lexical_util/
mul.rs

1//! Fast multiplication routines.
2
3use crate::num::{as_cast, UnsignedInteger};
4
5/// Multiply two unsigned, integral values, and return the high and low product.
6///
7/// The high product is the upper half of the product and the low product is the
8/// lower half. The `full` type is the full type size, while the `half` type is
9/// the type with exactly half the bits.
10#[inline(always)]
11pub fn mul<Full, Half>(x: Full, y: Full) -> (Full, Full)
12where
13    Full: UnsignedInteger,
14    Half: UnsignedInteger,
15{
16    // Extract high-and-low masks.
17    let x1 = x >> Half::BITS as i32;
18    let x0 = x & as_cast(Half::MAX);
19    let y1 = y >> Half::BITS as i32;
20    let y0 = y & as_cast(Half::MAX);
21
22    let w0 = x0 * y0;
23    let tmp = (x1 * y0) + (w0 >> Half::BITS as i32);
24    let w1 = tmp & as_cast(Half::MAX);
25    let w2 = tmp >> Half::BITS as i32;
26    let w1 = w1 + x0 * y1;
27    let hi = (x1 * y1) + w2 + (w1 >> Half::BITS as i32);
28    let lo = x.wrapping_mul(y);
29
30    (hi, lo)
31}
32
33/// Multiply two unsigned, integral values, and return the high product.
34///
35/// The high product is the upper half of the product. The `full` type is the
36/// full type size, while the `half` type is the type with exactly half the
37/// bits.
38#[inline(always)]
39pub fn mulhi<Full, Half>(x: Full, y: Full) -> Full
40where
41    Full: UnsignedInteger,
42    Half: UnsignedInteger,
43{
44    // Extract high-and-low masks.
45    let x1 = x >> Half::BITS as i32;
46    let x0 = x & as_cast(Half::MAX);
47    let y1 = y >> Half::BITS as i32;
48    let y0 = y & as_cast(Half::MAX);
49
50    let w0 = x0 * y0;
51    let m = (x0 * y1) + (w0 >> Half::BITS as i32);
52    let w1 = m & as_cast(Half::MAX);
53    let w2 = m >> Half::BITS as i32;
54
55    let w3 = (x1 * y0 + w1) >> Half::BITS as i32;
56
57    x1 * y1 + w2 + w3
58}
lexical_util/mul.rs

lexical_util/
mul.rs
