, (scm_t_bits)prim);
   SCM_SET_SMOB_OBJECT_N (z, 2, name);
   SCM_SET_SMOB_OBJECT_N (z, 3, type);
   SCM_SET_SMOB_OBJECT_N (z, 4, binding);
diff --git a/libguile/numbers.h b/libguile/numbers.h
index cef2b863b..b7bcfe48c 100644
--- a/libguile/numbers.h
+++ b/libguile/numbers.h
@@ -128,9 +128,9 @@ typedef scm_t_int32 scm_t_wchar;
 #define SCM_REALP(x) (SCM_HAS_TYP16 (x, scm_tc16_real))
 #define SCM_COMPLEXP(x) (SCM_HAS_TYP16 (x, scm_tc16_complex))
 
-#define SCM_REAL_VALUE(x) (((scm_t_double *) SCM2PTR (x))->real)
-#define SCM_COMPLEX_REAL(x) (((scm_t_complex *) SCM2PTR (x))->real)
-#define SCM_COMPLEX_IMAG(x) (((scm_t_complex *) SCM2PTR (x))->imag)
+#define SCM_REAL_VALUE(x) (((scm_t_double *) SCM_HEAP_OBJECT_BASE (x))->real)
+#define SCM_COMPLEX_REAL(x) (((scm_t_complex *) SCM_HEAP_OBJECT_BASE (x))->real)
+#define SCM_COMPLEX_IMAG(x) (((scm_t_complex *) SCM_HEAP_OBJECT_BASE (x))->imag)
 
 /* Each bignum is just an mpz_t stored in a double cell starting at word 1. */
 #define SCM_I_BIG_MPZ(x) (*((mpz_t *) (SCM_CELL_OBJECT_LOC((x),1))))
diff --git a/libguile/ports.c b/libguile/ports.c
index 0aacacc06..02f4ea1d9 100644
--- a/libguile/ports.c
+++ b/libguile/ports.c
@@ -676,7 +676,7 @@ scm_c_make_port_with_encoding (scm_t_bits tag, unsigned long mode_bits,
   entry->alist = SCM_EOL;
 
   if (SCM_PORT_DESCRIPTOR (ret)->free)
-    scm_i_set_finalizer (SCM2PTR (ret), finalize_port, NULL);
+    scm_i_set_finalizer (SCM_HEAP_OBJECT_BASE (ret), finalize_port, NULL);
 
   if (SCM_PORT_DESCRIPTOR (ret)->flags & SCM_PORT_TYPE_HAS_FLUSH)
     scm_weak_set_add_x (scm_i_port_weak_set, ret);
diff --git a/libguile/smob.c b/libguile/smob.c
index c2347f3be..6dc2dfd83 100644
--- a/libguile/smob.c
+++ b/libguile/smob.c
@@ -1,4 +1,4 @@
-/* Copyright (C) 1995,1996,1998,1999,2000,2001, 2003, 2004, 2006, 2009, 2010, 2011, 2012 Free Software Foundation, Inc.
+/* Copyright (C) 1995,1996,1998,1999,2000,2001, 2003, 2004, 2006, 2009, 2010, 2011, 2012, 2013 Free Software Foundation, Inc.
  * 
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public License
@@ -414,7 +414,7 @@ scm_i_new_smob (scm_t_bits tc, scm_t_bits data)
   SCM_SET_CELL_WORD_0 (ret, tc);
 
   if (scm_smobs[smobnum].free)
-    scm_i_set_finalizer (SCM2PTR (ret), finalize_smob, NULL);
+    scm_i_set_finalizer (SCM_HEAP_OBJECT_BASE (ret), finalize_smob, NULL);
 
   return ret;
 }
@@ -441,7 +441,7 @@ scm_i_new_double_smob (scm_t_bits tc, scm_t_bits data1,
   SCM_SET_CELL_WORD_0 (ret, tc);
 
   if (scm_smobs[smobnum].free)
-    scm_i_set_finalizer (SCM2PTR (ret), finalize_smob, NULL);
+    scm_i_set_finalizer (SCM_HEAP_OBJECT_BASE (ret), finalize_smob, NULL);
 
   return ret;
 }
diff --git a/libguile/struct.c b/libguile/struct.c
index 1a30fefc5..ed75b8ba5 100644
--- a/libguile/struct.c
+++ b/libguile/struct.c
@@ -1,4 +1,4 @@
-/* Copyright (C) 1996,1997,1998,1999,2000,2001, 2003, 2004, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Free Software Foundation, Inc.
+/* Copyright (C) 1996,1997,1998,1999,2000,2001, 2003, 2004, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Free Software Foundation, Inc.
  * 
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public License
@@ -445,7 +445,8 @@ scm_i_alloc_struct (scm_t_bits *vtable_data, int n_words)
   /* vtable_data can be null when making a vtable vtable */
   if (vtable_data && vtable_data[scm_vtable_index_instance_finalize])
     /* Register a finalizer for the newly created instance.  */
-    scm_i_set_finalizer (SCM2PTR (ret), struct_finalizer_trampoline, NULL);
+    scm_i_set_finalizer (SCM_HEAP_OBJECT_BASE (ret),
+                         struct_finalizer_trampoline, NULL);
 
   return ret;
 }
diff --git a/libguile/tags.h b/libguile/tags.h
index a194ea0be..043bdbdce 100644
--- a/libguile/tags.h
+++ b/libguile/tags.h
@@ -391,6 +391,17 @@ typedef union SCM { struct { scm_t_bits n; } n; } SCM;
 #define scm_tc3_tc7_2		 7
 
 
+/* As we have seen, heap objects have a tag in their three lowest bits.
+   If you have a heap object and want the pointer to the start of the
+   object, perhaps for GC purposes, you need to mask off the low bits,
+   which is what SCM_HEAP_OBJECT_BASE does.
+
+   Note that you can avoid this macro if you know the specific type of
+   the object (pair, struct, or other).
+ */
+#define SCM_HEAP_OBJECT_BASE(x) ((scm_t_bits*)((SCM_UNPACK (x)) & ~7))
+
+
 /* Definitions for tc7: */
 
 #define SCM_ITAG7(x) 		(127 & SCM_UNPACK (x))
diff --git a/libguile/weak-set.c b/libguile/weak-set.c
index d648dbd34..e127cc83f 100644
--- a/libguile/weak-set.c
+++ b/libguile/weak-set.c
@@ -1,4 +1,4 @@
-/* Copyright (C) 2011, 2012 Free Software Foundation, Inc.
+/* Copyright (C) 2011, 2012, 2013 Free Software Foundation, Inc.
  * 
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public License
@@ -580,7 +580,7 @@ weak_set_add_x (scm_t_weak_set *set, unsigned long hash,
 
   if (SCM_HEAP_OBJECT_P (obj))
     SCM_I_REGISTER_DISAPPEARING_LINK ((void **) &entries[k].key,
-                                      (void *) SCM2PTR (obj));
+                                      (void *) SCM_HEAP_OBJECT_BASE (obj));
 
   return obj;
 }
@@ -743,7 +743,7 @@ scm_c_register_weak_gc_callback (SCM obj, void (*callback) (SCM))
 
   weak[0] = SCM_UNPACK_POINTER (obj);
   weak[1] = (void*)callback;
-  GC_GENERAL_REGISTER_DISAPPEARING_LINK (weak, SCM2PTR (obj));
+  GC_GENERAL_REGISTER_DISAPPEARING_LINK (weak, SCM_HEAP_OBJECT_BASE (obj));
 
 #ifdef HAVE_GC_SET_START_CALLBACK
   scm_c_hook_add (&scm_after_gc_c_hook, weak_gc_hook, weak, 0);
diff --git a/libguile/weak-table.c b/libguile/weak-table.c
index 9ef6674e1..85d587a45 100644
--- a/libguile/weak-table.c
+++ b/libguile/weak-table.c
@@ -1,4 +1,4 @@
-/* Copyright (C) 2011, 2012 Free Software Foundation, Inc.
+/* Copyright (C) 2011, 2012, 2013 Free Software Foundation, Inc.
  * 
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public License
@@ -131,13 +131,13 @@ register_disappearing_links (scm_t_weak_entry *entry,
       && (kind == SCM_WEAK_TABLE_KIND_KEY
           || kind == SCM_WEAK_TABLE_KIND_BOTH))
     SCM_I_REGISTER_DISAPPEARING_LINK ((void **) &entry->key,
-                                      SCM2PTR (k));
+                                      SCM_HEAP_OBJECT_BASE (k));
 
   if (SCM_UNPACK (v) && SCM_HEAP_OBJECT_P (v)
       && (kind == SCM_WEAK_TABLE_KIND_VALUE
           || kind == SCM_WEAK_TABLE_KIND_BOTH))
     SCM_I_REGISTER_DISAPPEARING_LINK ((void **) &entry->value,
-                                      SCM2PTR (v));
+                                      SCM_HEAP_OBJECT_BASE (v));
 }
 
 static void
@@ -162,7 +162,7 @@ move_disappearing_links (scm_t_weak_entry *from, scm_t_weak_entry *to,
       GC_move_disappearing_link ((void **) &from->key, (void **) &to->key);
 #else
       GC_unregister_disappearing_link ((void **) &from->key);
-      SCM_I_REGISTER_DISAPPEARING_LINK ((void **) &to->key, SCM2PTR (key));
+      SCM_I_REGISTER_DISAPPEARING_LINK ((void **) &to->key, SCM_HEAP_OBJECT_BASE (key));
 #endif
     }
 
@@ -173,7 +173,7 @@ move_disappearing_links (scm_t_weak_entry *from, scm_t_weak_entry *to,
       GC_move_disappearing_link ((void **) &from->value, (void **) &to->value);
 #else
       GC_unregister_disappearing_link ((void **) &from->value);
-      SCM_I_REGISTER_DISAPPEARING_LINK ((void **) &to->value, SCM2PTR (value));
+      SCM_I_REGISTER_DISAPPEARING_LINK ((void **) &to->value, SCM_HEAP_OBJECT_BASE (value));
 #endif
     }
 }
@@ -328,7 +328,7 @@ mark_weak_key_table (GC_word *addr, struct GC_ms_entry *mark_stack_ptr,
     if (entries[k].hash && entries[k].key)
       {
         SCM value = SCM_PACK (entries[k].value);
-        mark_stack_ptr = GC_MARK_AND_PUSH ((GC_word*) SCM2PTR (value),
+        mark_stack_ptr = GC_MARK_AND_PUSH ((GC_word*) SCM_HEAP_OBJECT_BASE (value),
                                            mark_stack_ptr, mark_stack_limit,
                                            NULL);
       }
@@ -347,7 +347,7 @@ mark_weak_value_table (GC_word *addr, struct GC_ms_entry *mark_stack_ptr,
     if (entries[k].hash && entries[k].value)
       {
         SCM key = SCM_PACK (entries[k].key);
-        mark_stack_ptr = GC_MARK_AND_PUSH ((GC_word*) SCM2PTR (key),
+        mark_stack_ptr = GC_MARK_AND_PUSH ((GC_word*) SCM_HEAP_OBJECT_BASE (key),
                                            mark_stack_ptr, mark_stack_limit,
                                            NULL);
       }
@@ -864,7 +864,7 @@ scm_c_register_weak_gc_callback (SCM obj, void (*callback) (SCM))
 
   weak[0] = SCM_UNPACK_POINTER (obj);
   weak[1] = (void*)callback;
-  GC_GENERAL_REGISTER_DISAPPEARING_LINK (weak, SCM2PTR (obj));
+  GC_GENERAL_REGISTER_DISAPPEARING_LINK (weak, SCM_HEAP_OBJECT_BASE (obj));
 
 #ifdef HAVE_GC_TABLE_START_CALLBACK
   scm_c_hook_add (&scm_after_gc_c_hook, weak_gc_hook, weak, 0);
diff --git a/libguile/weak-vector.c b/libguile/weak-vector.c
index 3e90b3d57..17c574a07 100644
--- a/libguile/weak-vector.c
+++ b/libguile/weak-vector.c
@@ -1,4 +1,4 @@
-/* Copyright (C) 1995,1996,1998,2000,2001, 2003, 2006, 2008, 2009, 2010, 2011, 2012 Free Software Foundation, Inc.
+/* Copyright (C) 1995,1996,1998,2000,2001, 2003, 2006, 2008, 2009, 2010, 2011, 2012, 2013 Free Software Foundation, Inc.
  * 
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public License
@@ -177,7 +177,7 @@ scm_c_weak_vector_set_x (SCM wv, size_t k, SCM x)
 
   if (SCM_HEAP_OBJECT_P (x))
     SCM_I_REGISTER_DISAPPEARING_LINK ((void **) &elts[k],
-                                      SCM2PTR (x));
+                                      SCM_HEAP_OBJECT_BASE (x));
 }