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Using x86 intrinsics for carry-less multiplication #544

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Using x86 intrinsics for carry-less multiplication #544

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18 changes: 12 additions & 6 deletions src/ff/galois_field.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,7 @@ use crate::{
use bitvec::prelude::{BitArr, Lsb0};
use generic_array::GenericArray;
use typenum::{Unsigned, U1, U4, U5};
use core::arch::x86_64::{_mm_clmulepi64_si128, _mm_set_epi64x, _mm_extract_epi64};

// Bit store type definitions
type U8_1 = BitArr!(for 8, in u8, Lsb0);
Expand Down Expand Up @@ -171,12 +172,17 @@ macro_rules! bit_array_impl {
type Output = Self;
fn mul(self, rhs: Self) -> Self::Output {
debug_assert!(2 * $bits < u128::BITS);
let a = <Self as GaloisField>::as_u128(self);
let mut product = 0;
for i in 0..Self::BITS {
let bit = u128::from(rhs[i]);
product ^= bit * (a << i);
}
let a = _mm_set_epi64x(
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Could be wrong but I think this wouldn't work on ARM because it uses different instruction set for carry-less multiplies. Lots of folks use M1 Mac (ARM) so we probably need to support both x86 and ARM and define 3 sets of multiplication instructions: no clmul support (basically what we had before), x86 with clmul support and ARM with clmul support.

In other words, something like this should be enough

#[cfg(all(
    target_arch = "x86_64",
    target_feature = "pclmulqdq"
))]
fn mul(a: 64, b: 64) -> u128 // or [u64; 2]
{
   ... Intel intrinsics  
}

#[cfg(all(
    target_arch="aarch64",
    target_feature="neon"
)]
fn mul(a: 64, b: 64) -> u128 // or [u64; 2]
{
   ... ARM intrinsics  
}

#[cfg(not(any(all(
    target_arch="aarch64",
    target_feature="neon"), all(
    target_arch = "x86_64",
    target_feature = "pclmulqdq"
)))]
fn mul(a: 64, b: 64) -> u128 // or [u64; 2] 
{
   no hardware support, standard multiply
}

some reference materials and I would be happy to help if needed:
rust-lang/stdarch#318
https://github.com/geky/gf256/blob/master/src/xmul.rs

0,
i64::try_from(<Self as GaloisField>::as_u128(self)).unwrap(),
);
let b = _mm_set_epi64x(
0,
i64::try_from(<Self as GaloisField>::as_u128(rhs)).unwrap(),
);
let intrinsic_result = _mm_clmulepi64_si128(a, b, 0);
let intermediate_result = _mm_extract_epi64(intrinsic_result, 0);
let mut product = u128::try_from(intermediate_result).unwrap();

let poly = <Self as GaloisField>::POLYNOMIAL;
while (u128::BITS - product.leading_zeros()) > $bits {
Expand Down