Prevent invalid register mode changes in combine

This patch stops combine from changing the mode of an existing register
in-place if doing so would change the size of the underlying register
allocation size, as given by REGMODE_NATURAL_SIZE.  Without this,
many tests fail in adjust_reg_mode after SVE is added.  One example
is gcc.c-torture/compile/20090401-1.c.

2017-10-13  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* combine.c (can_change_dest_mode): Reject changes in
	REGMODE_NATURAL_SIZE.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r253717

gcc/ChangeLog

@@ -2,6 +2,13 @@
 	    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>
+
 	* cfgexpand.c (expand_debug_expr): Use GET_MODE_UNIT_BITSIZE.
 	(expand_debug_source_expr): Likewise.
 	* combine.c (combine_simplify_rtx): Likewise.

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.  */