Make sure that statements such as:

  strcpy (dst, "/gnu/store/…");

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

--- 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
@@ -3192,6 +3192,42 @@ determine_block_size (tree len, rtx len_
 			  GET_MODE_MASK (GET_MODE (len_rtx)));
 }
 
+/* Return true if STR contains the string "/gnu/store".  */
+
+static bool
+store_reference_p (tree str)
+{
+  if (TREE_CODE (str) == ADDR_EXPR)
+    str = TREE_OPERAND (str, 0);
+
+  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;
+}
+
 /* Helper function to do the actual work for expand_builtin_memcpy.  */
 
 static rtx
@@ -3207,6 +3243,13 @@ expand_builtin_memcpy_args (tree dest, t
   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
+     <http://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;