Speed up MSMs for non-GPU accelerated MSMs and architectures that don't support GPU/semolina (#126)

* WASM target support

* fast multiexp for WASM

* add parallelisation for MSM

https://github.com/zcash/halo2/blob/main/halo2_proofs/src/arithmetic.rs

Cargo.toml

@@ -23,7 +23,6 @@ rand_chacha = "0.3"
 itertools = "0.9.0"
 subtle = "2.4"
 pasta_curves = { version = "0.4.0", features = ["repr-c"] }
-pasta-msm = "0.1.3"
 neptune = { version = "8.1.0", default-features = false }
 generic-array = "0.14.4"
 num-bigint = { version = "0.4", features = ["serde", "rand"] }
@@ -47,4 +46,7 @@ name = "compressed-snark"
 harness = false
 
 [features]
-default = [ "bellperson/default", "neptune/default" ]
+default = ["bellperson/default", "neptune/default"]
+
+[target.'cfg(any(target_arch = "x86_64", target_arch = "aarch64"))'.dependencies]
+pasta-msm = "0.1.3"

src/pasta.rs

@@ -16,9 +16,138 @@ use pasta_curves::{
 };
 use rand::SeedableRng;
 use rand_chacha::ChaCha20Rng;
-use rayon::prelude::*;
 use sha3::Shake256;
-use std::{io::Read, ops::Mul};
+use std::io::Read;
+
+//////////////////////////////////////Shared MSM code for Pasta curves///////////////////////////////////////////////
+
+/// Native implementation of fast multiexp for platforms that do not support pasta_msm/semolina
+/// Forked from zcash/halo2
+fn cpu_multiexp_serial<C: CurveAffine>(coeffs: &[C::Scalar], bases: &[C], acc: &mut C::Curve) {
+  use ff::PrimeField;
+  let coeffs: Vec<_> = coeffs.iter().map(|a| a.to_repr()).collect();
+
+  let c = if bases.len() < 4 {
+    1
+  } else if bases.len() < 32 {
+    3
+  } else {
+    (f64::from(bases.len() as u32)).ln().ceil() as usize
+  };
+
+  fn get_at<F: PrimeField>(segment: usize, c: usize, bytes: &F::Repr) -> usize {
+    let skip_bits = segment * c;
+    let skip_bytes = skip_bits / 8;
+
+    if skip_bytes >= 32 {
+      return 0;
+    }
+
+    let mut v = [0; 8];
+    for (v, o) in v.iter_mut().zip(bytes.as_ref()[skip_bytes..].iter()) {
+      *v = *o;
+    }
+
+    let mut tmp = u64::from_le_bytes(v);
+    tmp >>= skip_bits - (skip_bytes * 8);
+    tmp %= 1 << c;
+
+    tmp as usize
+  }
+
+  let segments = (256 / c) + 1;
+
+  for current_segment in (0..segments).rev() {
+    for _ in 0..c {
+      *acc = acc.double();
+    }
+
+    #[derive(Clone, Copy)]
+    enum Bucket<C: CurveAffine> {
+      None,
+      Affine(C),
+      Projective(C::Curve),
+    }
+
+    impl<C: CurveAffine> Bucket<C> {
+      fn add_assign(&mut self, other: &C) {
+        *self = match *self {
+          Bucket::None => Bucket::Affine(*other),
+          Bucket::Affine(a) => Bucket::Projective(a + *other),
+          Bucket::Projective(mut a) => {
+            a += *other;
+            Bucket::Projective(a)
+          }
+        }
+      }
+
+      fn add(self, mut other: C::Curve) -> C::Curve {
+        match self {
+          Bucket::None => other,
+          Bucket::Affine(a) => {
+            other += a;
+            other
+          }
+          Bucket::Projective(a) => other + a,
+        }
+      }
+    }
+
+    let mut buckets: Vec<Bucket<C>> = vec![Bucket::None; (1 << c) - 1];
+
+    for (coeff, base) in coeffs.iter().zip(bases.iter()) {
+      let coeff = get_at::<C::Scalar>(current_segment, c, coeff);
+      if coeff != 0 {
+        buckets[coeff - 1].add_assign(base);
+      }
+    }
+
+    // Summation by parts
+    // e.g. 3a + 2b + 1c = a +
+    //                    (a) + b +
+    //                    ((a) + b) + c
+    let mut running_sum = C::Curve::identity();
+    for exp in buckets.into_iter().rev() {
+      running_sum = exp.add(running_sum);
+      *acc += &running_sum;
+    }
+  }
+}
+
+/// Performs a multi-exponentiation operation without GPU acceleration.
+///
+/// This function will panic if coeffs and bases have a different length.
+///
+/// This will use multithreading if beneficial.
+/// Forked from zcash/halo2
+fn cpu_best_multiexp<C: CurveAffine>(coeffs: &[C::Scalar], bases: &[C]) -> C::Curve {
+  assert_eq!(coeffs.len(), bases.len());
+
+  let num_threads = rayon::current_num_threads();
+  if coeffs.len() > num_threads {
+    let chunk = coeffs.len() / num_threads;
+    let num_chunks = coeffs.chunks(chunk).len();
+    let mut results = vec![C::Curve::identity(); num_chunks];
+    rayon::scope(|scope| {
+      let chunk = coeffs.len() / num_threads;
+
+      for ((coeffs, bases), acc) in coeffs
+        .chunks(chunk)
+        .zip(bases.chunks(chunk))
+        .zip(results.iter_mut())
+      {
+        scope.spawn(move |_| {
+          cpu_multiexp_serial(coeffs, bases, acc);
+        });
+      }
+    });
+    results.iter().fold(C::Curve::identity(), |a, b| a + b)
+  } else {
+    let mut acc = C::Curve::identity();
+    cpu_multiexp_serial(coeffs, bases, &mut acc);
+    acc
+  }
+}
 
 //////////////////////////////////////Pallas///////////////////////////////////////////////
 
@@ -43,6 +172,7 @@ impl Group for pallas::Point {
   type RO = PoseidonRO<Self::Base, Self::Scalar>;
   type ROCircuit = PoseidonROCircuit<Self::Base>;
 
+  #[cfg(any(target_arch = "x86_64", target_arch = "aarch64"))]
   fn vartime_multiscalar_mul(
     scalars: &[Self::Scalar],
     bases: &[Self::PreprocessedGroupElement],
@@ -50,14 +180,18 @@ impl Group for pallas::Point {
     if scalars.len() >= 128 {
       pasta_msm::pallas(bases, scalars)
     } else {
-      scalars
-        .par_iter()
-        .zip(bases)
-        .map(|(scalar, base)| base.mul(scalar))
-        .reduce(Ep::group_zero, |x, y| x + y)
+      cpu_best_multiexp(scalars, bases)
     }
   }
 
+  #[cfg(not(any(target_arch = "x86_64", target_arch = "aarch64")))]
+  fn vartime_multiscalar_mul(
+    scalars: &[Self::Scalar],
+    bases: &[Self::PreprocessedGroupElement],
+  ) -> Self {
+    cpu_best_multiexp(scalars, bases)
+  }
+
   fn preprocessed(&self) -> Self::PreprocessedGroupElement {
     self.to_affine()
   }
@@ -153,6 +287,7 @@ impl Group for vesta::Point {
   type RO = PoseidonRO<Self::Base, Self::Scalar>;
   type ROCircuit = PoseidonROCircuit<Self::Base>;
 
+  #[cfg(any(target_arch = "x86_64", target_arch = "aarch64"))]
   fn vartime_multiscalar_mul(
     scalars: &[Self::Scalar],
     bases: &[Self::PreprocessedGroupElement],
@@ -160,14 +295,18 @@ impl Group for vesta::Point {
     if scalars.len() >= 128 {
       pasta_msm::vesta(bases, scalars)
     } else {
-      scalars
-        .par_iter()
-        .zip(bases)
-        .map(|(scalar, base)| base.mul(scalar))
-        .reduce(Eq::group_zero, |x, y| x + y)
+      cpu_best_multiexp(scalars, bases)
     }
   }
 
+  #[cfg(not(any(target_arch = "x86_64", target_arch = "aarch64")))]
+  fn vartime_multiscalar_mul(
+    scalars: &[Self::Scalar],
+    bases: &[Self::PreprocessedGroupElement],
+  ) -> Self {
+    cpu_best_multiexp(scalars, bases)
+  }
+
   fn compress(&self) -> Self::CompressedGroupElement {
     VestaCompressedElementWrapper::new(self.to_bytes())
   }