More arithmetic optimizations

src/fri/recursive_verifier.rs

@@ -139,7 +139,13 @@ impl<F: Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         let precomputed_reduced_evals = with_context!(
             self,
             "precompute reduced evaluations",
-            PrecomputedReducedEvalsTarget::from_os_and_alpha(os, alpha, self)
+            PrecomputedReducedEvalsTarget::from_os_and_alpha(
+                os,
+                alpha,
+                common_data.degree_bits,
+                zeta,
+                self
+            )
         );
 
         for (i, round_proof) in proof.query_round_proofs.iter().enumerate() {
@@ -204,8 +210,8 @@ impl<F: Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         &mut self,
         proof: &FriInitialTreeProofTarget,
         alpha: ExtensionTarget<D>,
-        zeta: ExtensionTarget<D>,
         subgroup_x: Target,
+        vanish_zeta: ExtensionTarget<D>,
         precomputed_reduced_evals: PrecomputedReducedEvalsTarget<D>,
         common_data: &CommonCircuitData<F, D>,
     ) -> ExtensionTarget<D> {
@@ -218,7 +224,6 @@ impl<F: Extendable<D>, const D: usize> CircuitBuilder<F, D> {
                 - config.rate_bits
         );
         let subgroup_x = self.convert_to_ext(subgroup_x);
-        let vanish_zeta = self.sub_extension(subgroup_x, zeta);
         let mut alpha = ReducingFactorTarget::new(alpha);
         let mut sum = self.zero_extension();
 
@@ -255,19 +260,19 @@ impl<F: Extendable<D>, const D: usize> CircuitBuilder<F, D> {
             .collect::<Vec<_>>();
         let zs_composition_eval = alpha.reduce_base(&zs_evals, self);
 
-        let g = self.constant_extension(F::Extension::primitive_root_of_unity(degree_log));
-        let zeta_right = self.mul_extension(g, zeta);
-        let interpol_val = self.interpolate2(
-            [
-                (zeta, precomputed_reduced_evals.zs),
-                (zeta_right, precomputed_reduced_evals.zs_right),
-            ],
-            subgroup_x,
+        let interpol_val = self.mul_add_extension(
+            vanish_zeta,
+            precomputed_reduced_evals.slope,
+            precomputed_reduced_evals.zs,
         );
-        let (zs_numerator, vanish_zeta_right) =
-            self.sub_two_extension(zs_composition_eval, interpol_val, subgroup_x, zeta_right);
-        let zs_denominator = self.mul_extension(vanish_zeta, vanish_zeta_right);
-        sum = alpha.shift(sum, self);
+        let (zs_numerator, vanish_zeta_right) = self.sub_two_extension(
+            zs_composition_eval,
+            interpol_val,
+            subgroup_x,
+            precomputed_reduced_evals.zeta_right,
+        );
+        let (mut sum, zs_denominator) =
+            alpha.shift_and_mul(sum, vanish_zeta, vanish_zeta_right, self);
         sum = self.div_add_extension(zs_numerator, zs_denominator, sum);
 
         sum
@@ -307,12 +312,25 @@ impl<F: Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         );
 
         // `subgroup_x` is `subgroup[x_index]`, i.e., the actual field element in the domain.
-        let mut subgroup_x = with_context!(self, "compute x from its index", {
-            let g = self.constant(F::MULTIPLICATIVE_GROUP_GENERATOR);
+        let (mut subgroup_x, vanish_zeta) = with_context!(self, "compute x from its index", {
+            let g = self.constant(F::coset_shift());
             let phi = self.constant(F::primitive_root_of_unity(n_log));
 
             let phi = self.exp_from_bits(phi, x_index_bits.iter().rev());
-            self.mul(g, phi)
+            let g_ext = self.convert_to_ext(g);
+            let phi_ext = self.convert_to_ext(phi);
+            let zero = self.zero_extension();
+            let tmp = self.double_arithmetic_extension(
+                F::ONE,
+                F::NEG_ONE,
+                g_ext,
+                phi_ext,
+                zero,
+                g_ext,
+                phi_ext,
+                zeta,
+            );
+            (tmp.0 .0[0], tmp.1)
         });
 
         // old_eval is the last derived evaluation; it will be checked for consistency with its
@@ -323,8 +341,8 @@ impl<F: Extendable<D>, const D: usize> CircuitBuilder<F, D> {
             self.fri_combine_initial(
                 &round_proof.initial_trees_proof,
                 alpha,
-                zeta,
                 subgroup_x,
+                vanish_zeta,
                 precomputed_reduced_evals,
                 common_data,
             )
@@ -393,12 +411,16 @@ struct PrecomputedReducedEvalsTarget<const D: usize> {
     pub single: ExtensionTarget<D>,
     pub zs: ExtensionTarget<D>,
     pub zs_right: ExtensionTarget<D>,
+    pub slope: ExtensionTarget<D>,
+    pub zeta_right: ExtensionTarget<D>,
 }
 
 impl<const D: usize> PrecomputedReducedEvalsTarget<D> {
     fn from_os_and_alpha<F: Extendable<D>>(
         os: &OpeningSetTarget<D>,
         alpha: ExtensionTarget<D>,
+        degree_log: usize,
+        zeta: ExtensionTarget<D>,
         builder: &mut CircuitBuilder<F, D>,
     ) -> Self {
         let mut alpha = ReducingFactorTarget::new(alpha);
@@ -416,10 +438,16 @@ impl<const D: usize> PrecomputedReducedEvalsTarget<D> {
         let zs = alpha.reduce(&os.plonk_zs, builder);
         let zs_right = alpha.reduce(&os.plonk_zs_right, builder);
 
+        let g = builder.constant_extension(F::Extension::primitive_root_of_unity(degree_log));
+        let zeta_right = builder.mul_extension(g, zeta);
+        let (numerator, denominator) = builder.sub_two_extension(zs_right, zs, zeta_right, zeta);
+
         Self {
             single,
             zs,
             zs_right,
+            slope: builder.div_extension(numerator, denominator),
+            zeta_right,
         }
     }
 }

src/util/reducing.rs

@@ -211,9 +211,23 @@ impl<const D: usize> ReducingFactorTarget<D> {
         F: Extendable<D>,
     {
         let exp = builder.exp_u64_extension(self.base, self.count);
-        let tmp = builder.mul_extension(exp, x);
         self.count = 0;
-        tmp
+        builder.mul_extension(exp, x)
+    }
+
+    pub fn shift_and_mul<F>(
+        &mut self,
+        x: ExtensionTarget<D>,
+        a: ExtensionTarget<D>,
+        b: ExtensionTarget<D>,
+        builder: &mut CircuitBuilder<F, D>,
+    ) -> (ExtensionTarget<D>, ExtensionTarget<D>)
+    where
+        F: Extendable<D>,
+    {
+        let exp = builder.exp_u64_extension(self.base, self.count);
+        self.count = 0;
+        builder.mul_two_extension(exp, x, a, b)
     }
 
     pub fn reset(&mut self) {