Merge pull request #373 from mir-protocol/mul_gate

Multiplication gate

src/gadgets/arithmetic.rs

@@ -1,5 +1,7 @@
 use std::borrow::Borrow;
 
+use itertools::Itertools;
+
 use crate::field::extension_field::Extendable;
 use crate::field::field_types::{PrimeField, RichField};
 use crate::gates::arithmetic_base::ArithmeticGate;
@@ -206,11 +208,11 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
 
     /// Multiply `n` `Target`s.
     pub fn mul_many(&mut self, terms: &[Target]) -> Target {
-        let terms_ext = terms
+        terms
             .iter()
-            .map(|&t| self.convert_to_ext(t))
-            .collect::<Vec<_>>();
-        self.mul_many_extension(&terms_ext).to_target_array()[0]
+            .copied()
+            .fold1(|acc, t| self.mul(acc, t))
+            .unwrap_or_else(|| self.one())
     }
 
     /// Exponentiate `base` to the power of `2^power_log`.

src/gadgets/arithmetic_extension.rs

@@ -1,8 +1,11 @@
+use itertools::Itertools;
+
 use crate::field::extension_field::target::{ExtensionAlgebraTarget, ExtensionTarget};
 use crate::field::extension_field::FieldExtension;
 use crate::field::extension_field::{Extendable, OEF};
 use crate::field::field_types::{Field, PrimeField, RichField};
 use crate::gates::arithmetic_extension::ArithmeticExtensionGate;
+use crate::gates::multiplication_extension::MulExtensionGate;
 use crate::iop::generator::{GeneratedValues, SimpleGenerator};
 use crate::iop::target::Target;
 use crate::iop::witness::{PartitionWitness, Witness};
@@ -41,13 +44,19 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
             return result;
         }
 
+        let result = if self.target_as_constant_ext(addend) == Some(F::Extension::ZERO) {
+            // If the addend is zero, we use a multiplication gate.
+            self.compute_mul_extension_operation(operation)
+        } else {
+            // Otherwise, we use an arithmetic gate.
+            self.compute_arithmetic_extension_operation(operation)
+        };
         // Otherwise, we must actually perform the operation using an ArithmeticExtensionGate slot.
-        let result = self.add_arithmetic_extension_operation(operation);
         self.arithmetic_results.insert(operation, result);
         result
     }
 
-    fn add_arithmetic_extension_operation(
+    fn compute_arithmetic_extension_operation(
         &mut self,
         operation: ExtensionArithmeticOperation<F, D>,
     ) -> ExtensionTarget<D> {
@@ -70,6 +79,22 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         ExtensionTarget::from_range(gate, ArithmeticExtensionGate::<D>::wires_ith_output(i))
     }
 
+    fn compute_mul_extension_operation(
+        &mut self,
+        operation: ExtensionArithmeticOperation<F, D>,
+    ) -> ExtensionTarget<D> {
+        let (gate, i) = self.find_mul_gate(operation.const_0);
+        let wires_multiplicand_0 =
+            ExtensionTarget::from_range(gate, MulExtensionGate::<D>::wires_ith_multiplicand_0(i));
+        let wires_multiplicand_1 =
+            ExtensionTarget::from_range(gate, MulExtensionGate::<D>::wires_ith_multiplicand_1(i));
+
+        self.connect_extension(operation.multiplicand_0, wires_multiplicand_0);
+        self.connect_extension(operation.multiplicand_1, wires_multiplicand_1);
+
+        ExtensionTarget::from_range(gate, MulExtensionGate::<D>::wires_ith_output(i))
+    }
+
     /// Checks for special cases where the value of
     /// `const_0 * multiplicand_0 * multiplicand_1 + const_1 * addend`
     /// can be determined without adding an `ArithmeticGate`.
@@ -273,11 +298,11 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
 
     /// Multiply `n` `ExtensionTarget`s.
     pub fn mul_many_extension(&mut self, terms: &[ExtensionTarget<D>]) -> ExtensionTarget<D> {
-        let mut product = self.one_extension();
-        for &term in terms {
-            product = self.mul_extension(product, term);
-        }
-        product
+        terms
+            .iter()
+            .copied()
+            .fold1(|acc, t| self.mul_extension(acc, t))
+            .unwrap_or_else(|| self.one_extension())
     }
 
     /// Like `mul_add`, but for `ExtensionTarget`s.

src/gates/mod.rs

@@ -14,6 +14,7 @@ pub mod gate_tree;
 pub mod gmimc;
 pub mod insertion;
 pub mod interpolation;
+pub mod multiplication_extension;
 pub mod noop;
 pub mod poseidon;
 pub(crate) mod poseidon_mds;

src/gates/multiplication_extension.rs

@@ -0,0 +1,204 @@
+use std::ops::Range;
+
+use crate::field::extension_field::target::ExtensionTarget;
+use crate::field::extension_field::Extendable;
+use crate::field::extension_field::FieldExtension;
+use crate::field::field_types::RichField;
+use crate::gates::gate::Gate;
+use crate::iop::generator::{GeneratedValues, SimpleGenerator, WitnessGenerator};
+use crate::iop::target::Target;
+use crate::iop::witness::{PartitionWitness, Witness};
+use crate::plonk::circuit_builder::CircuitBuilder;
+use crate::plonk::circuit_data::CircuitConfig;
+use crate::plonk::vars::{EvaluationTargets, EvaluationVars, EvaluationVarsBase};
+
+/// A gate which can perform a weighted multiplication, i.e. `result = c0 x y`. If the config
+/// supports enough routed wires, it can support several such operations in one gate.
+#[derive(Debug)]
+pub struct MulExtensionGate<const D: usize> {
+    /// Number of multiplications performed by the gate.
+    pub num_ops: usize,
+}
+
+impl<const D: usize> MulExtensionGate<D> {
+    pub fn new_from_config(config: &CircuitConfig) -> Self {
+        Self {
+            num_ops: Self::num_ops(config),
+        }
+    }
+
+    /// Determine the maximum number of operations that can fit in one gate for the given config.
+    pub(crate) fn num_ops(config: &CircuitConfig) -> usize {
+        let wires_per_op = 3 * D;
+        config.num_routed_wires / wires_per_op
+    }
+
+    pub fn wires_ith_multiplicand_0(i: usize) -> Range<usize> {
+        3 * D * i..3 * D * i + D
+    }
+    pub fn wires_ith_multiplicand_1(i: usize) -> Range<usize> {
+        3 * D * i + D..3 * D * i + 2 * D
+    }
+    pub fn wires_ith_output(i: usize) -> Range<usize> {
+        3 * D * i + 2 * D..3 * D * i + 3 * D
+    }
+}
+
+impl<F: RichField + Extendable<D>, const D: usize> Gate<F, D> for MulExtensionGate<D> {
+    fn id(&self) -> String {
+        format!("{:?}", self)
+    }
+
+    fn eval_unfiltered(&self, vars: EvaluationVars<F, D>) -> Vec<F::Extension> {
+        let const_0 = vars.local_constants[0];
+
+        let mut constraints = Vec::new();
+        for i in 0..self.num_ops {
+            let multiplicand_0 = vars.get_local_ext_algebra(Self::wires_ith_multiplicand_0(i));
+            let multiplicand_1 = vars.get_local_ext_algebra(Self::wires_ith_multiplicand_1(i));
+            let output = vars.get_local_ext_algebra(Self::wires_ith_output(i));
+            let computed_output = (multiplicand_0 * multiplicand_1).scalar_mul(const_0);
+
+            constraints.extend((output - computed_output).to_basefield_array());
+        }
+
+        constraints
+    }
+
+    fn eval_unfiltered_base(&self, vars: EvaluationVarsBase<F>) -> Vec<F> {
+        let const_0 = vars.local_constants[0];
+
+        let mut constraints = Vec::new();
+        for i in 0..self.num_ops {
+            let multiplicand_0 = vars.get_local_ext(Self::wires_ith_multiplicand_0(i));
+            let multiplicand_1 = vars.get_local_ext(Self::wires_ith_multiplicand_1(i));
+            let output = vars.get_local_ext(Self::wires_ith_output(i));
+            let computed_output = (multiplicand_0 * multiplicand_1).scalar_mul(const_0);
+
+            constraints.extend((output - computed_output).to_basefield_array());
+        }
+
+        constraints
+    }
+
+    fn eval_unfiltered_recursively(
+        &self,
+        builder: &mut CircuitBuilder<F, D>,
+        vars: EvaluationTargets<D>,
+    ) -> Vec<ExtensionTarget<D>> {
+        let const_0 = vars.local_constants[0];
+
+        let mut constraints = Vec::new();
+        for i in 0..self.num_ops {
+            let multiplicand_0 = vars.get_local_ext_algebra(Self::wires_ith_multiplicand_0(i));
+            let multiplicand_1 = vars.get_local_ext_algebra(Self::wires_ith_multiplicand_1(i));
+            let output = vars.get_local_ext_algebra(Self::wires_ith_output(i));
+            let computed_output = {
+                let mul = builder.mul_ext_algebra(multiplicand_0, multiplicand_1);
+                builder.scalar_mul_ext_algebra(const_0, mul)
+            };
+
+            let diff = builder.sub_ext_algebra(output, computed_output);
+            constraints.extend(diff.to_ext_target_array());
+        }
+
+        constraints
+    }
+
+    fn generators(
+        &self,
+        gate_index: usize,
+        local_constants: &[F],
+    ) -> Vec<Box<dyn WitnessGenerator<F>>> {
+        (0..self.num_ops)
+            .map(|i| {
+                let g: Box<dyn WitnessGenerator<F>> = Box::new(
+                    MulExtensionGenerator {
+                        gate_index,
+                        const_0: local_constants[0],
+                        i,
+                    }
+                    .adapter(),
+                );
+                g
+            })
+            .collect::<Vec<_>>()
+    }
+
+    fn num_wires(&self) -> usize {
+        self.num_ops * 3 * D
+    }
+
+    fn num_constants(&self) -> usize {
+        1
+    }
+
+    fn degree(&self) -> usize {
+        3
+    }
+
+    fn num_constraints(&self) -> usize {
+        self.num_ops * D
+    }
+}
+
+#[derive(Clone, Debug)]
+struct MulExtensionGenerator<F: RichField + Extendable<D>, const D: usize> {
+    gate_index: usize,
+    const_0: F,
+    i: usize,
+}
+
+impl<F: RichField + Extendable<D>, const D: usize> SimpleGenerator<F>
+    for MulExtensionGenerator<F, D>
+{
+    fn dependencies(&self) -> Vec<Target> {
+        MulExtensionGate::<D>::wires_ith_multiplicand_0(self.i)
+            .chain(MulExtensionGate::<D>::wires_ith_multiplicand_1(self.i))
+            .map(|i| Target::wire(self.gate_index, i))
+            .collect()
+    }
+
+    fn run_once(&self, witness: &PartitionWitness<F>, out_buffer: &mut GeneratedValues<F>) {
+        let extract_extension = |range: Range<usize>| -> F::Extension {
+            let t = ExtensionTarget::from_range(self.gate_index, range);
+            witness.get_extension_target(t)
+        };
+
+        let multiplicand_0 =
+            extract_extension(MulExtensionGate::<D>::wires_ith_multiplicand_0(self.i));
+        let multiplicand_1 =
+            extract_extension(MulExtensionGate::<D>::wires_ith_multiplicand_1(self.i));
+
+        let output_target = ExtensionTarget::from_range(
+            self.gate_index,
+            MulExtensionGate::<D>::wires_ith_output(self.i),
+        );
+
+        let computed_output = (multiplicand_0 * multiplicand_1).scalar_mul(self.const_0);
+
+        out_buffer.set_extension_target(output_target, computed_output)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use anyhow::Result;
+
+    use crate::field::goldilocks_field::GoldilocksField;
+    use crate::gates::gate_testing::{test_eval_fns, test_low_degree};
+    use crate::gates::multiplication_extension::MulExtensionGate;
+    use crate::plonk::circuit_data::CircuitConfig;
+
+    #[test]
+    fn low_degree() {
+        let gate = MulExtensionGate::new_from_config(&CircuitConfig::standard_recursion_config());
+        test_low_degree::<GoldilocksField, _, 4>(gate);
+    }
+
+    #[test]
+    fn eval_fns() -> Result<()> {
+        let gate = MulExtensionGate::new_from_config(&CircuitConfig::standard_recursion_config());
+        test_eval_fns::<GoldilocksField, _, 4>(gate)
+    }
+}

src/plonk/circuit_builder.rs

@@ -20,6 +20,7 @@ use crate::gates::arithmetic_u32::{U32ArithmeticGate, NUM_U32_ARITHMETIC_OPS};
 use crate::gates::constant::ConstantGate;
 use crate::gates::gate::{Gate, GateInstance, GateRef, PrefixedGate};
 use crate::gates::gate_tree::Tree;
+use crate::gates::multiplication_extension::MulExtensionGate;
 use crate::gates::noop::NoopGate;
 use crate::gates::public_input::PublicInputGate;
 use crate::gates::random_access::RandomAccessGate;
@@ -769,6 +770,8 @@ pub struct BatchedGates<F: RichField + Extendable<D>, const D: usize> {
     pub(crate) free_arithmetic: HashMap<(F, F), (usize, usize)>,
     pub(crate) free_base_arithmetic: HashMap<(F, F), (usize, usize)>,
 
+    pub(crate) free_mul: HashMap<F, (usize, usize)>,
+
     /// A map `b -> (g, i)` from `b` bits to an available random access gate of that size with gate
     /// index `g` and already using `i` random accesses.
     pub(crate) free_random_access: HashMap<usize, (usize, usize)>,
@@ -793,6 +796,7 @@ impl<F: RichField + Extendable<D>, const D: usize> BatchedGates<F, D> {
         Self {
             free_arithmetic: HashMap::new(),
             free_base_arithmetic: HashMap::new(),
+            free_mul: HashMap::new(),
             free_random_access: HashMap::new(),
             current_switch_gates: Vec::new(),
             current_u32_arithmetic_gate: None,
@@ -865,6 +869,33 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         (gate, i)
     }
 
+    /// Finds the last available arithmetic gate with the given constants or add one if there aren't any.
+    /// Returns `(g,i)` such that there is an arithmetic gate with the given constants at index
+    /// `g` and the gate's `i`-th operation is available.
+    pub(crate) fn find_mul_gate(&mut self, const_0: F) -> (usize, usize) {
+        let (gate, i) = self
+            .batched_gates
+            .free_mul
+            .get(&const_0)
+            .copied()
+            .unwrap_or_else(|| {
+                let gate = self.add_gate(
+                    MulExtensionGate::new_from_config(&self.config),
+                    vec![const_0],
+                );
+                (gate, 0)
+            });
+
+        // Update `free_arithmetic` with new values.
+        if i < MulExtensionGate::<D>::num_ops(&self.config) - 1 {
+            self.batched_gates.free_mul.insert(const_0, (gate, i + 1));
+        } else {
+            self.batched_gates.free_mul.remove(&const_0);
+        }
+
+        (gate, i)
+    }
+
     /// Finds the last available random access gate with the given `vec_size` or add one if there aren't any.
     /// Returns `(g,i)` such that there is a random access gate with the given `vec_size` at index
     /// `g` and the gate's `i`-th random access is available.
@@ -1021,6 +1052,22 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         assert!(self.batched_gates.free_arithmetic.is_empty());
     }
 
+    /// Fill the remaining unused arithmetic operations with zeros, so that all
+    /// `ArithmeticExtensionGenerator`s are run.
+    fn fill_mul_gates(&mut self) {
+        let zero = self.zero_extension();
+        for (c0, (_gate, i)) in self.batched_gates.free_mul.clone() {
+            for _ in i..MulExtensionGate::<D>::num_ops(&self.config) {
+                // If we directly wire in zero, an optimization will skip doing anything and return
+                // zero. So we pass in a virtual target and connect it to zero afterward.
+                let dummy = self.add_virtual_extension_target();
+                self.arithmetic_extension(c0, F::ZERO, dummy, dummy, zero);
+                self.connect_extension(dummy, zero);
+            }
+        }
+        assert!(self.batched_gates.free_mul.is_empty());
+    }
+
     /// Fill the remaining unused random access operations with zeros, so that all
     /// `RandomAccessGenerator`s are run.
     fn fill_random_access_gates(&mut self) {
@@ -1110,6 +1157,7 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
     fn fill_batched_gates(&mut self) {
         self.fill_arithmetic_gates();
         self.fill_base_arithmetic_gates();
+        self.fill_mul_gates();
         self.fill_random_access_gates();
         self.fill_switch_gates();
         self.fill_u32_arithmetic_gates();