fix(ssa refactor): pad radix result (#1730)

* fix(ssa refactor): pad radix result

* chore(ssa refactor): cp working test

* fix(ssa refactor): padding always at end

crates/nargo_cli/tests/test_data_ssa_refactor/to_le_bytes/Nargo.toml

@@ -0,0 +1,5 @@
+[package]
+authors = [""]
+compiler_version = "0.1"
+
+[dependencies]

crates/nargo_cli/tests/test_data_ssa_refactor/to_le_bytes/Prover.toml

@@ -0,0 +1 @@
+x = "2040124"

crates/nargo_cli/tests/test_data_ssa_refactor/to_le_bytes/src/main.nr

@@ -0,0 +1,14 @@
+use dep::std;
+
+fn main(x : Field) -> pub [u8; 4] {
+    // The result of this byte array will be little-endian
+    let byte_array = x.to_le_bytes(31);
+    let mut first_four_bytes = [0; 4];
+    for i in 0..4 {
+        first_four_bytes[i] = byte_array[i];
+    }
+    // Issue #617 fix
+    // We were incorrectly mapping our output array from bit decomposition functions during acir generation
+    first_four_bytes[3] = byte_array[31];
+    first_four_bytes
+}

crates/noirc_evaluator/src/ssa_refactor/acir_gen/acir_ir/acir_variable.rs

@@ -592,7 +592,8 @@ impl AcirContext {
 
         let input_expr = &self.vars[input_var].to_expression();
 
-        let limbs = self.acir_ir.radix_le_decompose(input_expr, radix, limb_count)?;
+        let bit_size = u32::BITS - (radix - 1).leading_zeros();
+        let limbs = self.acir_ir.radix_le_decompose(input_expr, radix, limb_count, bit_size)?;
 
         let mut limb_vars = vecmap(limbs, |witness| {
             let witness = self.add_data(AcirVarData::Witness(witness));
@@ -603,6 +604,16 @@ impl AcirContext {
             limb_vars.reverse();
         }
 
+        // For legacy reasons (see #617) the to_radix interface supports 256 bits even though
+        // FieldElement::max_num_bits() is only 254 bits. Any limbs beyond the specified count
+        // become zero padding.
+        let max_decomposable_bits: u32 = 256;
+        let limb_count_with_padding = max_decomposable_bits / bit_size;
+        let zero = self.add_constant(FieldElement::zero());
+        while limb_vars.len() < limb_count_with_padding as usize {
+            limb_vars.push(AcirValue::Var(zero, result_element_type));
+        }
+
         Ok(vec![AcirValue::Array(limb_vars.into())])
     }
 

crates/noirc_evaluator/src/ssa_refactor/acir_gen/acir_ir/generated_acir.rs

@@ -269,8 +269,8 @@ impl GeneratedAcir {
         input_expr: &Expression,
         radix: u32,
         limb_count: u32,
+        bit_size: u32,
     ) -> Result<Vec<Witness>, AcirGenError> {
-        let bit_size = u32::BITS - (radix - 1).leading_zeros();
         let radix_big = BigUint::from(radix);
         assert_eq!(
             BigUint::from(2u128).pow(bit_size),