[PATCH 3/5] Use simplify_subreg_regno in combine.c:subst

[Richard Sandiford]

combine.c:subst should refuse to substitute a hard register
into a subreg if the new subreg would not be simplified to a
simple hard register, since the result would have to be reloaded.
This is more for optimisation than correctness, since in theory
the RA should be able to fix up any unsimplified subregs.


gcc/
        * combine.c (subst): Use simplify_subreg_regno rather than
        REG_CANNOT_CHANGE_MODE_P to detect invalid mode changes.

Index: gcc/combine.c
===================================================================
--- gcc/combine.c       2014-09-15 10:00:17.545330404 +0100
+++ gcc/combine.c       2014-09-15 10:00:17.545330404 +0100
@@ -5121,15 +5121,13 @@ #define COMBINE_RTX_EQUAL_P(X,Y)                        
\
                      )
                    return gen_rtx_CLOBBER (VOIDmode, const0_rtx);
 
-#ifdef CANNOT_CHANGE_MODE_CLASS
                  if (code == SUBREG
                      && REG_P (to)
                      && REGNO (to) < FIRST_PSEUDO_REGISTER
-                     && REG_CANNOT_CHANGE_MODE_P (REGNO (to),
-                                                  GET_MODE (to),
-                                                  GET_MODE (x)))
+                     && simplify_subreg_regno (REGNO (to), GET_MODE (to),
+                                               SUBREG_BYTE (x),
+                                               GET_MODE (x)) < 0)
                    return gen_rtx_CLOBBER (VOIDmode, const0_rtx);
-#endif
 
                  new_rtx = (unique_copy && n_occurrences ? copy_rtx (to) : to);
                  n_occurrences++;