fix: Don't remove necessary RC instructions in DIE pass (#6585)

Co-authored-by: Ary Borenszweig <[email protected]>
Co-authored-by: Tom French <[email protected]>
Co-authored-by: Maxim Vezenov <[email protected]>

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

@@ -127,8 +127,9 @@ impl Context {
                         .push(instructions_len - instruction_index - 1);
                 }
             } else {
-                use Instruction::*;
-                if matches!(instruction, IncrementRc { .. } | DecrementRc { .. }) {
+                // We can't remove rc instructions if they're loaded from a reference
+                // since we'd have no way of knowing whether the reference is still used.
+                if Self::is_inc_dec_instruction_on_known_array(instruction, &function.dfg) {
                     self.rc_instructions.push((*instruction_id, block_id));
                 } else {
                     instruction.for_each_value(|value| {
@@ -140,7 +141,6 @@ impl Context {
             rc_tracker.track_inc_rcs_to_remove(*instruction_id, function);
         }
 
-        self.instructions_to_remove.extend(rc_tracker.get_non_mutated_arrays());
         self.instructions_to_remove.extend(rc_tracker.rc_pairs_to_remove);
 
         // If there are some instructions that might trigger an out of bounds error,
@@ -337,6 +337,28 @@ impl Context {
 
         inserted_check
     }
+
+    /// True if this is a `Instruction::IncrementRc` or `Instruction::DecrementRc`
+    /// operating on an array directly from a `Instruction::MakeArray` or an
+    /// intrinsic known to return a fresh array.
+    fn is_inc_dec_instruction_on_known_array(
+        instruction: &Instruction,
+        dfg: &DataFlowGraph,
+    ) -> bool {
+        use Instruction::*;
+        if let IncrementRc { value } | DecrementRc { value } = instruction {
+            if let Value::Instruction { instruction, .. } = &dfg[*value] {
+                return match &dfg[*instruction] {
+                    MakeArray { .. } => true,
+                    Call { func, .. } => {
+                        matches!(&dfg[*func], Value::Intrinsic(_) | Value::ForeignFunction(_))
+                    }
+                    _ => false,
+                };
+            }
+        }
+        false
+    }
 }
 
 fn instruction_might_result_in_out_of_bounds(
@@ -510,10 +532,11 @@ struct RcTracker {
     // If we see an inc/dec RC pair within a block we can safely remove both instructions.
     rcs_with_possible_pairs: HashMap<Type, Vec<RcInstruction>>,
     rc_pairs_to_remove: HashSet<InstructionId>,
+
     // We also separately track all IncrementRc instructions and all arrays which have been mutably borrowed.
     // If an array has not been mutably borrowed we can then safely remove all IncrementRc instructions on that array.
     inc_rcs: HashMap<ValueId, HashSet<InstructionId>>,
-    mut_borrowed_arrays: HashSet<ValueId>,
+
     // The SSA often creates patterns where after simplifications we end up with repeat
     // IncrementRc instructions on the same value. We track whether the previous instruction was an IncrementRc,
     // and if the current instruction is also an IncrementRc on the same value we remove the current instruction.
@@ -566,35 +589,10 @@ impl RcTracker {
                         dec_rc.possibly_mutated = true;
                     }
                 }
-
-                self.mut_borrowed_arrays.insert(*array);
-            }
-            Instruction::Store { value, .. } => {
-                // We are very conservative and say that any store of an array value means it has the potential
-                // to be mutated. This is done due to the tracking of mutable borrows still being per block.
-                let typ = function.dfg.type_of_value(*value);
-                if matches!(&typ, Type::Array(..) | Type::Slice(..)) {
-                    self.mut_borrowed_arrays.insert(*value);
-                }
             }
             _ => {}
         }
     }
-
-    fn get_non_mutated_arrays(&self) -> HashSet<InstructionId> {
-        self.inc_rcs
-            .keys()
-            .filter_map(|value| {
-                if !self.mut_borrowed_arrays.contains(value) {
-                    Some(&self.inc_rcs[value])
-                } else {
-                    None
-                }
-            })
-            .flatten()
-            .copied()
-            .collect()
-    }
 }
 #[cfg(test)]
 mod test {
@@ -830,32 +828,23 @@ mod test {
     }
 
     #[test]
-    fn remove_inc_rcs_that_are_never_mutably_borrowed() {
+    fn does_not_remove_inc_or_dec_rc_of_if_they_are_loaded_from_a_reference() {
         let src = "
-            acir(inline) fn main f0 {
-              b0(v0: [Field; 2]):
-                inc_rc v0
-                inc_rc v0
-                inc_rc v0
-                v2 = array_get v0, index u32 0 -> Field
-                inc_rc v0
-                return v2
+            brillig(inline) fn borrow_mut f0 {
+              b0(v0: &mut [Field; 3]):
+                v1 = load v0 -> [Field; 3]
+                inc_rc v1 // this one shouldn't be removed
+                v2 = load v0 -> [Field; 3]
+                inc_rc v2 // this one shouldn't be removed
+                v3 = load v0 -> [Field; 3]
+                v6 = array_set v3, index u32 0, value Field 5
+                store v6 at v0
+                dec_rc v6
+                return
             }
             ";
         let ssa = Ssa::from_str(src).unwrap();
-        let main = ssa.main();
-
-        // The instruction count never includes the terminator instruction
-        assert_eq!(main.dfg[main.entry_block()].instructions().len(), 5);
-
-        let expected = "
-            acir(inline) fn main f0 {
-              b0(v0: [Field; 2]):
-                v2 = array_get v0, index u32 0 -> Field
-                return v2
-            }
-            ";
         let ssa = ssa.dead_instruction_elimination();
-        assert_normalized_ssa_equals(ssa, expected);
+        assert_normalized_ssa_equals(ssa, src);
     }
 }

test_programs/execution_success/reference_counts/src/main.nr

@@ -1,6 +1,6 @@
 fn main() {
     let mut array = [0, 1, 2];
-    assert_refcount(array, 1);
+    assert_refcount(array, 2);
 
     borrow(array, std::mem::array_refcount(array));
     borrow_mut(&mut array, std::mem::array_refcount(array));
@@ -13,13 +13,13 @@ fn borrow(array: [Field; 3], rc_before_call: u32) {
 }
 
 fn borrow_mut(array: &mut [Field; 3], rc_before_call: u32) {
-    assert_refcount(*array, rc_before_call + 0); // Issue! This should be rc_before_call + 1
+    assert_refcount(*array, rc_before_call + 1);
     array[0] = 5;
     println(array[0]);
 }
 
 fn copy_mut(mut array: [Field; 3], rc_before_call: u32) {
-    assert_refcount(array, rc_before_call + 0); // Issue! This should be rc_before_call + 1
+    assert_refcount(array, rc_before_call + 1);
     array[0] = 6;
     println(array[0]);
 }