perf: two useful reductions, direct felt2var (#1277)

crates/recursion/circuit/src/challenger.rs

@@ -115,8 +115,7 @@ pub fn reduce_32<C: Config>(builder: &mut Builder<C>, vals: &[Felt<C::F>]) -> Va
     let mut power = C::N::one();
     let result: Var<C::N> = builder.eval(C::N::zero());
     for val in vals.iter() {
-        let bits = builder.num2bits_f_circuit(*val);
-        let val = builder.bits2num_v_circuit(&bits);
+        let val = builder.felt2var_circuit(*val);
         builder.assign(result, result + val * power);
         power *= C::N::from_canonical_u64(1u64 << 32);
     }

crates/recursion/circuit/src/fri.rs

@@ -204,6 +204,10 @@ pub fn verify_query<C: Config>(
         let index_sibling: Var<_> = builder.eval(one - index_bits.clone()[offset]);
         let index_pair = &index_bits[(offset + 1)..];
 
+        // Reduce folded_eval (mod the BabyBear prime) since it gets used multiple times below and
+        // the reductions will be repeated.
+        builder.reduce_e(folded_eval);
+
         let evals_ext = [
             builder.select_ef(index_sibling, folded_eval, step.sibling_value),
             builder.select_ef(index_sibling, step.sibling_value, folded_eval),

crates/recursion/compiler/src/constraints/mod.rs

@@ -358,6 +358,10 @@ impl<C: Config + Debug> ConstraintCompiler<C> {
                     opcode: ConstraintOpcode::ReduceE,
                     args: vec![vec![a.id()]],
                 }),
+                DslIr::CircuitFelt2Var(a, b) => constraints.push(Constraint {
+                    opcode: ConstraintOpcode::CircuitFelt2Var,
+                    args: vec![vec![b.id()], vec![a.id()]],
+                }),
                 _ => panic!("unsupported {:?}", instruction),
             };
         }

crates/recursion/compiler/src/constraints/opcodes.rs

@@ -46,6 +46,7 @@ pub enum ConstraintOpcode {
     CommitVkeyHash,
     CommitCommitedValuesDigest,
     CircuitFelts2Ext,
+    CircuitFelt2Var,
     PermuteBabyBear,
     ReduceE,
 }

crates/recursion/compiler/src/ir/builder.rs

@@ -474,6 +474,12 @@ impl<C: Config> Builder<C> {
         self.operations.push(DslIr::ReduceE(ext));
     }
 
+    pub fn felt2var_circuit(&mut self, felt: Felt<C::F>) -> Var<C::N> {
+        let var = self.uninit();
+        self.operations.push(DslIr::CircuitFelt2Var(felt, var));
+        var
+    }
+
     pub fn cycle_tracker(&mut self, name: &str) {
         self.operations.push(DslIr::CycleTracker(name.to_string()));
     }

crates/recursion/compiler/src/ir/instructions.rs

@@ -193,6 +193,8 @@ pub enum DslIr<C: Config> {
     /// Decompose a field element into bits (bits = num2bits(felt)). Should only be used when
     /// target is a gnark circuit.
     CircuitNum2BitsF(Felt<C::F>, Vec<Var<C::N>>),
+    /// Convert a Felt to a Var in a circuit. Avoids decomposing to bits and then reconstructing.
+    CircuitFelt2Var(Felt<C::F>, Var<C::N>),
 
     // Hashing.
     /// Permutes an array of baby bear elements using Poseidon2 (output = p2_permute(array)).

crates/recursion/gnark-ffi/go/sp1/babybear/babybear.go

@@ -53,7 +53,19 @@ func Zero() Variable {
 	}
 }
 
+func One() Variable {
+	return Variable{
+		Value:  frontend.Variable("1"),
+		NbBits: 1,
+	}
+}
+
 func NewF(value string) Variable {
+	if value == "0" {
+		return Zero()
+	} else if value == "1" {
+		return One()
+	}
 	return Variable{
 		Value:  frontend.Variable(value),
 		NbBits: 31,
@@ -105,7 +117,7 @@ func (c *Chip) MulFConst(a Variable, b int) Variable {
 }
 
 func (c *Chip) negF(a Variable) Variable {
-	if a.NbBits == 31 {
+	if a.NbBits <= 31 {
 		return Variable{Value: c.api.Sub(modulus, a.Value), NbBits: 31}
 	}
 
@@ -283,7 +295,7 @@ func (p *Chip) reduceFast(x Variable) Variable {
 }
 
 func (p *Chip) ReduceSlow(x Variable) Variable {
-	if x.NbBits == 31 {
+	if x.NbBits <= 31 {
 		return x
 	}
 	return Variable{

crates/recursion/gnark-ffi/go/sp1/sp1.go

@@ -201,6 +201,8 @@ func (circuit *Circuit) Define(api frontend.API) error {
 			api.AssertIsEqual(circuit.CommitedValuesDigest, element)
 		case "CircuitFelts2Ext":
 			exts[cs.Args[0][0]] = babybear.Felts2Ext(felts[cs.Args[1][0]], felts[cs.Args[2][0]], felts[cs.Args[3][0]], felts[cs.Args[4][0]])
+		case "CircuitFelt2Var":
+			vars[cs.Args[0][0]] = fieldAPI.ReduceSlow(felts[cs.Args[1][0]]).Value
 		case "ReduceE":
 			exts[cs.Args[0][0]] = fieldAPI.ReduceE(exts[cs.Args[0][0]])
 		default: