combine: Tweak change_zero_ext

change_zero_ext handles (zero_extend:M1 (subreg:M2 (reg:M1) ...))
already; this patch extends it to also deal with any
(zero_extend:M1 (subreg:M2 (reg:M3) ...)) where the subreg is not
paradoxical.


	* combine.c (change_zero_ext): Also handle extends from a subreg
	to a mode bigger than that of the operand of the subreg.

From-SVN: r242950

gcc/ChangeLog

 2016-11-29  Segher Boessenkool  <segher@kernel.crashing.org>
 
+	* combine.c (change_zero_ext): Also handle extends from a subreg
+	to a mode bigger than that of the operand of the subreg.
+
+2016-11-29  Segher Boessenkool  <segher@kernel.crashing.org>
+
 	PR target/77687
 	* config/rs6000/rs6000.c (rs6000_emit_stack_reset): Emit the
 	stack_restore_tie insn instead of stack_tie, for the SVR4 and

gcc/combine.c

@@ -11251,11 +11251,13 @@ change_zero_ext (rtx pat)
       else if (GET_CODE (x) == ZERO_EXTEND
 	       && SCALAR_INT_MODE_P (mode)
 	       && GET_CODE (XEXP (x, 0)) == SUBREG
-	       && GET_MODE (SUBREG_REG (XEXP (x, 0))) == mode
+	       && !paradoxical_subreg_p (XEXP (x, 0))
 	       && subreg_lowpart_p (XEXP (x, 0)))
 	{
 	  size = GET_MODE_PRECISION (GET_MODE (XEXP (x, 0)));
 	  x = SUBREG_REG (XEXP (x, 0));
+	  if (GET_MODE (x) != mode)
+	    x = gen_lowpart_SUBREG (mode, x);
 	}
       else if (GET_CODE (x) == ZERO_EXTEND
 	       && SCALAR_INT_MODE_P (mode)