Hi Richard,
For the following (contrived) test:

void foo(int32x4_t v)
{
  v[3] = 0;
  return v;
}

-O2 code-gen:
foo:
        fmov    s1, wzr
        ins     v0.s[3], v1.s[0]
        ret

I suppose we can instead emit the following code-gen ?
foo:
     ins v0.s[3], wzr
     ret

combine produces:
Failed to match this instruction:
(set (reg:V4SI 95 [ v ])
    (vec_merge:V4SI (const_vector:V4SI [
                (const_int 0 [0]) repeated x4
            ])
        (reg:V4SI 97)
        (const_int 8 [0x8])))

So, I wrote the following pattern to match the above insn:
(define_insn "aarch64_simd_vec_set_zero<mode>"
  [(set (match_operand:VALL_F16 0 "register_operand" "=w")
        (vec_merge:VALL_F16
            (match_operand:VALL_F16 1 "const_dup0_operand" "w")
            (match_operand:VALL_F16 3 "register_operand" "0")
            (match_operand:SI 2 "immediate_operand" "i")))]
  "TARGET_SIMD"
  {
    int elt = ENDIAN_LANE_N (<nunits>, exact_log2 (INTVAL (operands[2])));
    operands[2] = GEN_INT ((HOST_WIDE_INT) 1 << elt);
    return "ins\\t%0.<Vetype>[%p2], wzr";
  }
)

which now matches the above insn produced by combine.
However, in reload dump, it creates a new insn for assigning
register to (const_vector (const_int 0)),
which results in:
(insn 19 8 13 2 (set (reg:V4SI 33 v1 [99])
        (const_vector:V4SI [
                (const_int 0 [0]) repeated x4
            ])) "wzr-test.c":8:1 1269 {*aarch64_simd_movv4si}
     (nil))
(insn 13 19 14 2 (set (reg/i:V4SI 32 v0)
        (vec_merge:V4SI (reg:V4SI 33 v1 [99])
            (reg:V4SI 32 v0 [97])
            (const_int 8 [0x8]))) "wzr-test.c":8:1 1808
{aarch64_simd_vec_set_zerov4si}
     (nil))

and eventually the code-gen:
foo:
        movi    v1.4s, 0
        ins     v0.s[3], wzr
        ret

To get rid of redundant assignment of 0 to v1, I tried to split the
above pattern
as in the attached patch. This works to emit code-gen:
foo:
        ins     v0.s[3], wzr
        ret

However, I am not sure if this is the right approach. Could you suggest,
if it'd be possible to get rid of UNSPEC_SETZERO in the patch ?

Thanks,
Prathamesh