Macro optimize_for_simd 

macro_rules! optimize_for_simd {
    (likely($expr:expr)) => { ... };
    (unlikely($expr:expr)) => { ... };
    (assume_aligned($ptr:expr, $align:expr)) => { ... };
    (assume_len($slice:expr, $len:expr)) => { ... };
    (prefetch_read($ptr:expr)) => { ... };
    (prefetch_write($ptr:expr)) => { ... };
}

Macro for compile-time optimization hints