re PR rtl-optimization/6086 (Reload misoptimizes DImode PREINC on PPC)

	PR optimization/6086
	* combine.c (combine_simplify_rtx): If simplify_rtx failed because
	of SUBREG of volatile MEM or because the MEM was mode dependent,
	return CLOBBER instead of unmodified SUBREG.

From-SVN: r51606

gcc/ChangeLog

+2002-03-30  Jakub Jelinek  <jakub@redhat.com>
+
+	PR optimization/6086
+	* combine.c (combine_simplify_rtx): If simplify_rtx failed because
+	of SUBREG of volatile MEM or because the MEM was mode dependent,
+	return CLOBBER instead of unmodified SUBREG.
+
 Sat Mar 30 14:08:55 CET 2002  Jan Hubicka  <jh@suse.cz>
 
 	* local-alloc.c (local_alloc): Avoid call of update_equiv_regs when not optimizing.

gcc/combine.c

@@ -3875,6 +3875,13 @@ combine_simplify_rtx (x, op0_mode, last, in_dest)
 	  return temp;
       }
 
+      /* Don't change the mode of the MEM if that would change the meaning
+	 of the address.  */
+      if (GET_CODE (SUBREG_REG (x)) == MEM
+	  && (MEM_VOLATILE_P (SUBREG_REG (x))
+	      || mode_dependent_address_p (XEXP (SUBREG_REG (x), 0))))
+	return gen_rtx_CLOBBER (mode, const0_rtx);
+
       /* Note that we cannot do any narrowing for non-constants since
 	 we might have been counting on using the fact that some bits were
 	 zero.  We now do this in the SET.  */