[PATCH 5/6] gcc/lower_subreg.c: Prevent decomposition if modes are not tieable

[Jonathan Wright <Jonathan.Wright@arm.com>]

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Hi,

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.

Bootstrapped and regression tested on:
* x86_64-pc-linux-gnu - no issues.
* aarch64-none-linux-gnu - two test failures which will be fixed by
  the next patch in this series. 

Ok for master?

Thanks,
Jonathan

---

gcc/ChangeLog:

2021-10-14  Jonathan Wright  <jonathan.wright@arm.com>

	* lower-subreg.c (simple_move): Prevent decomposition if
	modes are not tieable.

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diff --git a/gcc/lower-subreg.c b/gcc/lower-subreg.c
index 21078268ba0d241efc469fe71357d3b94f8935d6..f0dc63f485f1237d96ceeb0c75dca9aa8e053c6e 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