Function vmull_s8 

pub fn vmull_s8(a: int8x8_t, b: int8x8_t) -> int16x8_t

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

Signed multiply long Arm’s documentation