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perf: non-allocating mul_mod #373

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May 27, 2024
Merged

perf: non-allocating mul_mod #373

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6 changes: 2 additions & 4 deletions src/base_convert.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -95,8 +95,7 @@ impl<const BITS: usize, const LIMBS: usize> Uint<BITS, LIMBS> {
///
/// * [`BaseConvertError::InvalidBase`] if the base is less than 2.
/// * [`BaseConvertError::InvalidDigit`] if a digit is out of range.
/// * [`BaseConvertError::Overflow`] if the number is too large to
/// fit.
/// * [`BaseConvertError::Overflow`] if the number is too large to fit.
#[inline]
pub fn from_base_le<I>(base: u64, digits: I) -> Result<Self, BaseConvertError>
where
Expand Down Expand Up @@ -155,8 +154,7 @@ impl<const BITS: usize, const LIMBS: usize> Uint<BITS, LIMBS> {
///
/// * [`BaseConvertError::InvalidBase`] if the base is less than 2.
/// * [`BaseConvertError::InvalidDigit`] if a digit is out of range.
/// * [`BaseConvertError::Overflow`] if the number is too large to
/// fit.
/// * [`BaseConvertError::Overflow`] if the number is too large to fit.
#[inline]
pub fn from_base_be<I: IntoIterator<Item = u64>>(
base: u64,
Expand Down
23 changes: 13 additions & 10 deletions src/modular.rs
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Expand Up @@ -52,24 +52,28 @@ impl<const BITS: usize, const LIMBS: usize> Uint<BITS, LIMBS> {
/// some pre-computation.
#[inline]
#[must_use]
#[cfg(feature = "alloc")] // see comments below
pub fn mul_mod(self, rhs: Self, mut modulus: Self) -> Self {
if modulus == Self::ZERO {
return Self::ZERO;
}

// Allocate at least `nlimbs(2 * BITS)` limbs to store the product. This array
// casting is a workaround for `generic_const_exprs` not being stable.
let mut product = [[0u64; 2]; LIMBS];
let product_len = crate::nlimbs(2 * BITS);
debug_assert!(2 * LIMBS >= product_len);
// SAFETY: `[[u64; 2]; LIMBS] == [u64; 2 * LIMBS] >= [u64; nlimbs(2 * BITS)]`.
let product = unsafe {
core::slice::from_raw_parts_mut(product.as_mut_ptr().cast::<u64>(), product_len)
};

// Compute full product.
// The challenge here is that Rust doesn't allow us to create a
// `Uint<2 * BITS, _>` for the intermediate result. Otherwise
// we could just use a `widening_mul`. So instead we allocate from heap.
// Alternatively we could use `alloca`, but that is blocked on
// See <https://github.com/rust-lang/rust/issues/48055>
let mut product = vec![0; crate::nlimbs(2 * BITS)];
let overflow = algorithms::addmul(&mut product, self.as_limbs(), rhs.as_limbs());
let overflow = algorithms::addmul(product, self.as_limbs(), rhs.as_limbs());
debug_assert!(!overflow);

// Compute modulus using `div_rem`.
// This stores the remainder in the divisor, `modulus`.
algorithms::div(&mut product, &mut modulus.limbs);
algorithms::div(product, &mut modulus.limbs);

modulus
}
Expand All @@ -79,7 +83,6 @@ impl<const BITS: usize, const LIMBS: usize> Uint<BITS, LIMBS> {
/// Returns zero if the modulus is zero.
#[inline]
#[must_use]
#[cfg(feature = "alloc")] // see comments in mul_mod
pub fn pow_mod(mut self, mut exp: Self, modulus: Self) -> Self {
if modulus == Self::ZERO || modulus <= Self::from(1) {
// Also covers Self::BITS == 0
Expand Down
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