fix: Reduce memory usage in mem2reg (#7053)

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

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

@@ -1,4 +1,4 @@
-use vec_collections::VecSet;
+use vec_collections::{AbstractVecSet, VecSet};
 
 use crate::ssa::ir::value::ValueId;
 
@@ -55,6 +55,20 @@ impl AliasSet {
         }
     }
 
+    /// Returns true if calling `unify` would change something in this alias set.
+    ///
+    /// This is an optimization to avoid having to look up an entry ready to be modified in the [Block](crate::ssa::opt::mem2reg::block::Block),
+    /// because doing so would involve calling `Arc::make_mut` which clones the entry, ready for modification.
+    pub(super) fn should_unify(&self, other: &Self) -> bool {
+        if let (Some(self_aliases), Some(other_aliases)) = (&self.aliases, &other.aliases) {
+            // `unify` would extend `self_aliases` with `other_aliases`, so if `other_aliases` is a subset, then nothing would happen.
+            !other_aliases.is_subset(self_aliases)
+        } else {
+            // `unify` would set `aliases` to `None`, so if it's not `Some`, then nothing would happen.
+            self.aliases.is_some()
+        }
+    }
+
     /// Inserts a new alias into this set if it is not unknown
     pub(super) fn insert(&mut self, new_alias: ValueId) {
         if let Some(aliases) = &mut self.aliases {

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

@@ -128,6 +128,12 @@ impl Block {
         }
 
         for (expression, new_aliases) in &other.aliases {
+            // If nothing would change, then don't call `.entry(...).and_modify(...)` as it involves creating more `Arc`s.
+            if let Some(aliases) = self.aliases.get(expression) {
+                if !aliases.should_unify(new_aliases) {
+                    continue;
+                }
+            }
             let expression = expression.clone();
 
             self.aliases