feat: make Instruction::Truncate actually truncate in brillig

noir-lang · Jan 10, 2024 · a781dbc · a781dbc
1 parent 8e11c54
commit a781dbc
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Showing 2 changed files with 18 additions and 5 deletions.
diff --git a/compiler/noirc_evaluator/src/brillig/brillig_gen/brillig_block.rs b/compiler/noirc_evaluator/src/brillig/brillig_gen/brillig_block.rs
@@ -521,7 +521,7 @@ impl<'block> BrilligBlock<'block> {
                     unreachable!("unsupported function call type {:?}", dfg[*func])
                 }
             },
-            Instruction::Truncate { value, .. } => {
+            Instruction::Truncate { value, bit_size, .. } => {
                 let result_ids = dfg.instruction_results(instruction_id);
                 let destination_register = self.variables.define_register_variable(
                     self.function_context,
@@ -530,7 +530,11 @@ impl<'block> BrilligBlock<'block> {
                     dfg,
                 );
                 let source_register = self.convert_ssa_register_value(*value, dfg);
-                self.brillig_context.truncate_instruction(destination_register, source_register);
+                self.brillig_context.truncate_instruction(
+                    destination_register,
+                    source_register,
+                    *bit_size,
+                );
             }
             Instruction::Cast(value, target_type) => {
                 let result_ids = dfg.instruction_results(instruction_id);

diff --git a/compiler/noirc_evaluator/src/brillig/brillig_ir.rs b/compiler/noirc_evaluator/src/brillig/brillig_ir.rs
@@ -687,10 +687,19 @@ impl BrilligContext {
         &mut self,
         destination_of_truncated_value: RegisterIndex,
         value_to_truncate: RegisterIndex,
+        bit_size: u32,
     ) {
-        // Effectively a no-op because brillig already has implicit truncation on integer
-        // operations. We need only copy the value to it's destination.
-        self.mov_instruction(destination_of_truncated_value, value_to_truncate);
+        // The brillig VM performs all arithmetic operations modulo 2**bit_size
+        // So to truncate any value to a target bit size we can just issue a no-op arithmetic operation
+        // With bit size equal to target_bit_size
+        let zero_register = self.make_constant(Value::from(FieldElement::zero()));
+        self.binary_instruction(
+            value_to_truncate,
+            zero_register,
+            destination_of_truncated_value,
+            BrilligBinaryOp::Integer { op: BinaryIntOp::Add, bit_size },
+        );
+        self.deallocate_register(zero_register);
     }
 
     /// Emits a stop instruction