chore: strip initialization of unused memory blocks from ACIR (#2985)

Co-authored-by: Maxim Vezenov <[email protected]>

acvm-repo/acvm/src/compiler/optimizers/mod.rs

@@ -2,10 +2,13 @@ use acir::circuit::{Circuit, Opcode};
 
 mod general;
 mod redundant_range;
+mod unused_memory;
 
 pub(crate) use general::GeneralOptimizer;
 pub(crate) use redundant_range::RangeOptimizer;
 
+use self::unused_memory::UnusedMemoryOptimizer;
+
 use super::AcirTransformationMap;
 
 /// Applies [`ProofSystemCompiler`][crate::ProofSystemCompiler] independent optimizations to a [`Circuit`].
@@ -26,6 +29,11 @@ pub fn optimize(acir: Circuit) -> (Circuit, AcirTransformationMap) {
     // by applying the modifications done to the circuit opcodes and also to the opcode_positions (delete and insert)
     let acir_opcode_positions = acir.opcodes.iter().enumerate().map(|(i, _)| i).collect();
 
+    // Unused memory optimization pass
+    let memory_optimizer = UnusedMemoryOptimizer::new(acir);
+    let (acir, acir_opcode_positions) =
+        memory_optimizer.remove_unused_memory_initializations(acir_opcode_positions);
+
     // Range optimization pass
     let range_optimizer = RangeOptimizer::new(acir);
     let (acir, acir_opcode_positions) =

acvm-repo/acvm/src/compiler/optimizers/unused_memory.rs

@@ -0,0 +1,61 @@
+use acir::circuit::{opcodes::BlockId, Circuit, Opcode};
+use std::collections::HashSet;
+
+/// `UnusedMemoryOptimizer` will remove initializations of memory blocks which are unused.
+pub(crate) struct UnusedMemoryOptimizer {
+    unused_memory_initializations: HashSet<BlockId>,
+    circuit: Circuit,
+}
+
+impl UnusedMemoryOptimizer {
+    /// Creates a new `UnusedMemoryOptimizer ` by collecting unused memory init
+    /// opcodes from `Circuit`.
+    pub(crate) fn new(circuit: Circuit) -> Self {
+        let unused_memory_initializations = Self::collect_unused_memory_initializations(&circuit);
+        Self { circuit, unused_memory_initializations }
+    }
+
+    /// Creates a set of ids for memory blocks for which no [`Opcode::MemoryOp`]s exist.
+    ///
+    /// These memory blocks can be safely removed.
+    fn collect_unused_memory_initializations(circuit: &Circuit) -> HashSet<BlockId> {
+        let mut unused_memory_initialization = HashSet::new();
+
+        for opcode in &circuit.opcodes {
+            match opcode {
+                Opcode::MemoryInit { block_id, .. } => {
+                    unused_memory_initialization.insert(*block_id);
+                }
+                Opcode::MemoryOp { block_id, .. } => {
+                    unused_memory_initialization.remove(block_id);
+                }
+                _ => (),
+            }
+        }
+        unused_memory_initialization
+    }
+
+    /// Returns a `Circuit` where [`Opcode::MemoryInit`]s for unused memory blocks are dropped.
+    pub(crate) fn remove_unused_memory_initializations(
+        self,
+        order_list: Vec<usize>,
+    ) -> (Circuit, Vec<usize>) {
+        let mut new_order_list = Vec::with_capacity(order_list.len());
+        let mut optimized_opcodes = Vec::with_capacity(self.circuit.opcodes.len());
+        for (idx, opcode) in self.circuit.opcodes.iter().enumerate() {
+            match opcode {
+                Opcode::MemoryInit { block_id, .. }
+                    if self.unused_memory_initializations.contains(block_id) =>
+                {
+                    // Drop opcode
+                }
+                _ => {
+                    new_order_list.push(order_list[idx]);
+                    optimized_opcodes.push(opcode.clone());
+                }
+            }
+        }
+
+        (Circuit { opcodes: optimized_opcodes, ..self.circuit }, new_order_list)
+    }
+}