feat: optimize out unnecessary truncation instructions (#3717)

# Description

## Problem\*

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

## Summary\*

We're currently adding a double truncation when performing overflow
checks on left shifts. We could/should address this directly but as a
more general rule we can optimize out any truncation which is truncating
a value to fit into a type which is equal or larger to the source type.

I've also replaced a few instances of `insert_instruction` with the more
instruction-specific version to reduce verbosity.

## 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 <kevtheappdev@gmail.com>

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

@@ -314,16 +314,11 @@ impl FunctionBuilder {
                 (FieldElement::max_num_bits(), self.insert_binary(predicate, BinaryOp::Mul, pow))
             };
 
-        let instruction = Instruction::Binary(Binary { lhs, rhs: pow, operator: BinaryOp::Mul });
         if max_bit <= bit_size {
-            self.insert_instruction(instruction, None).first()
+            self.insert_binary(lhs, BinaryOp::Mul, pow)
         } else {
-            let result = self.insert_instruction(instruction, None).first();
-            self.insert_instruction(
-                Instruction::Truncate { value: result, bit_size, max_bit_size: max_bit },
-                None,
-            )
-            .first()
+            let result = self.insert_binary(lhs, BinaryOp::Mul, pow);
+            self.insert_truncate(result, bit_size, max_bit)
         }
     }
 

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

@@ -467,11 +467,23 @@ impl Instruction {
                 }
                 None
             }
-            Instruction::Truncate { value, bit_size, .. } => {
+            Instruction::Truncate { value, bit_size, max_bit_size } => {
                 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))
+                } else if let Value::Instruction { instruction, .. } = &dfg[dfg.resolve(*value)] {
+                    if let Instruction::Truncate { bit_size: src_bit_size, .. } = &dfg[*instruction]
+                    {
+                        // If we're truncating the value to fit into the same or larger bit size then this is a noop.
+                        if src_bit_size <= bit_size && src_bit_size <= max_bit_size {
+                            SimplifiedTo(*value)
+                        } else {
+                            None
+                        }
+                    } else {
+                        None
+                    }
                 } else {
                     None
                 }

compiler/noirc_evaluator/src/ssa/ssa_gen/context.rs

@@ -275,16 +275,11 @@ impl<'a> FunctionContext<'a> {
         let bit_width =
             self.builder.numeric_constant(FieldElement::from(2_i128.pow(bit_size)), Type::field());
         let sign_not = self.builder.insert_binary(one, BinaryOp::Sub, sign);
-        let as_field =
-            self.builder.insert_instruction(Instruction::Cast(input, Type::field()), None).first();
-        let sign_field =
-            self.builder.insert_instruction(Instruction::Cast(sign, Type::field()), None).first();
+        let as_field = self.builder.insert_cast(input, Type::field());
+        let sign_field = self.builder.insert_cast(sign, Type::field());
         let positive_predicate = self.builder.insert_binary(sign_field, BinaryOp::Mul, as_field);
         let two_complement = self.builder.insert_binary(bit_width, BinaryOp::Sub, as_field);
-        let sign_not_field = self
-            .builder
-            .insert_instruction(Instruction::Cast(sign_not, Type::field()), None)
-            .first();
+        let sign_not_field = self.builder.insert_cast(sign_not, Type::field());
         let negative_predicate =
             self.builder.insert_binary(sign_not_field, BinaryOp::Mul, two_complement);
         self.builder.insert_binary(positive_predicate, BinaryOp::Add, negative_predicate)
@@ -315,17 +310,8 @@ impl<'a> FunctionContext<'a> {
                 match operator {
                     BinaryOpKind::Add | BinaryOpKind::Subtract => {
                         // Result is computed modulo the bit size
-                        let mut result = self
-                            .builder
-                            .insert_instruction(
-                                Instruction::Truncate {
-                                    value: result,
-                                    bit_size,
-                                    max_bit_size: bit_size + 1,
-                                },
-                                None,
-                            )
-                            .first();
+                        let mut result =
+                            self.builder.insert_truncate(result, bit_size, bit_size + 1);
                         result = self.builder.insert_cast(result, Type::unsigned(bit_size));
 
                         self.check_signed_overflow(result, lhs, rhs, operator, bit_size, location);
@@ -335,17 +321,7 @@ impl<'a> FunctionContext<'a> {
                         // Result is computed modulo the bit size
                         let mut result =
                             self.builder.insert_cast(result, Type::unsigned(2 * bit_size));
-                        result = self
-                            .builder
-                            .insert_instruction(
-                                Instruction::Truncate {
-                                    value: result,
-                                    bit_size,
-                                    max_bit_size: 2 * bit_size,
-                                },
-                                None,
-                            )
-                            .first();
+                        result = self.builder.insert_truncate(result, bit_size, 2 * bit_size);
 
                         self.check_signed_overflow(result, lhs, rhs, operator, bit_size, location);
                         self.builder.insert_cast(result, result_type)
@@ -476,17 +452,16 @@ impl<'a> FunctionContext<'a> {
 
                 // Then we check the signed product fits in a signed integer of bit_size-bits
                 let not_same = self.builder.insert_binary(one, BinaryOp::Sub, same_sign);
-                let not_same_sign_field = self
-                    .builder
-                    .insert_instruction(Instruction::Cast(not_same, Type::unsigned(bit_size)), None)
-                    .first();
+                let not_same_sign_field =
+                    self.builder.insert_cast(not_same, Type::unsigned(bit_size));
                 let positive_maximum_with_offset =
                     self.builder.insert_binary(half_width, BinaryOp::Add, not_same_sign_field);
                 let product_overflow_check =
                     self.builder.insert_binary(product, BinaryOp::Lt, positive_maximum_with_offset);
-                self.builder.set_location(location).insert_instruction(
-                    Instruction::Constrain(product_overflow_check, one, Some(message)),
-                    None,
+                self.builder.set_location(location).insert_constrain(
+                    product_overflow_check,
+                    one,
+                    Some(message),
                 );
             }
             BinaryOpKind::ShiftLeft => unreachable!("shift is not supported for signed integer"),