Function vmull_s16 

pub fn vmull_s16(a: int16x4_t, b: int16x4_t) -> int32x4_t

Available on (AArch64 or ARM64EC) and (AArch64 or ARM64EC or target feature v7) and (ARM or AArch64 or ARM64EC) and target feature neon only.

Signed multiply long Arm’s documentation