fix(ssa): Track RC instructions when removing last loads

compiler/noirc_evaluator/src/ssa/opt/mem2reg.rs

@@ -66,7 +66,7 @@
 //!
 //! Repeating this algorithm for each block in the function in program order should result in
 //! optimizing out most known loads. However, identifying all aliases correctly has been proven
 //! undecidable in general (Landi, 1992). So this pass will not always optimize out all loads
 //! that could theoretically be optimized out. This pass can be performed at any time in the
 //! SSA optimization pipeline, although it will be more successful the simpler the program's CFG is.
 //! This pass is currently performed several times to enable other passes - most notably being
@@ -127,7 +127,7 @@
     /// Load and Store instructions that should be removed at the end of the pass.
     ///
     /// We avoid removing individual instructions as we go since removing elements
     /// from the middle of Vecs many times will be slower than a single call to `retain`.
     instructions_to_remove: HashSet<InstructionId>,
 
     /// Track a value's last load across all blocks.
@@ -142,6 +142,10 @@
     /// instruction that aliased that reference.
     /// If that store has been set for removal, we can also remove this instruction.
     aliased_references: HashMap<ValueId, HashSet<InstructionId>>,
+
+    /// Track whether the last instruction is an inc_rc/dec_rc instruction.
+    /// If it is we should not remove any repeat last loads.
+    inside_rc_reload: bool,
 }
 
 impl<'f> PerFunctionContext<'f> {
@@ -158,6 +162,7 @@
             last_loads: HashMap::default(),
             calls_reference_input: HashSet::default(),
             aliased_references: HashMap::default(),
+            inside_rc_reload: false,
         }
     }
 
@@ -435,7 +440,7 @@
                     let result = self.inserter.function.dfg.instruction_results(instruction)[0];
                     let previous_result =
                         self.inserter.function.dfg.instruction_results(*last_load)[0];
-                    if *previous_address == address {
+                    if *previous_address == address && !self.inside_rc_reload {
                         self.inserter.map_value(result, previous_result);
                         self.instructions_to_remove.insert(instruction);
                     }
@@ -553,6 +558,18 @@
             }
             _ => (),
         }
+
+        self.track_rc_reload_state(instruction);
+    }
+
+    fn track_rc_reload_state(&mut self, instruction: InstructionId) {
+        match &self.inserter.function.dfg[instruction] {
+            // We just had an increment or decrement to an array's reference counter
+            Instruction::IncrementRc { .. } | Instruction::DecrementRc { .. } => {
+                self.inside_rc_reload = true;
+            }
+            _ => self.inside_rc_reload = false,
+        }
     }
 
     fn contains_references(typ: &Type) -> bool {