perf(runtime): make declared-prototype reverse lookup O(1)

changelog.d/9214-decl-prototype-reverse-index.md

@@ -0,0 +1,33 @@
+**"Which class's `.prototype` is this object?" is now O(1)** (#9180). Every
+`Object.defineProperty`, `Object.getOwnPropertyDescriptor` and `delete` asked
+`class_id_for_decl_prototype_object` about its receiver, and the answer was a
+linear scan of every materialized declared-class prototype. The comment above
+it explained that the table was small and the path was a cold reflection path;
+a bundled application falsifies both — esbuild's `__export(exports, { … })`
+runs `defineProperty` thousands of times during module init, the receivers are
+never prototypes, and a miss walked the whole table. It was 3.10% of
+`cc --help`.
+
+The registry now carries its own inverse. `CLASS_DECL_PROTOTYPE_OBJECTS` holds
+a `DeclPrototypeTable` whose two maps are private to one file, so the six
+existing mutation sites — the store, both GC root scanners, the per-slot GC
+step, the test reset and the test seeds — go through methods that update both
+directions together, and a seventh cannot be written without editing that file.
+That matters more than it sounds: an earlier pointer-keyed cache with
+hand-placed invalidation went stale at the sites it missed, and the symptom was
+not a crash but `getOwnPropertyDescriptor(C.prototype, "g")` quietly returning
+`undefined` where node returns an accessor descriptor.
+
+Two things keep it honest beyond privacy. The reverse map is an exact inverse
+only while one address belongs to one class id and is never re-pointed;
+`insert` is where either could break, checks both while inserting with one
+extra reverse lookup, and on anything unusual abandons the index for good and
+answers from the same authoritative forward-table scan as before. In a
+`debug_assertions` build every reverse lookup is compared against that scan, so
+the whole runtime test suite is checking the index rather than trusting it.
+
+Measured on Linux with 400 declared-class prototypes materialized, per
+operation on non-prototype receivers: `getOwnPropertyDescriptor` 420 → 200 ns
+(2.10×), `defineProperty` 1585 → 1055 ns (1.50×), `delete` 2685 → 2445 ns. The
+scan's signature was the slope — `getOwnPropertyDescriptor` cost 180/210/305/420
+ns at 0/50/200/400 prototypes before, and is flat at ~200 ns after.

crates/perry-runtime/src/object/class_gc_roots.rs

@@ -54,9 +54,9 @@ pub fn scan_class_inheritance_roots_mut(visitor: &mut crate::gc::RuntimeRootVisi
     CLASS_DECL_PROTOTYPE_OBJECTS.with(|table| {
         if let Ok(mut guard) = table.write() {
             if let Some(map) = guard.as_mut() {
-                for ptr in map.values_mut() {
+                map.visit_root_slots(|ptr| {
                     visitor.visit_usize_slot(ptr);
-                }
+                });
             }
         }
     });
@@ -85,9 +85,10 @@ pub(crate) fn test_seed_class_inheritance_roots(proto_cid: u32, proto_ptr: usize
 #[cfg(test)]
 pub(crate) fn test_seed_decl_class_prototype_root(class_id: u32, proto_ptr: usize) {
     CLASS_DECL_PROTOTYPE_OBJECTS.with(|table| {
-        let mut guard = table.write().unwrap();
-        guard
-            .get_or_insert_with(std::collections::HashMap::new)
+        table
+            .write()
+            .unwrap()
+            .get_or_insert_with(Default::default)
             .insert(class_id, proto_ptr);
     });
 }
@@ -122,7 +123,7 @@ pub(crate) fn test_decl_class_prototype_root(class_id: u32) -> usize {
             .read()
             .unwrap()
             .as_ref()
-            .and_then(|m| m.get(&class_id).copied())
+            .and_then(|m| m.get(class_id))
             .unwrap_or(0)
     })
 }
@@ -148,7 +149,7 @@ pub(crate) fn test_clear_class_inheritance_roots(proto_cid: u32, closure_cid: u3
     });
     CLASS_DECL_PROTOTYPE_OBJECTS.with(|table| {
         if let Some(m) = table.write().unwrap().as_mut() {
-            m.remove(&proto_cid);
+            m.remove(proto_cid);
         }
     });
     CLASS_PARENT_CLOSURES.with(|table| {

crates/perry-runtime/src/object/class_registry.rs

@@ -46,6 +46,7 @@ mod builtin_alias_construct;
 mod class_meta;
 mod construct;
 pub(crate) use construct::scan_current_new_target_root_mut;
+pub mod decl_prototype_table;
 mod dispatch;
 mod function_prototype;
 mod gc_roots;