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Use carrying_{mul|add} in num::bignum #92960

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Jan 18, 2022
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Use carrying_{mul|add} in num::bignum #92960

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42 changes: 5 additions & 37 deletions library/core/src/num/bignum.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -19,18 +19,8 @@
)]
#![macro_use]

use crate::intrinsics;

/// Arithmetic operations required by bignums.
pub trait FullOps: Sized {
/// Returns `(carry', v')` such that `carry' * 2^W + v' = self + other + carry`,
/// where `W` is the number of bits in `Self`.
fn full_add(self, other: Self, carry: bool) -> (bool /* carry */, Self);

/// Returns `(carry', v')` such that `carry' * 2^W + v' = self * other + carry`,
/// where `W` is the number of bits in `Self`.
fn full_mul(self, other: Self, carry: Self) -> (Self /* carry */, Self);

/// Returns `(carry', v')` such that `carry' * 2^W + v' = self * other + other2 + carry`,
/// where `W` is the number of bits in `Self`.
fn full_mul_add(self, other: Self, other2: Self, carry: Self) -> (Self /* carry */, Self);
Expand All @@ -45,22 +35,6 @@ macro_rules! impl_full_ops {
($($ty:ty: add($addfn:path), mul/div($bigty:ident);)*) => (
$(
impl FullOps for $ty {
fn full_add(self, other: $ty, carry: bool) -> (bool, $ty) {
// This cannot overflow; the output is between `0` and `2 * 2^nbits - 1`.
// FIXME: will LLVM optimize this into ADC or similar?
let (v, carry1) = intrinsics::add_with_overflow(self, other);
let (v, carry2) = intrinsics::add_with_overflow(v, if carry {1} else {0});
(carry1 || carry2, v)
}

fn full_mul(self, other: $ty, carry: $ty) -> ($ty, $ty) {
// This cannot overflow;
// the output is between `0` and `2^nbits * (2^nbits - 1)`.
// FIXME: will LLVM optimize this into ADC or similar?
let v = (self as $bigty) * (other as $bigty) + (carry as $bigty);
((v >> <$ty>::BITS) as $ty, v as $ty)
}

fn full_mul_add(self, other: $ty, other2: $ty, carry: $ty) -> ($ty, $ty) {
// This cannot overflow;
// the output is between `0` and `2^nbits * (2^nbits - 1)`.
Expand Down Expand Up @@ -173,12 +147,11 @@ macro_rules! define_bignum {
pub fn add<'a>(&'a mut self, other: &$name) -> &'a mut $name {
use crate::cmp;
use crate::iter;
use crate::num::bignum::FullOps;

let mut sz = cmp::max(self.size, other.size);
let mut carry = false;
for (a, b) in iter::zip(&mut self.base[..sz], &other.base[..sz]) {
let (c, v) = (*a).full_add(*b, carry);
let (v, c) = (*a).carrying_add(*b, carry);
*a = v;
carry = c;
}
Expand All @@ -191,13 +164,11 @@ macro_rules! define_bignum {
}

pub fn add_small(&mut self, other: $ty) -> &mut $name {
use crate::num::bignum::FullOps;

let (mut carry, v) = self.base[0].full_add(other, false);
let (v, mut carry) = self.base[0].carrying_add(other, false);
self.base[0] = v;
let mut i = 1;
while carry {
let (c, v) = self.base[i].full_add(0, carry);
let (v, c) = self.base[i].carrying_add(0, carry);
self.base[i] = v;
carry = c;
i += 1;
Expand All @@ -212,12 +183,11 @@ macro_rules! define_bignum {
pub fn sub<'a>(&'a mut self, other: &$name) -> &'a mut $name {
use crate::cmp;
use crate::iter;
use crate::num::bignum::FullOps;

let sz = cmp::max(self.size, other.size);
let mut noborrow = true;
for (a, b) in iter::zip(&mut self.base[..sz], &other.base[..sz]) {
let (c, v) = (*a).full_add(!*b, noborrow);
let (v, c) = (*a).carrying_add(!*b, noborrow);
*a = v;
noborrow = c;
}
Expand All @@ -229,12 +199,10 @@ macro_rules! define_bignum {
/// Multiplies itself by a digit-sized `other` and returns its own
/// mutable reference.
pub fn mul_small(&mut self, other: $ty) -> &mut $name {
use crate::num::bignum::FullOps;

let mut sz = self.size;
let mut carry = 0;
for a in &mut self.base[..sz] {
let (c, v) = (*a).full_mul(other, carry);
let (v, c) = (*a).carrying_mul(other, carry);
*a = v;
carry = c;
}
Expand Down