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Improve division performance 

Before:
test divide_0      ... bench:       4,058 ns/iter (+/- 255)
test divide_1      ... bench:     304,507 ns/iter (+/- 28,063)
test divide_2      ... bench: 668,293,969 ns/iter (+/- 25,383,239)

After:
test divide_0      ... bench:         874 ns/iter (+/- 71)
test divide_1      ... bench:      16,641 ns/iter (+/- 1,205)
test divide_2      ... bench:   1,336,888 ns/iter (+/- 77,450)
This commit is contained in:
Kent Overstreet 2015-12-10 12:40:45 -09:00
parent 08b0022aab
commit 79928b3185
1 changed files with 107 additions and 95 deletions
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196
src/bigint.rs
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@ -158,6 +158,20 @@ fn mac_with_carry(a: BigDigit, b: BigDigit, c: BigDigit, carry: &mut BigDigit) -
lo
}
/// Divide a two digit numerator by a one digit divisor, returns quotient and remainder:
///
/// Note: the caller must ensure that both the quotient and remainder will fit into a single digit.
/// This is _not_ true for an arbitrary numerator/denominator.
///
/// (This function also matches what the x86 divide instruction does).
#[inline]
fn div_wide(hi: BigDigit, lo: BigDigit, divisor: BigDigit) -> (BigDigit, BigDigit) {
debug_assert!(hi < divisor);
let lhs = big_digit::to_doublebigdigit(hi, lo);
let rhs = divisor as DoubleBigDigit;
((lhs / rhs) as BigDigit, (lhs % rhs) as BigDigit)
}
/// A big unsigned integer type.
///
/// A `BigUint`-typed value `BigUint { data: vec!(a, b, c) }` represents a number
@ -822,6 +836,18 @@ impl<'a, 'b> Mul<&'b BigUint> for &'a BigUint {
}
}
fn div_rem_digit(mut a: BigUint, b: BigDigit) -> (BigUint, BigDigit) {
let mut rem = 0;
for d in a.data.iter_mut().rev() {
let (q, r) = div_wide(rem, *d, b);
*d = q;
rem = r;
}
(a.normalize(), rem)
}
forward_all_binop_to_ref_ref!(impl Div for BigUint, div);
impl<'a, 'b> Div<&'b BigUint> for &'a BigUint {
@ -917,83 +943,94 @@ impl Integer for BigUint {
if self.is_zero() { return (Zero::zero(), Zero::zero()); }
if *other == One::one() { return (self.clone(), Zero::zero()); }
/* Required or the q_len calculation below can underflow: */
match self.cmp(other) {
Less => return (Zero::zero(), self.clone()),
Equal => return (One::one(), Zero::zero()),
Greater => {} // Do nothing
}
let mut shift = 0;
let mut n = *other.data.last().unwrap();
while n < (1 << big_digit::BITS - 2) {
n <<= 1;
shift += 1;
}
assert!(shift < big_digit::BITS);
let (d, m) = div_mod_floor_inner(self << shift, other << shift);
return (d, m >> shift);
/*
* This algorithm is from Knuth, TAOCP vol 2 section 4.3, algorithm D:
*
* First, normalize the arguments so the highest bit in the highest digit of the divisor is
* set: the main loop uses the highest digit of the divisor for generating guesses, so we
* want it to be the largest number we can efficiently divide by.
*/
let shift = other.data.last().unwrap().leading_zeros() as usize;
let mut a = self << shift;
let b = other << shift;
/*
* The algorithm works by incrementally calculating "guesses", q0, for part of the
* remainder. Once we have any number q0 such that q0 * b <= a, we can set
*
* q += q0
* a -= q0 * b
*
* and then iterate until a < b. Then, (q, a) will be our desired quotient and remainder.
*
* q0, our guess, is calculated by dividing the last few digits of a by the last digit of b
* - this should give us a guess that is "close" to the actual quotient, but is possibly
* greater than the actual quotient. If q0 * b > a, we simply use iterated subtraction
* until we have a guess such that q0 & b <= a.
*/
fn div_mod_floor_inner(a: BigUint, b: BigUint) -> (BigUint, BigUint) {
let mut m = a;
let mut d: BigUint = Zero::zero();
let mut n = 1;
while m >= b {
let (d0, d_unit, b_unit) = div_estimate(&m, &b, n);
let mut d0 = d0;
let mut prod = &b * &d0;
while prod > m {
// FIXME(#5992): assignment operator overloads
// d0 -= &d_unit
d0 = d0 - &d_unit;
// FIXME(#5992): assignment operator overloads
// prod -= &b_unit;
prod = prod - &b_unit
}
if d0.is_zero() {
n = 2;
let bn = *b.data.last().unwrap();
let q_len = a.data.len() - b.data.len() + 1;
let mut q: BigUint = BigUint { data: Vec::with_capacity(q_len) };
q.data.extend(repeat(0).take(q_len));
/*
* We reuse the same temporary to avoid hitting the allocator in our inner loop - this is
* sized to hold a0 (in the common case; if a particular digit of the quotient is zero a0
* can be bigger).
*/
let mut tmp: BigUint = BigUint { data: Vec::with_capacity(2) };
for j in (0..q_len).rev() {
/*
* When calculating our next guess q0, we don't need to consider the digits below j
* + b.data.len() - 1: we're guessing digit j of the quotient (i.e. q0 << j) from
* digit bn of the divisor (i.e. bn << (b.data.len() - 1) - so the product of those
* two numbers will be zero in all digits up to (j + b.data.len() - 1).
*/
let offset = j + b.data.len() - 1;
if offset >= a.data.len() {
continue;
}
n = 1;
// FIXME(#5992): assignment operator overloads
// d += d0;
d = d + d0;
// FIXME(#5992): assignment operator overloads
// m -= prod;
m = m - prod;
}
return (d, m);
}
/* just avoiding a heap allocation: */
let mut a0 = tmp;
a0.data.truncate(0);
a0.data.extend(a.data[offset..].iter().cloned());
fn div_estimate(a: &BigUint, b: &BigUint, n: usize)
-> (BigUint, BigUint, BigUint) {
if a.data.len() < n {
return (Zero::zero(), Zero::zero(), (*a).clone());
}
/*
* q0 << j * big_digit::BITS is our actual quotient estimate - we do the shifts
* implicitly at the end, when adding and subtracting to a and q. Not only do we
* save the cost of the shifts, the rest of the arithmetic gets to work with
* smaller numbers.
*/
let (mut q0, _) = div_rem_digit(a0, bn);
let mut prod = &b * &q0;
let an = &a.data[a.data.len() - n ..];
let bn = *b.data.last().unwrap();
let mut d = Vec::with_capacity(an.len());
let mut carry = 0;
for elt in an.iter().rev() {
let ai = big_digit::to_doublebigdigit(carry, *elt);
let di = ai / (bn as DoubleBigDigit);
assert!(di < big_digit::BASE);
carry = (ai % (bn as DoubleBigDigit)) as BigDigit;
d.push(di as BigDigit)
}
d.reverse();
let shift = (a.data.len() - an.len()) - (b.data.len() - 1);
if shift == 0 {
return (BigUint::new(d), One::one(), (*b).clone());
}
while cmp_slice(&prod.data[..], &a.data[j..]) == Greater {
let one: BigUint = One::one();
return (BigUint::new(d).shl_unit(shift),
one.shl_unit(shift),
b.shl_unit(shift));
q0 = q0 - one;
prod = prod - &b;
}
add2(&mut q.data[j..], &q0.data[..]);
sub2(&mut a.data[j..], &prod.data[..]);
a = a.normalize();
tmp = q0;
}
debug_assert!(a < b);
(q.normalize(), a >> shift)
}
/// Calculates the Greatest Common Divisor (GCD) of the number and `other`.
@ -1146,43 +1183,18 @@ fn to_str_radix_reversed(u: &BigUint, radix: u32) -> Vec<u8> {
vec![b'0']
} else {
let mut res = Vec::new();
let mut digits = u.data.to_vec();
let mut digits = u.clone();
while !digits.is_empty() {
let rem = div_rem_in_place(&mut digits, radix);
res.push(to_digit(rem as u8));
// If we finished the most significant digit, drop it
if let Some(&0) = digits.last() {
digits.pop();
}
while digits != Zero::zero() {
let (q, r) = div_rem_digit(digits, radix as BigDigit);
res.push(to_digit(r as u8));
digits = q;
}
res
}
}
fn div_rem_in_place(digits: &mut [BigDigit], divisor: BigDigit) -> BigDigit {
let mut rem = 0;
for d in digits.iter_mut().rev() {
let (q, r) = full_div_rem(*d, divisor, rem);
*d = q;
rem = r;
}
rem
}
fn full_div_rem(a: BigDigit, b: BigDigit, borrow: BigDigit) -> (BigDigit, BigDigit) {
let lo = a as DoubleBigDigit;
let hi = borrow as DoubleBigDigit;
let lhs = lo | (hi << big_digit::BITS);
let rhs = b as DoubleBigDigit;
((lhs / rhs) as BigDigit, (lhs % rhs) as BigDigit)
}
fn to_digit(b: u8) -> u8 {
match b {
0 ... 9 => b'0' + b,