Dumb down FRI (#529)

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benches/fri.rs

@@ -1,6 +1,8 @@
 use criterion::{black_box, criterion_group, criterion_main, Criterion};
 use stwo::core::backend::CPUBackend;
 use stwo::core::fields::m31::BaseField;
+use stwo::core::fields::qm31::SecureField;
+use stwo::core::fields::secure_column::SecureColumn;
 use stwo::core::fri::FriOps;
 use stwo::core::poly::circle::CanonicCoset;
 use stwo::core::poly::line::{LineDomain, LineEvaluation};
@@ -10,9 +12,13 @@ fn folding_benchmark(c: &mut Criterion) {
     let domain = LineDomain::new(CanonicCoset::new(LOG_SIZE + 1).half_coset());
     let evals = LineEvaluation::new(
         domain,
-        vec![BaseField::from_u32_unchecked(712837213).into(); 1 << LOG_SIZE],
+        SecureColumn {
+            columns: std::array::from_fn(|i| {
+                vec![BaseField::from_u32_unchecked(i as u32); 1 << LOG_SIZE]
+            }),
+        },
     );
-    let alpha = BaseField::from_u32_unchecked(12389).into();
+    let alpha = SecureField::from_u32_unchecked(2213980, 2213981, 2213982, 2213983);
     c.bench_function("fold_line", |b| {
         b.iter(|| {
             black_box(CPUBackend::fold_line(black_box(&evals), black_box(alpha)));

src/core/backend/cpu/fri.rs

@@ -1,31 +1,25 @@
-use std::iter::zip;
-
 use super::CPUBackend;
 use crate::core::fft::ibutterfly;
-use crate::core::fields::m31::BaseField;
 use crate::core::fields::qm31::SecureField;
-use crate::core::fields::{ExtensionOf, Field, FieldExpOps};
+use crate::core::fields::FieldExpOps;
 use crate::core::fri::{FriOps, CIRCLE_TO_LINE_FOLD_STEP, FOLD_STEP};
-use crate::core::poly::circle::CircleEvaluation;
+use crate::core::poly::circle::SecureEvaluation;
 use crate::core::poly::line::LineEvaluation;
-use crate::core::poly::BitReversedOrder;
 use crate::core::utils::bit_reverse_index;
 
 impl FriOps for CPUBackend {
-    fn fold_line(
-        eval: &LineEvaluation<Self, SecureField, BitReversedOrder>,
-        alpha: SecureField,
-    ) -> LineEvaluation<Self, SecureField, BitReversedOrder> {
+    fn fold_line(eval: &LineEvaluation<Self>, alpha: SecureField) -> LineEvaluation<Self> {
         let n = eval.len();
         assert!(n >= 2, "Evaluation too small");
 
         let domain = eval.domain();
 
         let folded_values = eval
             .values
+            .into_iter()
             .array_chunks()
             .enumerate()
-            .map(|(i, &[f_x, f_neg_x])| {
+            .map(|(i, [f_x, f_neg_x])| {
                 // TODO(andrew): Inefficient. Update when domain twiddles get stored in a buffer.
                 let x = domain.at(bit_reverse_index(i << FOLD_STEP, domain.log_size()));
 
@@ -37,22 +31,20 @@ impl FriOps for CPUBackend {
 
         LineEvaluation::new(domain.double(), folded_values)
     }
-    fn fold_circle_into_line<F: Field>(
-        dst: &mut LineEvaluation<Self, SecureField, BitReversedOrder>,
-        src: &CircleEvaluation<Self, F, BitReversedOrder>,
+    fn fold_circle_into_line(
+        dst: &mut LineEvaluation<Self>,
+        src: &SecureEvaluation<Self>,
         alpha: SecureField,
-    ) where
-        F: ExtensionOf<BaseField>,
-        SecureField: ExtensionOf<F> + Field,
-    {
+    ) {
         assert_eq!(src.len() >> CIRCLE_TO_LINE_FOLD_STEP, dst.len());
 
         let domain = src.domain;
         let alpha_sq = alpha * alpha;
 
-        zip(&mut dst.values, src.array_chunks())
+        src.into_iter()
+            .array_chunks()
             .enumerate()
-            .for_each(|(i, (dst, &[f_p, f_neg_p]))| {
+            .for_each(|(i, [f_p, f_neg_p])| {
                 // TODO(andrew): Inefficient. Update when domain twiddles get stored in a buffer.
                 let p = domain.at(bit_reverse_index(
                     i << CIRCLE_TO_LINE_FOLD_STEP,
@@ -64,7 +56,7 @@ impl FriOps for CPUBackend {
                 ibutterfly(&mut f0_px, &mut f1_px, p.y.inverse());
                 let f_prime = alpha * f1_px + f0_px;
 
-                *dst = *dst * alpha_sq + f_prime;
+                dst.values.set(i, dst.values.at(i) * alpha_sq + f_prime);
             });
     }
 }

src/core/backend/cpu/mod.rs

@@ -8,7 +8,6 @@ use std::fmt::Debug;
 use super::{Backend, Column, ColumnOps, FieldOps};
 use crate::core::fields::Field;
 use crate::core::poly::circle::{CircleEvaluation, CirclePoly};
-use crate::core::poly::line::LineEvaluation;
 use crate::core::utils::bit_reverse;
 
 #[derive(Copy, Clone, Debug)]
@@ -49,8 +48,6 @@ impl<T: Debug + Clone + Default> Column<T> for Vec<T> {
 
 pub type CPUCirclePoly = CirclePoly<CPUBackend>;
 pub type CPUCircleEvaluation<F, EvalOrder> = CircleEvaluation<CPUBackend, F, EvalOrder>;
-// TODO(spapini): Remove the EvalOrder on LineEvaluation.
-pub type CPULineEvaluation<F, EvalOrder> = LineEvaluation<CPUBackend, F, EvalOrder>;
 
 #[cfg(test)]
 mod tests {

src/core/commitment_scheme/prover.rs

@@ -22,7 +22,6 @@ use crate::commitment_scheme::blake2_hash::{Blake2sHash, Blake2sHasher};
 use crate::commitment_scheme::blake2_merkle::Blake2sMerkleHasher;
 use crate::commitment_scheme::prover::{MerkleDecommitment, MerkleProver};
 use crate::core::channel::Channel;
-use crate::core::poly::circle::SecureEvaluation;
 
 type MerkleHasher = Blake2sMerkleHasher;
 type ProofChannel = Blake2sChannel;
@@ -89,13 +88,6 @@ impl CommitmentSchemeProver {
         let columns = self.evaluations().flatten();
         let quotients = compute_fri_quotients(&columns, &samples.flatten(), channel.draw_felt());
 
-        // TODO(spapini): Conversion to CircleEvaluation can be removed when FRI supports
-        // SecureColumn.
-        let quotients = quotients
-            .into_iter()
-            .map(SecureEvaluation::to_cpu)
-            .collect_vec();
-
         // Run FRI commitment phase on the oods quotients.
         let fri_config = FriConfig::new(LOG_LAST_LAYER_DEGREE_BOUND, LOG_BLOWUP_FACTOR, N_QUERIES);
         let fri_prover =

src/core/commitment_scheme/quotients.rs

@@ -96,7 +96,7 @@ pub fn fri_answers(
     random_coeff: SecureField,
     query_domain_per_log_size: BTreeMap<u32, SparseSubCircleDomain>,
     queried_values_per_column: &[Vec<BaseField>],
-) -> Result<Vec<SparseCircleEvaluation<SecureField>>, VerificationError> {
+) -> Result<Vec<SparseCircleEvaluation>, VerificationError> {
     izip!(column_log_sizes, samples, queried_values_per_column)
         .sorted_by_key(|(log_size, ..)| Reverse(*log_size))
         .group_by(|(log_size, ..)| *log_size)
@@ -121,7 +121,7 @@ pub fn fri_answers_for_log_size(
     random_coeff: SecureField,
     query_domain: &SparseSubCircleDomain,
     queried_values_per_column: &[&Vec<BaseField>],
-) -> Result<SparseCircleEvaluation<SecureField>, VerificationError> {
+) -> Result<SparseCircleEvaluation, VerificationError> {
     let commitment_domain = CanonicCoset::new(log_size).circle_domain();
     let sample_batches = ColumnSampleBatch::new_vec(samples);
     for queried_values in queried_values_per_column {

src/core/fields/secure_column.rs

@@ -44,4 +44,56 @@ impl<B: FieldOps<BaseField>> SecureColumn<B> {
     pub fn is_empty(&self) -> bool {
         self.columns[0].is_empty()
     }
+
+    pub fn to_cpu(&self) -> SecureColumn<CPUBackend> {
+        SecureColumn {
+            columns: self.columns.clone().map(|c| c.to_vec()),
+        }
+    }
+}
+
+pub struct SecureColumnIter<'a> {
+    column: &'a SecureColumn<CPUBackend>,
+    index: usize,
+}
+impl Iterator for SecureColumnIter<'_> {
+    type Item = SecureField;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.index < self.column.len() {
+            let value = self.column.at(self.index);
+            self.index += 1;
+            Some(value)
+        } else {
+            None
+        }
+    }
+}
+impl<'a> IntoIterator for &'a SecureColumn<CPUBackend> {
+    type Item = SecureField;
+    type IntoIter = SecureColumnIter<'a>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        SecureColumnIter {
+            column: self,
+            index: 0,
+        }
+    }
+}
+impl FromIterator<SecureField> for SecureColumn<CPUBackend> {
+    fn from_iter<I: IntoIterator<Item = SecureField>>(iter: I) -> Self {
+        let mut columns = std::array::from_fn(|_| vec![]);
+        for value in iter.into_iter() {
+            let vals = value.to_m31_array();
+            for j in 0..SECURE_EXTENSION_DEGREE {
+                columns[j].push(vals[j]);
+            }
+        }
+        SecureColumn { columns }
+    }
+}
+impl From<SecureColumn<CPUBackend>> for Vec<SecureField> {
+    fn from(column: SecureColumn<CPUBackend>) -> Self {
+        column.into_iter().collect()
+    }
 }