Change gate method for extra constants (#528)

* Change gate method for extra constants

* Rename

* feedback

plonky2/src/gates/constant.rs

@@ -6,7 +6,7 @@ use crate::gates::packed_util::PackedEvaluableBase;
 use crate::gates::util::StridedConstraintConsumer;
 use crate::hash::hash_types::RichField;
 use crate::iop::ext_target::ExtensionTarget;
-use crate::iop::generator::{ConstantGenerator, WitnessGenerator};
+use crate::iop::generator::WitnessGenerator;
 use crate::plonk::circuit_builder::CircuitBuilder;
 use crate::plonk::vars::{
     EvaluationTargets, EvaluationVars, EvaluationVarsBase, EvaluationVarsBaseBatch,
@@ -95,16 +95,9 @@ impl<F: RichField + Extendable<D>, const D: usize> Gate<F, D> for ConstantGate {
         self.num_consts
     }
 
-    fn extra_constants(&self, gate_index: usize) -> Vec<ConstantGenerator<F>> {
+    fn extra_constant_wires(&self) -> Vec<(usize, usize)> {
         (0..self.num_consts)
-            .map(|i| {
-                ConstantGenerator::new(
-                    gate_index,
-                    self.const_input(i),
-                    self.wire_output(i),
-                    F::ZERO,
-                )
-            })
+            .map(|i| (self.const_input(i), self.wire_output(i)))
             .collect()
     }
 }

plonky2/src/gates/gate.rs

@@ -12,7 +12,7 @@ use crate::gates::selectors::UNUSED_SELECTOR;
 use crate::gates::util::StridedConstraintConsumer;
 use crate::hash::hash_types::RichField;
 use crate::iop::ext_target::ExtensionTarget;
-use crate::iop::generator::{ConstantGenerator, WitnessGenerator};
+use crate::iop::generator::WitnessGenerator;
 use crate::plonk::circuit_builder::CircuitBuilder;
 use crate::plonk::vars::{
     EvaluationTargets, EvaluationVars, EvaluationVarsBase, EvaluationVarsBaseBatch,
@@ -181,8 +181,12 @@ pub trait Gate<F: RichField + Extendable<D>, const D: usize>: 'static + Send + S
             .len()
     }
 
-    /// Extra constants (i.e. constants that are not used in the gate's core functionality) generated by the gate.
-    fn extra_constants(&self, _gate_index: usize) -> Vec<ConstantGenerator<F>> {
+    /// Enables gates to store some "routed constants", if they have both unused constants and
+    /// unused routed wires.
+    ///
+    /// Each entry in the returned `Vec` has the form `(constant_index, wire_index)`. `wire_index`
+    /// must correspond to a *routed* wire.
+    fn extra_constant_wires(&self) -> Vec<(usize, usize)> {
         vec![]
     }
 }

plonky2/src/gates/random_access.rs

@@ -10,9 +10,7 @@ use crate::gates::packed_util::PackedEvaluableBase;
 use crate::gates::util::StridedConstraintConsumer;
 use crate::hash::hash_types::RichField;
 use crate::iop::ext_target::ExtensionTarget;
-use crate::iop::generator::{
-    ConstantGenerator, GeneratedValues, SimpleGenerator, WitnessGenerator,
-};
+use crate::iop::generator::{GeneratedValues, SimpleGenerator, WitnessGenerator};
 use crate::iop::target::Target;
 use crate::iop::wire::Wire;
 use crate::iop::witness::{PartitionWitness, Witness};
@@ -77,17 +75,17 @@ impl<F: RichField + Extendable<D>, const D: usize> RandomAccessGate<F, D> {
         (2 + self.vec_size()) * copy + 2 + i
     }
 
-    fn num_ram_wires(&self) -> usize {
+    fn start_extra_constants(&self) -> usize {
         (2 + self.vec_size()) * self.num_copies
     }
 
     fn wire_extra_constant(&self, i: usize) -> usize {
         debug_assert!(i < self.num_extra_constants);
-        self.num_ram_wires() + i
+        self.start_extra_constants() + i
     }
 
     pub fn num_routed_wires(&self) -> usize {
-        self.num_ram_wires() + self.num_extra_constants
+        self.start_extra_constants() + self.num_extra_constants
     }
 
     /// An intermediate wire where the prover gives the (purported) binary decomposition of the
@@ -255,9 +253,9 @@ impl<F: RichField + Extendable<D>, const D: usize> Gate<F, D> for RandomAccessGa
         self.num_copies * constraints_per_copy + self.num_extra_constants
     }
 
-    fn extra_constants(&self, gate_index: usize) -> Vec<ConstantGenerator<F>> {
+    fn extra_constant_wires(&self) -> Vec<(usize, usize)> {
         (0..self.num_extra_constants)
-            .map(|i| ConstantGenerator::new(gate_index, i, self.wire_extra_constant(i), F::ZERO))
+            .map(|i| (i, self.wire_extra_constant(i)))
             .collect()
     }
 }

plonky2/src/iop/generator.rs

@@ -328,27 +328,19 @@ impl<F: Field> SimpleGenerator<F> for NonzeroTestGenerator {
 
 /// Generator used to fill an extra constant.
 #[derive(Debug, Clone)]
-pub struct ConstantGenerator<F: Field> {
+pub(crate) struct ConstantGenerator<F: Field> {
     pub gate_index: usize,
     pub constant_index: usize,
-    pub target_index: usize,
+    pub wire_index: usize,
     pub constant: F,
 }
 
 impl<F: Field> ConstantGenerator<F> {
-    pub fn new(gate_index: usize, constant_index: usize, target_index: usize, constant: F) -> Self {
-        ConstantGenerator {
-            gate_index,
-            constant_index,
-            target_index,
-            constant,
-        }
-    }
-
     pub fn set_constant(&mut self, c: F) {
         self.constant = c;
     }
 }
+
 impl<F: Field> SimpleGenerator<F> for ConstantGenerator<F> {
     fn dependencies(&self) -> Vec<Target> {
         vec![]
@@ -356,7 +348,7 @@ impl<F: Field> SimpleGenerator<F> for ConstantGenerator<F> {
 
     fn run_once(&self, _witness: &PartitionWitness<F>, out_buffer: &mut GeneratedValues<F>) {
         out_buffer.set_target(
-            Target::wire(self.gate_index, self.target_index),
+            Target::wire(self.gate_index, self.wire_index),
             self.constant,
         );
     }

plonky2/src/plonk/circuit_builder.rs

@@ -219,10 +219,17 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         );
         constants.resize(gate_type.num_constants(), F::ZERO);
 
-        let index = self.gate_instances.len();
+        let gate_index = self.gate_instances.len();
 
         self.constant_generators
-            .extend(gate_type.extra_constants(index));
+            .extend(gate_type.extra_constant_wires().into_iter().map(
+                |(constant_index, wire_index)| ConstantGenerator {
+                    gate_index,
+                    constant_index,
+                    wire_index,
+                    constant: F::ZERO, // Placeholder; will be replaced later.
+                },
+            ));
 
         // Note that we can't immediately add this gate's generators, because the list of constants
         // could be modified later, i.e. in the case of `ConstantGate`. We will add them later in
@@ -237,7 +244,7 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
             constants,
         });
 
-        index
+        gate_index
     }
 
     fn check_gate_compatibility<G: Gate<F, D>>(&self, gate: &G) {
@@ -678,10 +685,7 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
             // Set the constant in the constant polynomial.
             self.gate_instances[const_gen.gate_index].constants[const_gen.constant_index] = c;
             // Generate a copy between the target and the routable wire.
-            self.connect(
-                Target::wire(const_gen.gate_index, const_gen.target_index),
-                t,
-            );
+            self.connect(Target::wire(const_gen.gate_index, const_gen.wire_index), t);
             // Set the constant in the generator (it's initially set with a dummy value).
             const_gen.set_constant(c);
             self.add_simple_generator(const_gen);