chore: use Brillig opcode when possible for less-than operations on f…

…ields

noir/noir-repo/acvm-repo/acir/src/circuit/black_box_functions.rs

@@ -186,6 +186,9 @@ pub enum BlackBoxFunc {
     /// - state: [(witness, 32); 8]
     /// - output: [(witness, 32); 8]
     Sha256Compression,
+
+    // Less than comparison between two field elements (only usable in unconstrained)
+    FieldLessThan,
 }
 
 impl std::fmt::Display for BlackBoxFunc {
@@ -218,6 +221,7 @@ impl BlackBoxFunc {
             BlackBoxFunc::BigIntToLeBytes => "bigint_to_le_bytes",
             BlackBoxFunc::Poseidon2Permutation => "poseidon2_permutation",
             BlackBoxFunc::Sha256Compression => "sha256_compression",
+            BlackBoxFunc::FieldLessThan => "field_less_than",
         }
     }
 
@@ -244,6 +248,7 @@ impl BlackBoxFunc {
             "bigint_to_le_bytes" => Some(BlackBoxFunc::BigIntToLeBytes),
             "poseidon2_permutation" => Some(BlackBoxFunc::Poseidon2Permutation),
             "sha256_compression" => Some(BlackBoxFunc::Sha256Compression),
+            "field_less_than" => Some(BlackBoxFunc::FieldLessThan),
             _ => None,
         }
     }

noir/noir-repo/acvm-repo/blackbox_solver/src/curve_specific_solver.rs

@@ -34,6 +34,7 @@ pub trait BlackBoxFunctionSolver<F> {
         _inputs: &[F],
         _len: u32,
     ) -> Result<Vec<F>, BlackBoxResolutionError>;
+    fn field_less_than(&self, _input_x: &F, _input_y: &F) -> Result<bool, BlackBoxResolutionError>;
 }
 
 pub struct StubbedBlackBoxSolver;
@@ -83,4 +84,7 @@ impl<F> BlackBoxFunctionSolver<F> for StubbedBlackBoxSolver {
     ) -> Result<Vec<F>, BlackBoxResolutionError> {
         Err(Self::fail(BlackBoxFunc::Poseidon2Permutation))
     }
+    fn field_less_than(&self, _input_x: &F, _input_y: &F) -> Result<bool, BlackBoxResolutionError> {
+        Err(Self::fail(BlackBoxFunc::FieldLessThan))
+    }
 }

noir/noir-repo/acvm-repo/bn254_blackbox_solver/src/lib.rs

@@ -74,4 +74,15 @@ impl BlackBoxFunctionSolver<FieldElement> for Bn254BlackBoxSolver {
     ) -> Result<Vec<FieldElement>, BlackBoxResolutionError> {
         poseidon2_permutation(inputs, len)
     }
+
+    fn field_less_than(
+        &self,
+        _input_x: &FieldElement,
+        _input_y: &FieldElement,
+    ) -> Result<bool, BlackBoxResolutionError> {
+        Err(BlackBoxResolutionError::Failed(
+            acir::BlackBoxFunc::FieldLessThan,
+            "field_less_than is not supported in acir, only in brillig/unconstrained".to_string(),
+        ))
+    }
 }

noir/noir-repo/compiler/noirc_evaluator/src/brillig/brillig_gen/brillig_black_box.rs

@@ -417,6 +417,19 @@ pub(crate) fn convert_black_box_call<F: AcirField + DebugToString, Registers: Re
                 unreachable!("ICE: AES128Encrypt expects three array arguments, one array result")
             }
         }
+        BlackBoxFunc::FieldLessThan => {
+            if let (
+                [BrilligVariable::SingleAddr(input_x), BrilligVariable::SingleAddr(input_y)],
+                [BrilligVariable::SingleAddr(output)],
+            ) = (function_arguments, function_results)
+            {
+                brillig_context.field_less_than_instruction(*input_x, *input_y, *output);
+            } else {
+                unreachable!(
+                    "ICE: FieldLessThan expects two register arguments one register result"
+                )
+            }
+        }
     }
 }
 

noir/noir-repo/compiler/noirc_evaluator/src/brillig/brillig_ir.rs

@@ -201,6 +201,14 @@ pub(crate) mod tests {
         ) -> Result<Vec<FieldElement>, BlackBoxResolutionError> {
             Ok(vec![0_u128.into(), 1_u128.into(), 2_u128.into(), 3_u128.into()])
         }
+
+        fn field_less_than(
+            &self,
+            _input_x: &FieldElement,
+            _input_y: &FieldElement,
+        ) -> Result<bool, BlackBoxResolutionError> {
+            Ok(true)
+        }
     }
 
     pub(crate) fn create_context(id: FunctionId) -> BrilligContext<FieldElement, Stack> {

noir/noir-repo/compiler/noirc_evaluator/src/brillig/brillig_ir/instructions.rs

@@ -38,6 +38,26 @@ impl<F: AcirField + DebugToString, Registers: RegisterAllocator> BrilligContext<
         self.binary(lhs, rhs, result, operation);
     }
 
+    /// Procresses a field less than instruction.
+    ///
+    /// Just performs an assertion on bit size and then forwards to binary_instruction().
+    pub(crate) fn field_less_than_instruction(
+        &mut self,
+        lhs: SingleAddrVariable,
+        rhs: SingleAddrVariable,
+        result: SingleAddrVariable,
+    ) {
+        let max_field_size = FieldElement::max_num_bits();
+        assert!(
+            lhs.bit_size == max_field_size && rhs.bit_size == max_field_size,
+            "Expected bit sizes lhs and rhs to be {}, got {} and {} for 'field less than' operation",
+            lhs.bit_size,
+            rhs.bit_size,
+            max_field_size,
+        );
+        self.binary_instruction(lhs, rhs, result, BrilligBinaryOp::LessThan);
+    }
+
     /// Processes a not instruction.
     ///
     /// Not is computed using a subtraction operation as there is no native not instruction

noir/noir-repo/compiler/noirc_evaluator/src/ssa/acir_gen/acir_ir/generated_acir.rs

@@ -343,6 +343,7 @@ impl<F: AcirField> GeneratedAcir<F> {
                     .expect("Compiler should generate correct size inputs"),
                 outputs: outputs.try_into().expect("Compiler should generate correct size outputs"),
             },
+            BlackBoxFunc::FieldLessThan => panic!("FieldLessThan is not supported in ACIR"),
         };
 
         self.push_opcode(AcirOpcode::BlackBoxFuncCall(black_box_func_call));
@@ -666,6 +667,8 @@ fn black_box_func_expected_input_size(name: BlackBoxFunc) -> Option<usize> {
 
         // FromLeBytes takes a variable array of bytes as input
         BlackBoxFunc::BigIntFromLeBytes => None,
+
+        BlackBoxFunc::FieldLessThan => panic!("FieldLessThan is not supported in ACIR"),
     }
 }
 
@@ -714,6 +717,8 @@ fn black_box_expected_output_size(name: BlackBoxFunc) -> Option<usize> {
 
         // AES encryption returns a variable number of outputs
         BlackBoxFunc::AES128Encrypt => None,
+
+        BlackBoxFunc::FieldLessThan => panic!("FieldLessThan is not supported in ACIR"),
     }
 }
 

noir/noir-repo/compiler/noirc_evaluator/src/ssa/ir/instruction/call.rs

@@ -572,6 +572,7 @@ fn simplify_black_box_func(
         }
         BlackBoxFunc::Sha256Compression => SimplifyResult::None, //TODO(Guillaume)
         BlackBoxFunc::AES128Encrypt => SimplifyResult::None,
+        BlackBoxFunc::FieldLessThan => panic!("FieldLessThan is not supported in ACIR"),
     }
 }
 

noir/noir-repo/noir_stdlib/src/field/bn254.nr

@@ -1,4 +1,5 @@
 use crate::runtime::is_unconstrained;
+use crate::field::bn254::field_less_than;
 
 // The low and high decomposition of the field modulus
 global PLO: Field = 53438638232309528389504892708671455233;
@@ -25,23 +26,30 @@ pub(crate) unconstrained fn decompose_hint(x: Field) -> (Field, Field) {
     compute_decomposition(x)
 }
 
+#[foreign(field_less_than)]
+pub fn field_less_than(_x: Field, _y: Field) -> bool {}
+
 fn compute_lt(x: Field, y: Field, num_bytes: u32) -> bool {
-    let x_bytes: [u8; 32] = x.to_le_bytes();
-    let y_bytes: [u8; 32] = y.to_le_bytes();
-    let mut x_is_lt = false;
-    let mut done = false;
-    for i in 0..num_bytes {
-        if (!done) {
-            let x_byte = x_bytes[num_bytes - 1 - i];
-            let y_byte = y_bytes[num_bytes - 1 - i];
-            let bytes_match = x_byte == y_byte;
-            if !bytes_match {
-                x_is_lt = x_byte < y_byte;
-                done = true;
+    if is_unconstrained() {
+        field_less_than(x, y)
+    } else {
+        let x_bytes: [u8; 32] = x.to_le_bytes();
+        let y_bytes: [u8; 32] = y.to_le_bytes();
+        let mut x_is_lt = false;
+        let mut done = false;
+        for i in 0..num_bytes {
+            if (!done) {
+                let x_byte = x_bytes[num_bytes - 1 - i];
+                let y_byte = y_bytes[num_bytes - 1 - i];
+                let bytes_match = x_byte == y_byte;
+                if !bytes_match {
+                    x_is_lt = x_byte < y_byte;
+                    done = true;
+                }
             }
         }
+        x_is_lt
     }
-    x_is_lt
 }
 
 fn compute_lte(x: Field, y: Field, num_bytes: u32) -> bool {

noir/noir-repo/tooling/lsp/src/solver.rs

@@ -50,4 +50,12 @@ impl BlackBoxFunctionSolver<acvm::FieldElement> for WrapperSolver {
     ) -> Result<Vec<acvm::FieldElement>, acvm::BlackBoxResolutionError> {
         self.0.poseidon2_permutation(inputs, len)
     }
+
+    fn field_less_than(
+        &self,
+        _input_x: &acvm::FieldElement,
+        _input_y: &acvm::FieldElement,
+    ) -> Result<bool, acvm::BlackBoxResolutionError> {
+        self.0.field_less_than(_input_x, _input_y)
+    }
 }