[gcc r12-4909] gcc/expmed.c: Ensure vector modes are tieable before extraction

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

https://gcc.gnu.org/g:4e5929e4575922015ff4634af4dea59c59a44f10

commit r12-4909-g4e5929e4575922015ff4634af4dea59c59a44f10
Author: Jonathan Wright <jonathan.wright@arm.com>
Date:   Mon Oct 11 23:02:16 2021 +0100

    gcc/expmed.c: Ensure vector modes are tieable before extraction
    
    Extracting a bitfield from a vector can be achieved by casting the
    vector to a new type whose elements are the same size as the desired
    bitfield, before generating a subreg. However, this is only an
    optimization if the original vector can be accessed in the new
    machine mode without first being copied - a condition denoted by the
    TARGET_MODES_TIEABLE_P hook.
    
    This patch adds a check to make sure that the vector modes are
    tieable before attempting to generate a subreg. This is a necessary
    prerequisite for a subsequent patch that will introduce new machine
    modes for Arm Neon vector-tuple types.
    
    gcc/ChangeLog:
    
    2021-10-11  Jonathan Wright  <jonathan.wright@arm.com>
    
            * expmed.c (extract_bit_field_1): Ensure modes are tieable.

Diff:
---
 gcc/expmed.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/gcc/expmed.c b/gcc/expmed.c
index bbdd0e71d20..4abce11b647 100644
--- a/gcc/expmed.c
+++ b/gcc/expmed.c
@@ -1735,7 +1735,8 @@ extract_bit_field_1 (rtx str_rtx, poly_uint64 bitsize, poly_uint64 bitnum,
       FOR_EACH_MODE_FROM (new_mode, new_mode)
 	if (known_eq (GET_MODE_SIZE (new_mode), GET_MODE_SIZE (GET_MODE (op0)))
 	    && known_eq (GET_MODE_UNIT_SIZE (new_mode), GET_MODE_SIZE (tmode))
-	    && targetm.vector_mode_supported_p (new_mode))
+	    && targetm.vector_mode_supported_p (new_mode)
+	    && targetm.modes_tieable_p (GET_MODE (op0), new_mode))
 	  break;
       if (new_mode != VOIDmode)
 	op0 = gen_lowpart (new_mode, op0);