feat: comparison for signed integers (#3873)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*
 
Implement comparison for signed integers.


## Additional Context



## Documentation\*

Check one:
- [X] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[Exceptional Case]** Documentation to be submitted in a separate
PR.

# PR Checklist\*

- [X] I have tested the changes locally.
- [X] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

Co-authored-by: kevaundray <[email protected]>

compiler/noirc_evaluator/src/ssa/acir_gen/acir_ir/acir_variable.rs

@@ -936,6 +936,56 @@ impl AcirContext {
         Ok(remainder)
     }
 
+    /// Returns an 'AcirVar' containing the boolean value lhs<rhs, assuming lhs and rhs are signed integers of size bit_count.
+    /// Like in the unsigned case, we compute the difference diff = lhs-rhs+2^n (and we avoid underflow)
+    /// The result depends on the diff and the signs of the inputs:
+    /// If same sign, lhs<rhs <=> diff<2^n, because the 2-complement representation keeps the ordering (e.g in 8 bits -1 is 255 > -2 = 254)
+    /// If not, lhs positive => diff > 2^n
+    /// and lhs negative => diff <= 2^n => diff < 2^n (because signs are not the same, so lhs != rhs and so diff != 2^n)
+    pub(crate) fn less_than_signed(
+        &mut self,
+        lhs: AcirVar,
+        rhs: AcirVar,
+        bit_count: u32,
+        predicate: AcirVar,
+    ) -> Result<AcirVar, RuntimeError> {
+        let pow_last = self.add_constant(FieldElement::from(1_u128 << (bit_count - 1)));
+        let pow = self.add_constant(FieldElement::from(1_u128 << (bit_count)));
+
+        // We check whether the inputs have same sign or not by computing the XOR of their bit sign
+        let lhs_sign = self.div_var(
+            lhs,
+            pow_last,
+            AcirType::NumericType(NumericType::Unsigned { bit_size: bit_count }),
+            predicate,
+        )?;
+        let rhs_sign = self.div_var(
+            rhs,
+            pow_last,
+            AcirType::NumericType(NumericType::Unsigned { bit_size: bit_count }),
+            predicate,
+        )?;
+        let same_sign = self.xor_var(
+            lhs_sign,
+            rhs_sign,
+            AcirType::NumericType(NumericType::Signed { bit_size: 1 }),
+        )?;
+
+        // We compute the input difference
+        let no_underflow = self.add_var(lhs, pow)?;
+        let diff = self.sub_var(no_underflow, rhs)?;
+
+        // We check the 'bit sign' of the difference
+        let diff_sign = self.less_than_var(diff, pow, bit_count + 1, predicate)?;
+
+        // Then the result is simply diff_sign XOR same_sign (can be checked with a truth table)
+        self.xor_var(
+            diff_sign,
+            same_sign,
+            AcirType::NumericType(NumericType::Signed { bit_size: 1 }),
+        )
+    }
+
     /// Returns an `AcirVar` which will be `1` if lhs >= rhs
     /// and `0` otherwise.
     pub(crate) fn more_than_eq_var(

compiler/noirc_evaluator/src/ssa/acir_gen/mod.rs

@@ -1569,12 +1569,17 @@ impl Context {
             // Note: that this produces unnecessary constraints when
             // this Eq instruction is being used for a constrain statement
             BinaryOp::Eq => self.acir_context.eq_var(lhs, rhs),
-            BinaryOp::Lt => self.acir_context.less_than_var(
-                lhs,
-                rhs,
-                bit_count,
-                self.current_side_effects_enabled_var,
-            ),
+            BinaryOp::Lt => match binary_type {
+                AcirType::NumericType(NumericType::Signed { .. }) => self
+                    .acir_context
+                    .less_than_signed(lhs, rhs, bit_count, self.current_side_effects_enabled_var),
+                _ => self.acir_context.less_than_var(
+                    lhs,
+                    rhs,
+                    bit_count,
+                    self.current_side_effects_enabled_var,
+                ),
+            },
             BinaryOp::Xor => self.acir_context.xor_var(lhs, rhs, binary_type),
             BinaryOp::And => self.acir_context.and_var(lhs, rhs, binary_type),
             BinaryOp::Or => self.acir_context.or_var(lhs, rhs, binary_type),

test_programs/execution_success/signed_comparison/Nargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "signed_comparison"
+type = "bin"
+authors = [""]
+
+[dependencies]

test_programs/execution_success/signed_comparison/Prover.toml

@@ -0,0 +1,3 @@
+x = "5"
+y = "8"
+z = "-15"

test_programs/execution_success/signed_comparison/src/main.nr

@@ -0,0 +1,13 @@
+use dep::std;
+
+fn main(mut x: i8, mut y: i8, z: i8) {
+    let mut s1: i8 = 5;
+    let mut s2: i8 = 8;
+    assert(-1 as i8 < 0);
+    assert(x < y);
+    assert(-x < y);
+    assert(-y < -x);
+    assert((z > x) == false);
+    assert(x <= s1);
+    assert(z < x - y - s2);
+}