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 --git a/gcc/expmed.c b/gcc/expmed.c
index bbdd0e7..4abce11 100644 (file)
--- 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);