https://bugs.launchpad.net/gcc-linaro/+bug/728315
https://lists.openwrt.org/pipermail/openwrt-devel/2011-March/009847.html

--- a/gcc/ee.c
+++ b/gcc/ee.c
@@ -209,7 +209,11 @@

       *regno = REGNO (reg);

-      if (paradoxical_subreg_p (use))
+      /* Non-paradoxical SUBREGs of promoted vars guarantee that the
+	 upper (elided) bits of the inner register have a particular value.
+	 For our purposes, such SUBREGs act as a full reference to the
+	 inner register.  */
+      if (paradoxical_subreg_p (use) || SUBREG_PROMOTED_VAR_P (use))
         *size = GET_MODE_BITSIZE (GET_MODE (reg));
       else
         *size = subreg_lsb (use) + GET_MODE_BITSIZE (GET_MODE (use));
