Improve multiply performance

The main idea here is to do as much as possible with slices, instead of
allocating new BigUints (= heap allocations).

Current performance:

multiply_0:	10,507 ns/iter (+/- 987)
multiply_1:	2,788,734 ns/iter (+/- 100,079)
multiply_2:	69,923,515 ns/iter (+/- 4,550,902)

After this patch, we get:

multiply_0:	364 ns/iter (+/- 62)
multiply_1:	34,085 ns/iter (+/- 1,179)
multiply_2:	3,753,883 ns/iter (+/- 46,876)
This commit is contained in:
Kent Overstreet 2015-12-10 15:25:42 -09:00
parent 496ae0337c
commit 08b0022aab
1 changed files with 219 additions and 70 deletions

View File

@ -148,6 +148,16 @@ fn sbb(a: BigDigit, b: BigDigit, borrow: &mut BigDigit) -> BigDigit {
lo
}
#[inline]
fn mac_with_carry(a: BigDigit, b: BigDigit, c: BigDigit, carry: &mut BigDigit) -> BigDigit {
let (hi, lo) = big_digit::from_doublebigdigit(
(a as DoubleBigDigit) +
(b as DoubleBigDigit) * (c as DoubleBigDigit) +
(*carry as DoubleBigDigit));
*carry = hi;
lo
}
/// A big unsigned integer type.
///
/// A `BigUint`-typed value `BigUint { data: vec!(a, b, c) }` represents a number
@ -172,18 +182,25 @@ impl PartialOrd for BigUint {
}
}
fn cmp_slice(a: &[BigDigit], b: &[BigDigit]) -> Ordering {
debug_assert!(a.last() != Some(&0));
debug_assert!(b.last() != Some(&0));
let (a_len, b_len) = (a.len(), b.len());
if a_len < b_len { return Less; }
if a_len > b_len { return Greater; }
for (&ai, &bi) in a.iter().rev().zip(b.iter().rev()) {
if ai < bi { return Less; }
if ai > bi { return Greater; }
}
return Equal;
}
impl Ord for BigUint {
#[inline]
fn cmp(&self, other: &BigUint) -> Ordering {
let (s_len, o_len) = (self.data.len(), other.data.len());
if s_len < o_len { return Less; }
if s_len > o_len { return Greater; }
for (&self_i, &other_i) in self.data.iter().rev().zip(other.data.iter().rev()) {
if self_i < other_i { return Less; }
if self_i > other_i { return Greater; }
}
return Equal;
cmp_slice(&self.data[..], &other.data[..])
}
}
@ -608,80 +625,202 @@ impl<'a> Sub<&'a BigUint> for BigUint {
}
}
fn sub_sign(a: &[BigDigit], b: &[BigDigit]) -> BigInt {
// Normalize:
let a = &a[..a.iter().rposition(|&x| x != 0).map_or(0, |i| i + 1)];
let b = &b[..b.iter().rposition(|&x| x != 0).map_or(0, |i| i + 1)];
forward_all_binop_to_val_ref_commutative!(impl Mul for BigUint, mul);
impl<'a> Mul<&'a BigUint> for BigUint {
type Output = BigUint;
fn mul(self, other: &BigUint) -> BigUint {
if self.is_zero() || other.is_zero() { return Zero::zero(); }
let (s_len, o_len) = (self.data.len(), other.data.len());
if s_len == 1 { return mul_digit(other.clone(), self.data[0]); }
if o_len == 1 { return mul_digit(self, other.data[0]); }
// Using Karatsuba multiplication
// (a1 * base + a0) * (b1 * base + b0)
// = a1*b1 * base^2 +
// (a1*b1 + a0*b0 - (a1-b0)*(b1-a0)) * base +
// a0*b0
let half_len = cmp::max(s_len, o_len) / 2;
let (s_hi, s_lo) = cut_at(self, half_len);
let (o_hi, o_lo) = cut_at(other.clone(), half_len);
let ll = &s_lo * &o_lo;
let hh = &s_hi * &o_hi;
let mm = {
let (s1, n1) = sub_sign(s_hi, s_lo);
let (s2, n2) = sub_sign(o_hi, o_lo);
match (s1, s2) {
(Equal, _) | (_, Equal) => &hh + &ll,
(Less, Greater) | (Greater, Less) => &hh + &ll + (n1 * n2),
(Less, Less) | (Greater, Greater) => &hh + &ll - (n1 * n2)
match cmp_slice(a, b) {
Greater => {
let mut ret = BigUint::from_slice(a);
sub2(&mut ret.data[..], b);
BigInt::from_biguint(Plus, ret.normalize())
},
Less => {
let mut ret = BigUint::from_slice(b);
sub2(&mut ret.data[..], a);
BigInt::from_biguint(Minus, ret.normalize())
},
_ => Zero::zero(),
}
};
}
return ll + mm.shl_unit(half_len) + hh.shl_unit(half_len * 2);
forward_all_binop_to_ref_ref!(impl Mul for BigUint, mul);
/// Three argument multiply accumulate:
/// acc += b * c
fn mac_digit(acc: &mut [BigDigit], b: &[BigDigit], c: BigDigit) {
if c == 0 { return; }
fn mul_digit(a: BigUint, n: BigDigit) -> BigUint {
if n == 0 { return Zero::zero(); }
if n == 1 { return a; }
let mut b_iter = b.iter();
let mut carry = 0;
let mut prod = a.data;
for a in &mut prod {
let d = (*a as DoubleBigDigit)
* (n as DoubleBigDigit)
+ (carry as DoubleBigDigit);
let (hi, lo) = big_digit::from_doublebigdigit(d);
carry = hi;
*a = lo;
for ai in acc.iter_mut() {
if let Some(bi) = b_iter.next() {
*ai = mac_with_carry(*ai, *bi, c, &mut carry);
} else if carry != 0 {
*ai = mac_with_carry(*ai, 0, c, &mut carry);
} else {
break;
}
if carry != 0 { prod.push(carry); }
BigUint::new(prod)
}
#[inline]
fn cut_at(mut a: BigUint, n: usize) -> (BigUint, BigUint) {
let mid = cmp::min(a.data.len(), n);
let hi = BigUint::from_slice(&a.data[mid ..]);
a.data.truncate(mid);
(hi, BigUint::new(a.data))
}
assert!(carry == 0);
}
#[inline]
fn sub_sign(a: BigUint, b: BigUint) -> (Ordering, BigUint) {
match a.cmp(&b) {
Less => (Less, b - a),
Greater => (Greater, a - b),
_ => (Equal, Zero::zero())
/// Three argument multiply accumulate:
/// acc += b * c
fn mac3(acc: &mut [BigDigit], b: &[BigDigit], c: &[BigDigit]) {
let (x, y) = if b.len() < c.len() { (b, c) } else { (c, b) };
/*
* Karatsuba multiplication is slower than long multiplication for small x and y:
*/
if x.len() <= 4 {
for (i, xi) in x.iter().enumerate() {
mac_digit(&mut acc[i..], y, *xi);
}
} else {
/*
* Karatsuba multiplication:
*
* The idea is that we break x and y up into two smaller numbers that each have about half
* as many digits, like so (note that multiplying by b is just a shift):
*
* x = x0 + x1 * b
* y = y0 + y1 * b
*
* With some algebra, we can compute x * y with three smaller products, where the inputs to
* each of the smaller products have only about half as many digits as x and y:
*
* x * y = (x0 + x1 * b) * (y0 + y1 * b)
*
* x * y = x0 * y0
* + x0 * y1 * b
* + x1 * y0 * b
* + x1 * y1 * b^2
*
* Let p0 = x0 * y0 and p2 = x1 * y1:
*
* x * y = p0
* + (x0 * y1 + x1 * p0) * b
* + p2 * b^2
*
* The real trick is that middle term:
*
* x0 * y1 + x1 * y0
*
* = x0 * y1 + x1 * y0 - p0 + p0 - p2 + p2
*
* = x0 * y1 + x1 * y0 - x0 * y0 - x1 * y1 + p0 + p2
*
* Now we complete the square:
*
* = -(x0 * y0 - x0 * y1 - x1 * y0 + x1 * y1) + p0 + p2
*
* = -((x1 - x0) * (y1 - y0)) + p0 + p2
*
* Let p1 = (x1 - x0) * (y1 - y0), and substitute back into our original formula:
*
* x * y = p0
* + (p0 + p2 - p1) * b
* + p2 * b^2
*
* Where the three intermediate products are:
*
* p0 = x0 * y0
* p1 = (x1 - x0) * (y1 - y0)
* p2 = x1 * y1
*
* In doing the computation, we take great care to avoid unnecessary temporary variables
* (since creating a BigUint requires a heap allocation): thus, we rearrange the formula a
* bit so we can use the same temporary variable for all the intermediate products:
*
* x * y = p2 * b^2 + p2 * b
* + p0 * b + p0
* - p1 * b
*
* The other trick we use is instead of doing explicit shifts, we slice acc at the
* appropriate offset when doing the add.
*/
/*
* When x is smaller than y, it's significantly faster to pick b such that x is split in
* half, not y:
*/
let b = x.len() / 2;
let (x0, x1) = x.split_at(b);
let (y0, y1) = y.split_at(b);
/* We reuse the same BigUint for all the intermediate multiplies: */
let len = y.len() + 1;
let mut p: BigUint = BigUint { data: Vec::with_capacity(len) };
p.data.extend(repeat(0).take(len));
// p2 = x1 * y1
mac3(&mut p.data[..], x1, y1);
// Not required, but the adds go faster if we drop any unneeded 0s from the end:
p = p.normalize();
add2(&mut acc[b..], &p.data[..]);
add2(&mut acc[b * 2..], &p.data[..]);
// Zero out p before the next multiply:
p.data.truncate(0);
p.data.extend(repeat(0).take(len));
// p0 = x0 * y0
mac3(&mut p.data[..], x0, y0);
p = p.normalize();
add2(&mut acc[..], &p.data[..]);
add2(&mut acc[b..], &p.data[..]);
// p1 = (x1 - x0) * (y1 - y0)
// We do this one last, since it may be negative and acc can't ever be negative:
let j0 = sub_sign(x1, x0);
let j1 = sub_sign(y1, y0);
match j0.sign * j1.sign {
Plus => {
p.data.truncate(0);
p.data.extend(repeat(0).take(len));
mac3(&mut p.data[..], &j0.data.data[..], &j1.data.data[..]);
p = p.normalize();
sub2(&mut acc[b..], &p.data[..]);
},
Minus => {
mac3(&mut acc[b..], &j0.data.data[..], &j1.data.data[..]);
},
NoSign => (),
}
}
}
fn mul3(x: &[BigDigit], y: &[BigDigit]) -> BigUint {
let len = x.len() + y.len() + 1;
let mut prod: BigUint = BigUint { data: Vec::with_capacity(len) };
// resize isn't stable yet:
//prod.data.resize(len, 0);
prod.data.extend(repeat(0).take(len));
mac3(&mut prod.data[..], x, y);
prod.normalize()
}
impl<'a, 'b> Mul<&'b BigUint> for &'a BigUint {
type Output = BigUint;
#[inline]
fn mul(self, other: &BigUint) -> BigUint {
mul3(&self.data[..], &other.data[..])
}
}
forward_all_binop_to_ref_ref!(impl Div for BigUint, div);
@ -3131,6 +3270,16 @@ mod biguint_tests {
// Switching u and l should fail:
let _n: BigUint = rng.gen_biguint_range(&u, &l);
}
#[test]
fn test_sub_sign() {
use super::sub_sign;
let a = BigInt::from_str_radix("265252859812191058636308480000000", 10).unwrap();
let b = BigInt::from_str_radix("26525285981219105863630848000000", 10).unwrap();
assert_eq!(sub_sign(&a.data.data[..], &b.data.data[..]), &a - &b);
assert_eq!(sub_sign(&b.data.data[..], &a.data.data[..]), &b - &a);
}
}
#[cfg(test)]