diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 0e29ae8a95d3..3b897e2dde74 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,10 @@
+2017-10-13  Richard Sandiford  <richard.sandiford@linaro.org>
+	    Alan Hayward  <alan.hayward@arm.com>
+	    David Sherwood  <david.sherwood@arm.com>
+
+	* combine.c (can_change_dest_mode): Reject changes in
+	REGMODE_NATURAL_SIZE.
+
 2017-10-13  Richard Sandiford  <richard.sandiford@linaro.org>
 	    Alan Hayward  <alan.hayward@arm.com>
 	    David Sherwood  <david.sherwood@arm.com>
diff --git a/gcc/combine.c b/gcc/combine.c
index aa246e67babd..3b96d86bdb37 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -2475,6 +2475,12 @@ can_change_dest_mode (rtx x, int added_sets, machine_mode mode)
   if (!REG_P (x))
     return false;
 
+  /* Don't change between modes with different underlying register sizes,
+     since this could lead to invalid subregs.  */
+  if (REGMODE_NATURAL_SIZE (mode)
+      != REGMODE_NATURAL_SIZE (GET_MODE (x)))
+    return false;
+
   regno = REGNO (x);
   /* Allow hard registers if the new mode is legal, and occupies no more
      registers than the old mode.  */