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 --git a/gcc/lower-subreg.c b/gcc/lower-subreg.c
index 2107826..f0dc63f 100644 (file)
--- 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