1 files changed, 26 insertions, 1 deletions

diff --git a/src/alloc.c b/src/alloc.c
index 044e750413b..21c4db4ddca 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -1582,6 +1582,21 @@ make_number (EMACS_INT n)
 }
 #endif
 
+/* Convert the pointer-sized word P to EMACS_INT while preserving its
+   type and ptr fields.  */
+static Lisp_Object
+widen_to_Lisp_Object (void *p)
+{
+  intptr_t i = (intptr_t) p;
+#ifdef USE_LISP_UNION_TYPE
+  Lisp_Object obj;
+  obj.i = i;
+  return obj;
+#else
+  return i;
+#endif
+}
+
 /***********************************************************************
                           String Allocation
  ***********************************************************************/
@@ -4293,7 +4308,17 @@ mark_memory (void *start, void *end)
 
   for (pp = start; (void *) pp < end; pp++)
     for (i = 0; i < sizeof *pp; i += GC_POINTER_ALIGNMENT)
-      mark_maybe_pointer (*(void **) ((char *) pp + i));
+      {
+        void *w = *(void **) ((char *) pp + i);
+        mark_maybe_pointer (w);
+
+        /* A host where a Lisp_Object is wider than a pointer might
+           allocate a Lisp_Object in non-adjacent halves.  If
+           USE_LSB_TAG, the bottom half is not a valid pointer, so
+           widen it to to a Lisp_Object and check it that way.  */
+        if (sizeof w < sizeof (Lisp_Object))
+          mark_maybe_object (widen_to_Lisp_Object (w));
+      }
 }
 
 /* setjmp will work with GCC unless NON_SAVING_SETJMP is defined in