chore: pull out refactored methods from u128 branch (#7385)

compiler/noirc_evaluator/proptest-regressions/acir/acir_variable.txt

@@ -0,0 +1,7 @@
+# Seeds for failure cases proptest has generated in the past. It is
+# automatically read and these particular cases re-run before any
+# novel cases are generated.
+#
+# It is recommended to check this file in to source control so that
+# everyone who runs the test benefits from these saved cases.
+cc 5c73714aa60966f1e26e79441f4a3102fe768cc6c87c9421d44cbc4b9ffa8d66 # shrinks to bit_size = 0

compiler/noirc_evaluator/proptest-regressions/ssa/ir/instruction/binary.txt

@@ -0,0 +1,7 @@
+# Seeds for failure cases proptest has generated in the past. It is
+# automatically read and these particular cases re-run before any
+# novel cases are generated.
+#
+# It is recommended to check this file in to source control so that
+# everyone who runs the test benefits from these saved cases.
+cc 696c50cf99099048e0e1ea8d77561cc6cf49a71b480a05e0ef437d57f97c454a # shrinks to input = 0, bit_size = 0

compiler/noirc_evaluator/src/acir/acir_variable.rs

@@ -1026,8 +1026,7 @@ impl<F: AcirField, B: BlackBoxFunctionSolver<F>> AcirContext<F, B> {
         leading: AcirVar,
         max_bit_size: u32,
     ) -> Result<AcirVar, RuntimeError> {
-        let max_power_of_two =
-            self.add_constant(F::from(2_u128).pow(&F::from(max_bit_size as u128 - 1)));
+        let max_power_of_two = self.add_constant(power_of_two::<F>(max_bit_size - 1));
 
         let intermediate = self.sub_var(max_power_of_two, lhs)?;
         let intermediate = self.mul_var(intermediate, leading)?;
@@ -1057,8 +1056,7 @@ impl<F: AcirField, B: BlackBoxFunctionSolver<F>> AcirContext<F, B> {
         assert_ne!(bit_size, 0, "signed integer should have at least one bit");
 
         // 2^{max_bit size-1}
-        let max_power_of_two =
-            self.add_constant(F::from(2_u128).pow(&F::from(bit_size as u128 - 1)));
+        let max_power_of_two = self.add_constant(power_of_two::<F>(bit_size - 1));
         let zero = self.add_constant(F::zero());
         let one = self.add_constant(F::one());
 
@@ -1164,7 +1162,7 @@ impl<F: AcirField, B: BlackBoxFunctionSolver<F>> AcirContext<F, B> {
         max_bit_size: u32,
     ) -> Result<AcirVar, RuntimeError> {
         // 2^{rhs}
-        let divisor = self.add_constant(F::from(2_u128).pow(&F::from(rhs as u128)));
+        let divisor = self.add_constant(power_of_two::<F>(rhs));
         let one = self.add_constant(F::one());
 
         //  Computes lhs = 2^{rhs} * q + r
@@ -1257,7 +1255,7 @@ impl<F: AcirField, B: BlackBoxFunctionSolver<F>> AcirContext<F, B> {
         // TODO: perhaps this should be a user error, instead of an assert
         assert!(max_bits + 1 < F::max_num_bits());
 
-        let two_max_bits = self.add_constant(F::from(2_u128).pow(&F::from(max_bits as u128)));
+        let two_max_bits = self.add_constant(power_of_two::<F>(max_bits));
         let diff = self.sub_var(lhs, rhs)?;
         let comparison_evaluation = self.add_var(diff, two_max_bits)?;
 
@@ -1568,6 +1566,36 @@ impl<F: AcirField, B: BlackBoxFunctionSolver<F>> AcirContext<F, B> {
     }
 }
 
+/// Returns an `F` representing the value `2**power`
+///
+/// # Panics
+///
+/// Panics if `2**power` exceeds `F::modulus()`.
+pub(super) fn power_of_two<F: AcirField>(power: u32) -> F {
+    if power >= F::max_num_bits() {
+        panic!("Field cannot represent this power of two");
+    }
+    let full_bytes = power / 8;
+    let extra_bits = power % 8;
+    let most_significant_byte: u8 = match extra_bits % 8 {
+        0 => 0x01,
+        1 => 0x02,
+        2 => 0x04,
+        3 => 0x08,
+        4 => 0x10,
+        5 => 0x20,
+        6 => 0x40,
+        7 => 0x80,
+        _ => unreachable!("We cover the full range of x % 8"),
+    };
+
+    let bytes_be: Vec<u8> = std::iter::once(most_significant_byte)
+        .chain(std::iter::repeat(0).take(full_bytes as usize))
+        .collect();
+
+    F::from_be_bytes_reduce(&bytes_be)
+}
+
 /// Enum representing the possible values that a
 /// Variable can be given.
 #[derive(Debug, Eq, Clone)]
@@ -1656,3 +1684,29 @@ fn fits_in_one_identity<F: AcirField>(expr: &Expression<F>, width: ExpressionWid
 /// A Reference to an `AcirVarData`
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
 pub(crate) struct AcirVar(usize);
+
+#[cfg(test)]
+mod test {
+    use acvm::{AcirField, FieldElement};
+    use proptest::prelude::*;
+
+    use super::power_of_two;
+
+    #[test]
+    #[should_panic = "Field cannot represent this power of two"]
+    fn power_of_two_panics_on_overflow() {
+        power_of_two::<FieldElement>(FieldElement::max_num_bits());
+    }
+
+    proptest! {
+        #[test]
+        fn power_of_two_agrees_with_generic_impl(bit_size in (0..=128u32)) {
+            let power_of_two_general =
+                FieldElement::from(2_u128).pow(&FieldElement::from(bit_size));
+            let power_of_two_opt: FieldElement = power_of_two(bit_size);
+
+            prop_assert_eq!(power_of_two_opt, power_of_two_general);
+        }
+
+    }
+}

compiler/noirc_evaluator/src/acir/generated_acir.rs

@@ -356,13 +356,13 @@ impl<F: AcirField> GeneratedAcir<F> {
         limb_count: u32,
         bit_size: u32,
     ) -> Result<Vec<Witness>, RuntimeError> {
-        let radix_big = BigUint::from(radix);
-        let radix_range = BigUint::from(2u128)..=BigUint::from(256u128);
+        let radix_range = 2..=256;
         assert!(
-            radix_range.contains(&radix_big),
+            radix_range.contains(&radix),
             "ICE: Radix must be in the range 2..=256, but found: {:?}",
             radix
         );
+        let radix_big = BigUint::from(radix);
         assert_eq!(
             BigUint::from(2u128).pow(bit_size),
             radix_big,

compiler/noirc_evaluator/src/acir/mod.rs

@@ -51,7 +51,7 @@ use crate::ssa::{
     },
     ssa_gen::Ssa,
 };
-use acir_variable::{AcirContext, AcirType, AcirVar};
+use acir_variable::{power_of_two, AcirContext, AcirType, AcirVar};
 use generated_acir::BrilligStdlibFunc;
 pub(crate) use generated_acir::GeneratedAcir;
 use noirc_frontend::hir_def::types::Type as HirType;
@@ -2122,7 +2122,8 @@ impl<'a> Context<'a> {
                 ) {
                     // Subtractions must first have the integer modulus added before truncation can be
                     // applied. This is done in order to prevent underflow.
-                    let integer_modulus = self.acir_context.add_constant(2_u128.pow(bit_size));
+                    let integer_modulus = power_of_two::<FieldElement>(bit_size);
+                    let integer_modulus = self.acir_context.add_constant(integer_modulus);
                     var = self.acir_context.add_var(var, integer_modulus)?;
                 }
             }

compiler/noirc_evaluator/src/ssa/ir/instruction.rs

@@ -1,3 +1,4 @@
+use binary::truncate_field;
 use serde::{Deserialize, Serialize};
 use std::hash::{Hash, Hasher};
 
@@ -968,9 +969,8 @@ impl Instruction {
                     return SimplifiedTo(*value);
                 }
                 if let Some((numeric_constant, typ)) = dfg.get_numeric_constant_with_type(*value) {
-                    let integer_modulus = 2_u128.pow(*bit_size);
-                    let truncated = numeric_constant.to_u128() % integer_modulus;
-                    SimplifiedTo(dfg.make_constant(truncated.into(), typ))
+                    let truncated_field = truncate_field(numeric_constant, *bit_size);
+                    SimplifiedTo(dfg.make_constant(truncated_field, typ))
                 } else if let Value::Instruction { instruction, .. } = &dfg[dfg.resolve(*value)] {
                     match &dfg[*instruction] {
                         Instruction::Truncate { bit_size: src_bit_size, .. } => {

compiler/noirc_evaluator/src/ssa/ir/instruction/binary.rs

@@ -441,7 +441,7 @@ pub(crate) fn eval_constant_binary_op(
             }
             let result = function(lhs, rhs)?;
             // Check for overflow
-            if result >= 1 << bit_size {
+            if result != 0 && result.ilog2() >= bit_size {
                 return None;
             }
             result.into()
@@ -489,6 +489,7 @@ fn try_convert_field_element_to_signed_integer(field: FieldElement, bit_size: u3
     let signed_int = if is_positive {
         unsigned_int as i128
     } else {
+        assert!(bit_size < 128);
         let x = (1u128 << bit_size) - unsigned_int;
         -(x as i128)
     };
@@ -501,14 +502,45 @@ fn convert_signed_integer_to_field_element(int: i128, bit_size: u32) -> FieldEle
         FieldElement::from(int)
     } else {
         // We add an offset of `bit_size` bits to shift the negative values into the range [2^(bitsize-1), 2^bitsize)
+        assert!(bit_size < 128);
         let offset_int = (1i128 << bit_size) + int;
         FieldElement::from(offset_int)
     }
 }
 
+/// Truncates `int` to fit within `bit_size` bits.
 fn truncate(int: u128, bit_size: u32) -> u128 {
-    let max = 1 << bit_size;
-    int % max
+    if bit_size == 128 {
+        int
+    } else {
+        let max = 1 << bit_size;
+        int % max
+    }
+}
+
+pub(crate) fn truncate_field<F: AcirField>(int: F, bit_size: u32) -> F {
+    if bit_size == 0 {
+        return F::zero();
+    }
+    let num_bytes = bit_size.div_ceil(8);
+    let mut be_bytes: Vec<u8> =
+        int.to_be_bytes().into_iter().rev().take(num_bytes as usize).rev().collect();
+
+    // We need to apply a mask to the largest byte to handle non-divisible bit sizes.
+    let mask = match bit_size % 8 {
+        0 => 0xff,
+        1 => 0x01,
+        2 => 0x03,
+        3 => 0x07,
+        4 => 0x0f,
+        5 => 0x1f,
+        6 => 0x3f,
+        7 => 0x7f,
+        _ => unreachable!("We cover the full range of x % 8"),
+    };
+    be_bytes[0] &= mask;
+
+    F::from_be_bytes_reduce(&be_bytes)
 }
 
 impl BinaryOp {
@@ -588,8 +620,12 @@ mod test {
     use proptest::prelude::*;
 
     use super::{
-        convert_signed_integer_to_field_element, try_convert_field_element_to_signed_integer,
+        convert_signed_integer_to_field_element, truncate_field,
+        try_convert_field_element_to_signed_integer,
     };
+    use acvm::{AcirField, FieldElement};
+    use num_bigint::BigUint;
+    use num_traits::One;
 
     proptest! {
         #[test]
@@ -601,5 +637,18 @@ mod test {
 
             prop_assert_eq!(int, recovered_int);
         }
+
+        #[test]
+        fn truncate_field_agrees_with_bigint_modulo(input: u128, bit_size in (0..=253u32)) {
+            let field = FieldElement::from(input);
+            let truncated_as_field = truncate_field(field, bit_size);
+
+            let integer_modulus = BigUint::from(2_u128).pow(bit_size);
+            let truncated_as_bigint = BigUint::from(input)
+                        .modpow(&BigUint::one(), &integer_modulus);
+            let truncated_as_bigint = FieldElement::from_be_bytes_reduce(&truncated_as_bigint.to_bytes_be());
+            prop_assert_eq!(truncated_as_field, truncated_as_bigint);
+        }
+
     }
 }

compiler/noirc_printable_type/Cargo.toml

@@ -16,3 +16,4 @@ acvm.workspace = true
 serde.workspace = true
 
 [dev-dependencies]
+proptest.workspace = true

compiler/noirc_printable_type/src/lib.rs

@@ -97,7 +97,13 @@ fn to_string<F: AcirField>(value: &PrintableValue<F>, typ: &PrintableType) -> Op
             output.push_str(&format_field_string(*f));
         }
         (PrintableValue::Field(f), PrintableType::UnsignedInteger { width }) => {
-            let uint_cast = f.to_u128() & ((1 << width) - 1); // Retain the lower 'width' bits
+            // Retain the lower 'width' bits
+            debug_assert!(*width <= 128, "We don't currently support uints larger than u128");
+            let mut uint_cast = f.to_u128();
+            if *width != 128 {
+                uint_cast &= (1 << width) - 1;
+            };
+
             output.push_str(&uint_cast.to_string());
         }
         (PrintableValue::Field(f), PrintableType::SignedInteger { width }) => {
@@ -267,6 +273,10 @@ fn format_field_string<F: AcirField>(field: F) -> String {
 mod tests {
     use acvm::FieldElement;
 
+    use proptest::prelude::*;
+
+    use crate::to_string;
+
     use super::{PrintableType, PrintableValue, PrintableValueDisplay};
 
     #[test]
@@ -299,4 +309,16 @@ mod tests {
             PrintableValueDisplay::<FieldElement>::FmtString(template.to_string(), values);
         assert_eq!(display.to_string(), expected);
     }
+
+    proptest! {
+        #[test]
+        fn handles_decoding_u128_values(uint_value: u128) {
+            let value = PrintableValue::Field(FieldElement::from(uint_value));
+            let typ = PrintableType::UnsignedInteger { width: 128 };
+
+            let value_as_string = to_string(&value, &typ).unwrap();
+            // We want to match rust's stringification.
+            prop_assert_eq!(value_as_string, uint_value.to_string());
+        }
+    }
 }