feat(perf): Remove inc_rc instructions for arrays which are never mut…

…ably borrowed (#6168)

# Description

## Problem\*

Part of general effort to reduce Brillig bytecode sizes

## Summary\*

An array can be borrowed without being mutated. We code gen inc_rc
instructions quite pessimistically on every borrow or reassignment of an
array. If we know that the array is never mutated, it should be safe to
remove these inc_rc instructions.

During DIE we now track all inc_rc instructions in a block and if that
array is never used in an array set or a store, we mark its inc_rc
instruction for removal.

## Additional Context

## Documentation\*

Check one:
- [X] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** 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: Tom French <15848336+TomAFrench@users.noreply.github.com>

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

@@ -113,8 +113,12 @@ impl Context {
         // We track per block whether an IncrementRc instruction has a paired DecrementRc instruction
         // with the same value but no array set in between.
         // If we see an inc/dec RC pair within a block we can safely remove both instructions.
-        let mut inc_rcs: HashMap<Type, Vec<RcInstruction>> = HashMap::default();
-        let mut inc_rcs_to_remove = HashSet::default();
+        let mut rcs_with_possible_pairs: HashMap<Type, Vec<RcInstruction>> = HashMap::default();
+        let mut rc_pairs_to_remove = HashSet::default();
+        // 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.
+        let mut inc_rcs: HashMap<ValueId, HashSet<InstructionId>> = HashMap::default();
+        let mut borrowed_arrays: HashSet<ValueId> = HashSet::default();
 
         // Indexes of instructions that might be out of bounds.
         // We'll remove those, but before that we'll insert bounds checks for them.
@@ -146,14 +150,29 @@ impl Context {
             self.track_inc_rcs_to_remove(
                 *instruction_id,
                 function,
+                &mut rcs_with_possible_pairs,
+                &mut rc_pairs_to_remove,
                 &mut inc_rcs,
-                &mut inc_rcs_to_remove,
+                &mut borrowed_arrays,
             );
         }
 
-        for id in inc_rcs_to_remove {
-            self.instructions_to_remove.insert(id);
-        }
+        let non_mutated_arrays =
+            inc_rcs
+                .keys()
+                .filter_map(|value| {
+                    if !borrowed_arrays.contains(value) {
+                        Some(&inc_rcs[value])
+                    } else {
+                        None
+                    }
+                })
+                .flatten()
+                .copied()
+                .collect::<Vec<_>>();
+
+        self.instructions_to_remove.extend(non_mutated_arrays);
+        self.instructions_to_remove.extend(rc_pairs_to_remove);
 
         // If there are some instructions that might trigger an out of bounds error,
         // first add constrain checks. Then run the DIE pass again, which will remove those
@@ -181,36 +200,50 @@ impl Context {
         &self,
         instruction_id: InstructionId,
         function: &Function,
-        inc_rcs: &mut HashMap<Type, Vec<RcInstruction>>,
+        rcs_with_possible_pairs: &mut HashMap<Type, Vec<RcInstruction>>,
         inc_rcs_to_remove: &mut HashSet<InstructionId>,
+        inc_rcs: &mut HashMap<ValueId, HashSet<InstructionId>>,
+        borrowed_arrays: &mut HashSet<ValueId>,
     ) {
         let instruction = &function.dfg[instruction_id];
         // DIE loops over a block in reverse order, so we insert an RC instruction for possible removal
         // when we see a DecrementRc and check whether it was possibly mutated when we see an IncrementRc.
         match instruction {
             Instruction::IncrementRc { value } => {
-                if let Some(inc_rc) = pop_rc_for(*value, function, inc_rcs) {
+                if let Some(inc_rc) = pop_rc_for(*value, function, rcs_with_possible_pairs) {
                     if !inc_rc.possibly_mutated {
                         inc_rcs_to_remove.insert(inc_rc.id);
                         inc_rcs_to_remove.insert(instruction_id);
                     }
                 }
+
+                inc_rcs.entry(*value).or_default().insert(instruction_id);
             }
             Instruction::DecrementRc { value } => {
                 let typ = function.dfg.type_of_value(*value);
 
                 // We assume arrays aren't mutated until we find an array_set
-                let inc_rc =
+                let dec_rc =
                     RcInstruction { id: instruction_id, array: *value, possibly_mutated: false };
-                inc_rcs.entry(typ).or_default().push(inc_rc);
+                rcs_with_possible_pairs.entry(typ).or_default().push(dec_rc);
             }
             Instruction::ArraySet { array, .. } => {
                 let typ = function.dfg.type_of_value(*array);
-                if let Some(inc_rcs) = inc_rcs.get_mut(&typ) {
-                    for inc_rc in inc_rcs {
-                        inc_rc.possibly_mutated = true;
+                if let Some(dec_rcs) = rcs_with_possible_pairs.get_mut(&typ) {
+                    for dec_rc in dec_rcs {
+                        dec_rc.possibly_mutated = true;
                     }
                 }
+
+                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(..)) {
+                    borrowed_arrays.insert(*value);
+                }
             }
             _ => {}
         }
@@ -572,6 +605,8 @@ fn apply_side_effects(
 mod test {
     use std::sync::Arc;
 
+    use im::vector;
+
     use crate::ssa::{
         function_builder::FunctionBuilder,
         ir::{
@@ -779,4 +814,143 @@ mod test {
 
         assert_eq!(main.dfg[main.entry_block()].instructions().len(), 3);
     }
+
+    #[test]
+    fn keep_inc_rc_on_borrowed_array_store() {
+        // acir(inline) fn main f0 {
+        //     b0():
+        //       v2 = allocate
+        //       inc_rc [u32 0, u32 0]
+        //       store [u32 0, u32 0] at v2
+        //       inc_rc [u32 0, u32 0]
+        //       jmp b1()
+        //     b1():
+        //       v3 = load v2
+        //       v5 = array_set v3, index u32 0, value u32 1
+        //       return v5
+        //   }
+        let main_id = Id::test_new(0);
+
+        // Compiling main
+        let mut builder = FunctionBuilder::new("main".into(), main_id);
+        let zero = builder.numeric_constant(0u128, Type::unsigned(32));
+        let array_type = Type::Array(Arc::new(vec![Type::unsigned(32)]), 2);
+        let array = builder.array_constant(vector![zero, zero], array_type.clone());
+        let v2 = builder.insert_allocate(array_type.clone());
+        builder.increment_array_reference_count(array);
+        builder.insert_store(v2, array);
+        builder.increment_array_reference_count(array);
+
+        let b1 = builder.insert_block();
+        builder.terminate_with_jmp(b1, vec![]);
+        builder.switch_to_block(b1);
+
+        let v3 = builder.insert_load(v2, array_type);
+        let one = builder.numeric_constant(1u128, Type::unsigned(32));
+        let v5 = builder.insert_array_set(v3, zero, one);
+        builder.terminate_with_return(vec![v5]);
+
+        let ssa = builder.finish();
+        let main = ssa.main();
+
+        // The instruction count never includes the terminator instruction
+        assert_eq!(main.dfg[main.entry_block()].instructions().len(), 4);
+        assert_eq!(main.dfg[b1].instructions().len(), 2);
+
+        // We expect the output to be unchanged
+        let ssa = ssa.dead_instruction_elimination();
+        let main = ssa.main();
+
+        assert_eq!(main.dfg[main.entry_block()].instructions().len(), 4);
+        assert_eq!(main.dfg[b1].instructions().len(), 2);
+    }
+
+    #[test]
+    fn keep_inc_rc_on_borrowed_array_set() {
+        // acir(inline) fn main f0 {
+        //     b0(v0: [u32; 2]):
+        //       inc_rc v0
+        //       v3 = array_set v0, index u32 0, value u32 1
+        //       inc_rc v0
+        //       inc_rc v0
+        //       inc_rc v0
+        //       v4 = array_get v3, index u32 1
+        //       return v4
+        //   }
+        let main_id = Id::test_new(0);
+
+        // Compiling main
+        let mut builder = FunctionBuilder::new("main".into(), main_id);
+        let array_type = Type::Array(Arc::new(vec![Type::unsigned(32)]), 2);
+        let v0 = builder.add_parameter(array_type.clone());
+        builder.increment_array_reference_count(v0);
+        let zero = builder.numeric_constant(0u128, Type::unsigned(32));
+        let one = builder.numeric_constant(1u128, Type::unsigned(32));
+        let v3 = builder.insert_array_set(v0, zero, one);
+        builder.increment_array_reference_count(v0);
+        builder.increment_array_reference_count(v0);
+        builder.increment_array_reference_count(v0);
+
+        let v4 = builder.insert_array_get(v3, one, Type::unsigned(32));
+
+        builder.terminate_with_return(vec![v4]);
+
+        let ssa = builder.finish();
+        let main = ssa.main();
+
+        // The instruction count never includes the terminator instruction
+        assert_eq!(main.dfg[main.entry_block()].instructions().len(), 6);
+
+        // We expect the output to be unchanged
+        let ssa = ssa.dead_instruction_elimination();
+        let main = ssa.main();
+
+        assert_eq!(main.dfg[main.entry_block()].instructions().len(), 6);
+    }
+
+    #[test]
+    fn remove_inc_rcs_that_are_never_mutably_borrowed() {
+        // 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
+        //       inc_rc v0
+        //       return v2
+        //   }
+        let main_id = Id::test_new(0);
+
+        // Compiling main
+        let mut builder = FunctionBuilder::new("main".into(), main_id);
+        let v0 = builder.add_parameter(Type::Array(Arc::new(vec![Type::field()]), 2));
+        builder.increment_array_reference_count(v0);
+        builder.increment_array_reference_count(v0);
+        builder.increment_array_reference_count(v0);
+
+        let zero = builder.numeric_constant(0u128, Type::unsigned(32));
+        let v2 = builder.insert_array_get(v0, zero, Type::field());
+        builder.increment_array_reference_count(v0);
+        builder.terminate_with_return(vec![v2]);
+
+        let ssa = builder.finish();
+        let main = ssa.main();
+
+        // The instruction count never includes the terminator instruction
+        assert_eq!(main.dfg[main.entry_block()].instructions().len(), 5);
+
+        // Expected output:
+        //
+        // acir(inline) fn main f0 {
+        //     b0(v0: [Field; 2]):
+        //       v2 = array_get v0, index u32 0
+        //       return v2
+        //   }
+        let ssa = ssa.dead_instruction_elimination();
+        let main = ssa.main();
+
+        let instructions = main.dfg[main.entry_block()].instructions();
+        assert_eq!(instructions.len(), 1);
+        assert!(matches!(&main.dfg[instructions[0]], Instruction::ArrayGet { .. }));
+    }
 }