Constant IEEE_SIMD_LANE_WIDTH 

pub const IEEE_SIMD_LANE_WIDTH: usize = 4; // 4usize

Number of u32 lanes in one integer SIMD vector (128-bit register). Binary64 uses IEEE_SIMD_LANE_WIDTH / 2 u64 lanes. Scalar fallback uses the same width so wrappers can size buffers without cfg.