From 44ff4c5cde82e193f3f1a55742cebe0e25757a5b Mon Sep 17 00:00:00 2001
From: Maxim Vezenov <mvezenov@gmail.com>
Date: Fri, 27 Sep 2024 15:33:33 +0000
Subject: [PATCH] simplify signed casts, conservative in the field/unsigned ->
 signed case

---
 .../src/ssa/ir/instruction/cast.rs            | 22 +++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/compiler/noirc_evaluator/src/ssa/ir/instruction/cast.rs b/compiler/noirc_evaluator/src/ssa/ir/instruction/cast.rs
index d0ed5a1fa9c..ed588def1d7 100644
--- a/compiler/noirc_evaluator/src/ssa/ir/instruction/cast.rs
+++ b/compiler/noirc_evaluator/src/ssa/ir/instruction/cast.rs
@@ -27,10 +27,10 @@ pub(super) fn simplify_cast(
                 SimplifiedTo(value)
             }
             (
-                Type::Numeric(NumericType::Unsigned { .. }),
+                Type::Numeric(NumericType::Unsigned { .. } | NumericType::Signed { .. }),
                 Type::Numeric(NumericType::NativeField),
             ) => {
-                // Unsigned -> Field: redefine same constant as Field
+                // Unsigned/Signed -> Field: redefine same constant as Field
                 SimplifiedTo(dfg.make_constant(constant, dst_typ.clone()))
             }
             (
@@ -48,6 +48,24 @@ pub(super) fn simplify_cast(
                 let truncated = FieldElement::from_be_bytes_reduce(&truncated.to_bytes_be());
                 SimplifiedTo(dfg.make_constant(truncated, dst_typ.clone()))
             }
+            (
+                Type::Numeric(
+                    NumericType::NativeField
+                    | NumericType::Unsigned { .. }
+                    | NumericType::Signed { .. },
+                ),
+                Type::Numeric(NumericType::Signed { bit_size }),
+            ) => {
+                // Field/Unsigned -> signed
+                // We only simplify to signed when we are below the maximum signed integer of the destination type.
+                let integer_modulus = BigUint::from(2u128).pow(*bit_size - 1);
+                let constant_uint: BigUint = BigUint::from_bytes_be(&constant.to_be_bytes());
+                if constant_uint < integer_modulus {
+                    SimplifiedTo(dfg.make_constant(constant, dst_typ.clone()))
+                } else {
+                    None
+                }
+            }
             _ => None,
         }
     } else if *dst_typ == dfg.type_of_value(value) {