perf(mangler): optimize slot assignment with pre-computed ancestor sets

crates/oxc_mangler/src/lib.rs

@@ -318,10 +318,14 @@ impl<'t> Mangler<'t> {
         let mut slots = Vec::from_iter_in(iter::repeat_n(0, scoping.symbols_len()), temp_allocator);
 
         // Stores the lived scope ids for each slot. Keyed by slot number.
-        let mut slot_liveness: Vec<BitSet> = Vec::new_in(temp_allocator);
+        // Pre-allocate capacity based on number of symbols (upper bound for slots).
+        let mut slot_liveness: Vec<BitSet> =
+            Vec::with_capacity_in(scoping.symbols_len(), temp_allocator);
         let mut tmp_bindings = Vec::with_capacity_in(100, temp_allocator);
 
         let mut reusable_slots = Vec::new_in(temp_allocator);
+        // Pre-computed BitSet for ancestor membership tests - reused across iterations
+        let mut ancestor_set = BitSet::new_in(scoping.scopes_len(), temp_allocator);
         // Walk down the scope tree and assign a slot number for each symbol.
         // It is possible to do this in a loop over the symbol list,
         // but walking down the scope tree seems to generate a better code.
@@ -372,6 +376,14 @@ impl<'t> Mangler<'t> {
                 );
             }
 
+            // Pre-compute the set of ancestors from root to scope_id (exclusive) for O(1) membership tests.
+            // This avoids repeated `take_while` comparisons in the inner loop.
+            ancestor_set.clear();
+            for ancestor_id in scoping.scope_ancestors(scope_id).skip(1) {
+                ancestor_set.set_bit(ancestor_id.index());
+            }
+
+            let scope_id_index = scope_id.index();
             for (&symbol_id, &assigned_slot) in tmp_bindings.iter().zip(&reusable_slots) {
                 slots[symbol_id.index()] = assigned_slot;
 
@@ -391,16 +403,22 @@ impl<'t> Mangler<'t> {
                     .map(|reference| ast_nodes.get_node(reference.node_id()).scope_id());
 
                 // Calculate the scope ids that this symbol is alive in.
-                let lived_scope_ids = referenced_scope_ids
+                // For each used_scope_id, we walk up the ancestor chain and collect scopes
+                // that are descendants of scope_id (i.e., not in ancestor_set and not scope_id itself).
+                let slot_liveness_bitset = &mut slot_liveness[assigned_slot as usize];
+                for used_scope_id in referenced_scope_ids
                     .chain(redeclared_scope_ids)
                     .chain([scope_id, declared_scope_id])
-                    .flat_map(|used_scope_id| {
-                        scoping.scope_ancestors(used_scope_id).take_while(|s_id| *s_id != scope_id)
-                    });
-
-                // Since the slot is now assigned to this symbol, it is alive in all the scopes that this symbol is alive in.
-                for scope_id in lived_scope_ids {
-                    slot_liveness[assigned_slot as usize].set_bit(scope_id.index());
+                {
+                    for ancestor_id in scoping.scope_ancestors(used_scope_id) {
+                        let ancestor_index = ancestor_id.index();
+                        // Stop when we reach scope_id or any of its ancestors
+                        if ancestor_index == scope_id_index || ancestor_set.has_bit(ancestor_index)
+                        {
+                            break;
+                        }
+                        slot_liveness_bitset.set_bit(ancestor_index);
+                    }
                 }
             }
         }

tasks/track_memory_allocations/allocs_minifier.snap

@@ -1,14 +1,14 @@
 File                       | File size      || Sys allocs     | Sys reallocs   || Arena allocs   | Arena reallocs | Arena bytes    
 -------------------------------------------------------------------------------------------------------------------------------------------
-checker.ts                 | 2.92 MB        ||          83503 |          14179 ||         152592 |          28237
+checker.ts                 | 2.92 MB        ||          83502 |          14179 ||         152592 |          28237
 
-cal.com.tsx                | 1.06 MB        ||          40448 |           3032 ||          37138 |           4586
+cal.com.tsx                | 1.06 MB        ||          40447 |           3032 ||          37138 |           4586
 
 RadixUIAdoptionSection.jsx | 2.52 kB        ||             85 |              9 ||             30 |              6
 
 pdf.mjs                    | 567.30 kB      ||          20193 |           2899 ||          47453 |           7730
 
 antd.js                    | 6.69 MB        ||          99534 |          13524 ||         331649 |          69358
 
-binder.ts                  | 193.08 kB      ||           4760 |            974 ||           7075 |            824
+binder.ts                  | 193.08 kB      ||           4759 |            974 ||           7075 |            824