fix(ssa): Do not replace previously constrained values

compiler/noirc_evaluator/src/ssa/opt/constant_folding.rs

@@ -78,6 +78,7 @@ impl Context {
         // Cache of instructions without any side-effects along with their outputs.
         let mut cached_instruction_results: HashMap<Instruction, Vec<ValueId>> = HashMap::default();
         let mut constrained_values: HashMap<ValueId, ValueId> = HashMap::default();
+        let mut side_effects_enabled: bool = false;
 
         for instruction_id in instructions {
             Self::fold_constants_into_instruction(
@@ -86,6 +87,7 @@ impl Context {
                 instruction_id,
                 &mut cached_instruction_results,
                 &mut constrained_values,
+                &mut side_effects_enabled,
             );
         }
         self.block_queue.extend(function.dfg[block].successors());
@@ -97,8 +99,10 @@ impl Context {
         id: InstructionId,
         instruction_result_cache: &mut HashMap<Instruction, Vec<ValueId>>,
         constrained_values: &mut HashMap<ValueId, ValueId>,
+        side_effects_enabled: &mut bool,
     ) {
-        let instruction = Self::resolve_instruction(id, dfg, constrained_values);
+        let instruction =
+            Self::resolve_instruction(id, dfg, constrained_values, side_effects_enabled);
         let old_results = dfg.instruction_results(id).to_vec();
 
         // If a copy of this instruction exists earlier in the block, then reuse the previous results.
@@ -118,6 +122,7 @@ impl Context {
             dfg,
             instruction_result_cache,
             constrained_values,
+            side_effects_enabled,
         );
     }
 
@@ -126,6 +131,7 @@ impl Context {
         instruction_id: InstructionId,
         dfg: &DataFlowGraph,
         constrained_values: &mut HashMap<ValueId, ValueId>,
+        side_effects_enabled: &bool,
     ) -> Instruction {
         let instruction = dfg[instruction_id].clone();
 
@@ -134,20 +140,33 @@ impl Context {
         //
         // This allows us to reach a stable final `ValueId` for each instruction input as we add more
         // constraints to the cache.
+        //
+        // We also must check whether side effects are currently enabled. If they are enabled we do not want
+        // to replace constrained values as they could be reliant upon runtime values. The runtime values
+        // would then be replaced later in the SSA, possibly leading to a mismatched constant constraint failure
+        // when we should have a runtime constraint failure earlier in the SSA.
         fn resolve_cache(
             dfg: &DataFlowGraph,
             cache: &HashMap<ValueId, ValueId>,
             value_id: ValueId,
+            side_effects_enabled: &bool,
         ) -> ValueId {
             let resolved_id = dfg.resolve(value_id);
+            if *side_effects_enabled {
+                return resolved_id;
+            }
             match cache.get(&resolved_id) {
-                Some(cached_value) => resolve_cache(dfg, cache, *cached_value),
+                Some(cached_value) => {
+                    resolve_cache(dfg, cache, *cached_value, side_effects_enabled)
+                }
                 None => resolved_id,
             }
         }
 
         // Resolve any inputs to ensure that we're comparing like-for-like instructions.
-        instruction.map_values(|value_id| resolve_cache(dfg, constrained_values, value_id))
+        instruction.map_values(|value_id| {
+            resolve_cache(dfg, constrained_values, value_id, side_effects_enabled)
+        })
     }
 
     /// Pushes a new [`Instruction`] into the [`DataFlowGraph`] which applies any optimizations
@@ -186,31 +205,48 @@ impl Context {
         dfg: &DataFlowGraph,
         instruction_result_cache: &mut HashMap<Instruction, Vec<ValueId>>,
         constraint_cache: &mut HashMap<ValueId, ValueId>,
+        side_effects_enabled: &mut bool,
     ) {
-        // If the instruction was a constraint, then create a link between the two `ValueId`s
-        // to map from the more complex to the simpler value.
-        if let Instruction::Constrain(lhs, rhs, _) = instruction {
-            // These `ValueId`s should be fully resolved now.
-            match (&dfg[lhs], &dfg[rhs]) {
-                // Ignore trivial constraints
-                (Value::NumericConstant { .. }, Value::NumericConstant { .. }) => (),
-
-                // Prefer replacing with constants where possible.
-                (Value::NumericConstant { .. }, _) => {
-                    constraint_cache.insert(rhs, lhs);
-                }
-                (_, Value::NumericConstant { .. }) => {
-                    constraint_cache.insert(lhs, rhs);
+        match instruction {
+            Instruction::EnableSideEffects { condition } => {
+                if let Value::NumericConstant { constant, .. } = &dfg[condition] {
+                    if constant.is_one() {
+                        *side_effects_enabled = true;
+                    } else {
+                        *side_effects_enabled = false;
+                    }
+                } else {
+                    // If the `enable_side_effects` condition is not a constant we cannot know for sure whether
+                    // side effects are not enabled. Thus, we mark the side effects as being enabled to make sure
+                    // that we do not mistakenly replace constrained values dependent upon runtime side effects.
+                    *side_effects_enabled = true;
                 }
-                // Otherwise prefer block parameters over instruction results.
-                // This is as block parameters are more likely to be a single witness rather than a full expression.
-                (Value::Param { .. }, Value::Instruction { .. }) => {
-                    constraint_cache.insert(rhs, lhs);
-                }
-                (Value::Instruction { .. }, Value::Param { .. }) => {
-                    constraint_cache.insert(lhs, rhs);
+            }
+            Instruction::Constrain(lhs, rhs, _) => {
+                match (&dfg[lhs], &dfg[rhs]) {
+                    // Ignore trivial constraints
+                    (Value::NumericConstant { .. }, Value::NumericConstant { .. }) => (),
+
+                    // Prefer replacing with constants where possible.
+                    (Value::NumericConstant { .. }, _) => {
+                        constraint_cache.insert(rhs, lhs);
+                    }
+                    (_, Value::NumericConstant { .. }) => {
+                        constraint_cache.insert(lhs, rhs);
+                    }
+                    // Otherwise prefer block parameters over instruction results.
+                    // This is as block parameters are more likely to be a single witness rather than a full expression.
+                    (Value::Param { .. }, Value::Instruction { .. }) => {
+                        constraint_cache.insert(rhs, lhs);
+                    }
+                    (Value::Instruction { .. }, Value::Param { .. }) => {
+                        constraint_cache.insert(lhs, rhs);
+                    }
+                    (_, _) => (),
                 }
-                (_, _) => (),
+            }
+            _ => {
+                // The remaining instructions are not cached, so do nothing
             }
         }
 
@@ -408,16 +444,7 @@ mod test {
 
         // Compiling main
         let mut builder = FunctionBuilder::new("main".into(), main_id, RuntimeType::Acir);
-        let v0 = builder.add_parameter(Type::field());
-
-        let field_10 = builder.field_constant(10u128);
-        builder.insert_constrain(v0, field_10, None);
-
-        let field_1 = builder.field_constant(1u128);
-        let v1 = builder.insert_binary(v0, BinaryOp::Add, field_1);
-
-        let field_11 = builder.field_constant(11u128);
-        builder.insert_constrain(v1, field_11, None);
+        constrained_value_replacement_setup(&mut builder, false);
 
         let mut ssa = builder.finish();
         let main = ssa.main_mut();
@@ -442,4 +469,69 @@ mod test {
             &Instruction::Constrain(ValueId::test_new(0), ValueId::test_new(1), None)
         );
     }
+
+    #[test]
+    fn constrained_value_replacement_with_side_effects() {
+        // fn main f0 {
+        //   b0(v0: Field):
+        //     enable_side_effects u1 1
+        //     constrain v0 == Field 10
+        //     v1 = add v0, Field 1
+        //     constrain v1 == Field 11
+        // }
+        let main_id = Id::test_new(0);
+
+        // Compiling main
+        let mut builder = FunctionBuilder::new("main".into(), main_id, RuntimeType::Acir);
+        constrained_value_replacement_setup(&mut builder, true);
+
+        let mut ssa = builder.finish();
+        let main = ssa.main_mut();
+        let instructions = main.dfg[main.entry_block()].instructions();
+        assert_eq!(instructions.len(), 4);
+
+        // Expected output:
+        //
+        // fn main f0 {
+        //   b0(v0: Field):
+        //     enable_side_effects u1 1
+        //     constrain v0 == Field 10
+        //     v6 = add v0, Field 1
+        //     constrain v6 == Field 11
+        // }
+        let ssa = ssa.fold_constants();
+        println!("ssa: {ssa}");
+        let main = ssa.main();
+        let instructions = main.dfg[main.entry_block()].instructions();
+        assert_eq!(instructions.len(), 4);
+
+        assert_eq!(
+            &main.dfg[instructions[0]],
+            &Instruction::EnableSideEffects { condition: ValueId::test_new(1) }
+        );
+
+        assert_eq!(
+            &main.dfg[instructions[3]],
+            &Instruction::Constrain(ValueId::test_new(6), ValueId::test_new(5), None)
+        );
+    }
+
+    fn constrained_value_replacement_setup(builder: &mut FunctionBuilder, with_side_effects: bool) {
+        let v0 = builder.add_parameter(Type::field());
+
+        if with_side_effects {
+            let field_one = builder.numeric_constant(1u128, Type::unsigned(1));
+            let enable_side_effects = Instruction::EnableSideEffects { condition: field_one };
+            builder.insert_instruction(enable_side_effects, None);
+        }
+
+        let field_10 = builder.field_constant(10u128);
+        builder.insert_constrain(v0, field_10, None);
+
+        let field_1 = builder.field_constant(1u128);
+        let v1 = builder.insert_binary(v0, BinaryOp::Add, field_1);
+
+        let field_11 = builder.field_constant(11u128);
+        builder.insert_constrain(v1, field_11, None);
+    }
 }