Use df_read_modify_subreg_p in cprop.c

[Richard Sandiford]

local_cprop_find_used_regs punted on all multiword registers,
with the comment:

      /* Setting a subreg of a register larger than word_mode leaves
         the non-written words unchanged.  */

But this only applies if the outer mode is smaller than the
inner mode.  If they're the same size then writes to the subreg
are a normal full update.

This patch uses df_read_modify_subreg_p instead.  A later patch
adds more uses of the same routine, but this part had a (positive)
effect on code generation for the testsuite whereas the others
seemed to be simple clean-ups.

Tested on aarch64-linux-gnu and x86_64-linux-gnu.  OK to install?

Thanks,
Richard


[ This patch is part of the SVE series posted here:
  https://gcc.gnu.org/ml/gcc/2016-11/msg00030.html ]

gcc/
2016-11-15  Richard Sandiford  <richard.sandif...@arm.com>
            Alan Hayward  <alan.hayw...@arm.com>
            David Sherwood  <david.sherw...@arm.com>

        * cprop.c (local_cprop_find_used_regs): Use df_read_modify_subreg_p.

diff --git a/gcc/cprop.c b/gcc/cprop.c
index 6b4c0b8..31868a5 100644
--- a/gcc/cprop.c
+++ b/gcc/cprop.c
@@ -1161,9 +1161,7 @@ local_cprop_find_used_regs (rtx *xptr, void *data)
       return;
 
     case SUBREG:
-      /* Setting a subreg of a register larger than word_mode leaves
-        the non-written words unchanged.  */
-      if (GET_MODE_BITSIZE (GET_MODE (SUBREG_REG (x))) > BITS_PER_WORD)
+      if (df_read_modify_subreg_p (x))
        return;
       break;