[gcc r12-4911] gcc/lower_subreg.c: Prevent decomposition if modes are not tieable

[Jonathan Wright <jonwri01@gcc.gnu.org>]

https://gcc.gnu.org/g:511245325a4d3414a951e2d489112e8372eae1b1

commit r12-4911-g511245325a4d3414a951e2d489112e8372eae1b1
Author: Jonathan Wright <jonathan.wright@arm.com>
Date:   Thu Oct 14 13:49:02 2021 +0100

    gcc/lower_subreg.c: Prevent decomposition if modes are not tieable
    
    Preventing decomposition if modes are not tieable is necessary to
    stop AArch64 partial Neon structure modes being treated as packed in
    registers.
    
    This is a necessary prerequisite for a future AArch64 PCS change to
    maintain good code generation.
    
    gcc/ChangeLog:
    
    2021-10-14  Jonathan Wright  <jonathan.wright@arm.com>
    
            * lower-subreg.c (simple_move): Prevent decomposition if
            modes are not tieable.

Diff:
---
 gcc/lower-subreg.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/gcc/lower-subreg.c b/gcc/lower-subreg.c
index 21078268ba0..f0dc63f485f 100644
--- a/gcc/lower-subreg.c
+++ b/gcc/lower-subreg.c
@@ -383,8 +383,10 @@ simple_move (rtx_insn *insn, bool speed_p)
      non-integer mode for which there is no integer mode of the same
      size.  */
   mode = GET_MODE (SET_DEST (set));
+  scalar_int_mode int_mode;
   if (!SCALAR_INT_MODE_P (mode)
-      && !int_mode_for_size (GET_MODE_BITSIZE (mode), 0).exists ())
+      && (!int_mode_for_size (GET_MODE_BITSIZE (mode), 0).exists (&int_mode)
+	  || !targetm.modes_tieable_p (mode, int_mode)))
     return NULL_RTX;
 
   /* Reject PARTIAL_INT modes.  They are used for processor specific