Make sure that statements such as:

  strcpy (dst, "/gnu/store/…");

or

  static const char str[] = "/gnu/store/…";
  …
  strcpy (dst, str);

do not result in chunked /gnu/store strings that are undetectable by
Guix's GC and its grafting code.  See <https://bugs.gnu.org/24703>
and <https://bugs.gnu.org/30395>.

--- gcc-5.3.0/gcc/builtins.c	2016-10-18 10:50:46.080616285 +0200
+++ gcc-5.3.0/gcc/builtins.c	2016-11-09 15:26:43.693042737 +0100
@@ -3012,6 +3012,58 @@ determine_block_size (tree len, rtx len_rtx,
 			  GET_MODE_MASK (GET_MODE (len_rtx)));
 }
 
+extern void debug_tree (tree);
+
+/* Return true if STR contains the string "/gnu/store".  */
+
+bool
+store_reference_p (tree str)
+{
+  if (getenv ("GUIX_GCC_DEBUG") != NULL)
+    debug_tree (str);
+
+  if (TREE_CODE (str) == ADDR_EXPR)
+    str = TREE_OPERAND (str, 0);
+
+  if (TREE_CODE (str) == VAR_DECL
+      && TREE_STATIC (str)
+      && TREE_READONLY (str))
+    {
+      /* STR may be a 'static const' variable whose initial value
+         is a string constant.  See <https://bugs.gnu.org/30395>.  */
+      str = DECL_INITIAL (str);
+      if (str == NULL_TREE)
+        return false;
+    }
+
+  if (TREE_CODE (str) != STRING_CST)
+    return false;
+
+  int len;
+  const char *store;
+
+  store = getenv ("NIX_STORE") ? getenv ("NIX_STORE") : "/gnu/store";
+  len = strlen (store);
+
+  /* Size of the hash part of store file names, including leading slash and
+     trailing hyphen.  */
+  const int hash_len = 34;
+
+  if (TREE_STRING_LENGTH (str) < len + hash_len)
+    return false;
+
+  /* We cannot use 'strstr' because 'TREE_STRING_POINTER' returns a string
+     that is not necessarily NUL-terminated.  */
+
+  for (int i = 0; i < TREE_STRING_LENGTH (str) - (len + hash_len); i++)
+    {
+      if (strncmp (TREE_STRING_POINTER (str) + i, store, len) == 0)
+	return true;
+    }
+
+  return false;
+}
+
 /* Try to verify that the sizes and lengths of the arguments to a string
    manipulation function given by EXP are within valid bounds and that
    the operation does not lead to buffer overflow or read past the end.
@@ -3605,6 +3657,13 @@ expand_builtin_memory_copy_args (tree dest, tree src, tree len,
   unsigned HOST_WIDE_INT max_size;
   unsigned HOST_WIDE_INT probable_max_size;
 
+  /* Do not emit block moves, which translate to the 'movabs' instruction on
+     x86_64, when SRC refers to store items.  That way, store references
+     remain visible to the Guix GC and grafting code.  See
+     <https://bugs.gnu.org/24703>.  */
+  if (store_reference_p (src))
+    return NULL_RTX;
+
   /* If DEST is not a pointer type, call the normal function.  */
   if (dest_align == 0)
     return NULL_RTX;
--- gcc-5.5.0/gcc/gimple-fold.c	2018-03-20 11:36:16.709442004 +0100
+++ gcc-5.5.0/gcc/gimple-fold.c	2018-03-20 11:46:43.838487065 +0100
@@ -635,6 +635,8 @@ var_decl_component_p (tree var)
   return SSA_VAR_P (inner);
 }
 
+extern bool store_reference_p (tree);
+
 /* If the SIZE argument representing the size of an object is in a range
    of values of which exactly one is valid (and that is zero), return
    true, otherwise false.  */
@@ -742,6 +744,9 @@ gimple_fold_builtin_memory_op (gimple_stmt_iterator *gsi,
       off0 = build_int_cst (build_pointer_type_for_mode (char_type_node,
 							 ptr_mode, true), 0);
 
+      if (store_reference_p (src))
+        return false;
+
       /* If we can perform the copy efficiently with first doing all loads
          and then all stores inline it that way.  Currently efficiently
 	 means that we can load all the memory into a single integer