script: Use a u16 for prototype id to speed up Castable::is<T>() (#44364)

`Castable::is<T>()` calls `get_dom_class()` in a way that incurs a
roundtrip to the JS engine and
multiple pointer indirections:

self.reflector().get_jsobject().get()  -> JSObject
Object.shape -> BaseShape.clasp    -> JSClass
DOMJSClass.dom_class              -> DOMClass
interface_chain[depth] == ID       -> bool

Since the ID doesn't change after creation of reflectors, we can instead
store the ID on the reflector
and avoid most of that cost. The trick is to use a Depth First Search to
generate IDs that allow a
range check for `Castable::is<T>`. An example of IDs in the DFS looks
like:

EventTarget = 0
  Node = 1
    CharacterData = 2
      Text = 3
      CDATASection = 4
      Comment = 5
      ProcessingInstruction = 6
    Document = 7
      HTMLDocument = 8
      XMLDocument = 9

Testing: Green try run at
https://github.com/webbeef/servo/actions/runs/24640508040 and manual
testing with general browsing.

Signed-off-by: webbeef <me@webbeef.org>

components/script_bindings/codegen/codegen.py

@@ -72,6 +72,11 @@ from configuration import (
 )
 
 AUTOGENERATED_WARNING_COMMENT = "/* THIS FILE IS AUTOGENERATED - DO NOT EDIT */\n\n"
+
+# DFS-ordered prototype ID ranges, populated by GlobalGenRoots.PrototypeList()
+# and consumed by CGIDLInterface to generate PROTO_FIRST/PROTO_LAST constants.
+_proto_ranges: dict[str, tuple[int, int]] = {}
+
 ALLOWED_WARNING_LIST = [
     'non_camel_case_types',
     'non_upper_case_globals',
@@ -3395,6 +3400,7 @@ let val = PrivateValue(ptr::null());
 JS_SetReservedSlot(obj.get(), DOM_WEAK_SLOT, &val);
 """
 
+        name = self.descriptor.name
         return CGGeneric(f"""
 let raw = Root::new(MaybeUnreflectedDom::from_box(object));
 
@@ -3409,6 +3415,7 @@ assert!(!canonical_proto.is_null());
 
 {create}
 let root = raw.reflect_with(obj.get());
+root.reflector().set_proto_id(PrototypeList::ID::{name} as u16);
 
 {unforgeable}
 
@@ -3441,6 +3448,7 @@ class CGWrapGlobalMethod(CGAbstractMethod):
         members = [f"{function}::<D>(cx.into(), obj.handle(), {array.variableName()}.get(), obj.handle());"
                    for (function, array) in pairs if array.length() > 0]
         membersStr = "\n".join(members)
+        name = self.descriptor.name
 
         return CGGeneric(f"""
 unsafe {{
@@ -3459,6 +3467,7 @@ unsafe {{
     assert!(!obj.is_null());
 
     let root = raw.reflect_with(obj.get());
+    root.reflector().set_proto_id(PrototypeList::ID::{name} as u16);
 
     let _ac = JSAutoRealm::new(cx.raw_cx(), obj.get());
     rooted!(&in(cx) let mut canonical_proto = ptr::null_mut::<JSObject>());
@@ -3503,12 +3512,16 @@ class CGIDLInterface(CGThing):
             check = f"ptr::eq(class, unsafe {{ {bindingModule}::Class.get() }})"
         else:
             check = f"ptr::eq(class, unsafe {{ &{bindingModule}::Class.get().dom_class }})"
+        # Get DFS-ordered ID range for this interface (set by PrototypeList generation).
+        proto_first, proto_last = _proto_ranges.get(name, (0, 65535))
         return f"""
 impl IDLInterface for {name} {{
     #[inline]
     fn derives(class: &'static DOMClass) -> bool {{
         {check}
     }}
+    const PROTO_FIRST: u16 = {proto_first};
+    const PROTO_LAST: u16 = {proto_last};
 }}
 """
 
@@ -9142,8 +9155,51 @@ class GlobalGenRoots():
     @staticmethod
     def PrototypeList(config: Configuration) -> CGThing:
         # Prototype ID enum.
+        # Assign IDs in DFS preorder of the inheritance tree so that
+        # all descendants of a type occupy a contiguous ID range.
+        # This enables O(1) is::<T>() checks via range comparison.
         interfaces = config.getDescriptors(isCallback=False, isNamespace=False)
-        protos = [d.name for d in interfaces]
+
+        # Build a tree: parent_name -> sorted list of child names
+        children: dict[str | None, list[str]] = {}
+        name_set = set()
+        for d in interfaces:
+            name = d.name
+            name_set.add(name)
+            parent_name = d.prototypeChain[-2] if len(d.prototypeChain) >= 2 else None
+            children.setdefault(parent_name, []).append(name)
+
+        # Sort children alphabetically for determinism within each level
+        for key in children:
+            children[key].sort()
+
+        # DFS preorder traversal
+        protos: list[str] = []
+        # Track range for each prototype: (first_id, last_id)
+        proto_ranges: dict[str, tuple[int, int]] = {}
+
+        def dfs(name: str) -> None:
+            first_id = len(protos)
+            protos.append(name)
+            for child in children.get(name, []):
+                dfs(child)
+            proto_ranges[name] = (first_id, len(protos) - 1)
+
+        # Start from roots (interfaces with no parent in our set)
+        roots = sorted(children.get(None, []))
+        for root in roots:
+            dfs(root)
+
+        # Any interfaces not in the tree (shouldn't happen, but be safe)
+        for d in interfaces:
+            if d.name not in proto_ranges:
+                first_id = len(protos)
+                protos.append(d.name)
+                proto_ranges[d.name] = (first_id, first_id)
+
+        # Store ranges at module level for use by CGIDLInterface.
+        global _proto_ranges
+        _proto_ranges = proto_ranges
         constructors = sorted([MakeNativeName(d.name)
                                for d in config.getDescriptors(hasInterfaceObject=True)
                                if d.shouldHaveGetConstructorObjectMethod()])

components/script_bindings/conversions.rs

@@ -51,6 +51,11 @@ impl<T: ToJSValConvertible + ?Sized> SafeToJSValConvertible for T {
 pub trait IDLInterface {
     /// Returns whether the given DOM class derives that interface.
     fn derives(_: &'static DOMClass) -> bool;
+
+    /// First prototype ID in the DFS-ordered range for this interface and its descendants.
+    const PROTO_FIRST: u16 = 0;
+    /// Last prototype ID in the DFS-ordered range for this interface and its descendants.
+    const PROTO_LAST: u16 = u16::MAX;
 }
 
 /// A trait to mark an IDL interface as deriving from another one.

components/script_bindings/inheritance.rs

@@ -6,7 +6,7 @@
 
 use std::mem;
 
-use crate::conversions::{DerivedFrom, IDLInterface, get_dom_class};
+use crate::conversions::{DerivedFrom, IDLInterface};
 use crate::reflector::DomObject;
 use crate::script_runtime::runtime_is_alive;
 
@@ -26,8 +26,8 @@ pub trait Castable: IDLInterface + DomObject + Sized {
             "Attempting to interact with DOM objects after JS runtime has shut down."
         );
 
-        let class = unsafe { get_dom_class(self.reflector().get_jsobject().get()).unwrap() };
-        T::derives(class)
+        let id = self.reflector().proto_id();
+        id >= T::PROTO_FIRST && id <= T::PROTO_LAST
     }
 
     /// Cast a DOM object upwards to one of the interfaces it derives from.

components/script_bindings/reflector.rs

@@ -42,6 +42,8 @@ pub struct Reflector<T = ()> {
     object: Heap<*mut JSObject>,
     /// Associated memory size (of rust side). Used for memory reporting to SM.
     size: T,
+    /// Cached prototype ID for fast type checks.
+    proto_id: Cell<u16>,
 }
 
 unsafe impl<T> js::gc::Traceable for Reflector<T> {
@@ -62,6 +64,18 @@ impl<T> Reflector<T> {
         unsafe { HandleObject::from_raw(self.object.handle()) }
     }
 
+    /// Get the cached prototype ID.
+    #[inline]
+    pub fn proto_id(&self) -> u16 {
+        self.proto_id.get()
+    }
+
+    /// Set the cached prototype ID.
+    #[inline]
+    pub fn set_proto_id(&self, id: u16) {
+        self.proto_id.set(id);
+    }
+
     /// Initialize the reflector. (May be called only once.)
     ///
     /// # Safety
@@ -88,6 +102,7 @@ impl<T: AssociatedMemorySize> Reflector<T> {
     pub fn new() -> Reflector<T> {
         Reflector {
             object: Heap::default(),
+            proto_id: Cell::new(u16::MAX),
             size: T::default(),
         }
     }

tests/unit/script/size_of.rs

@@ -29,11 +29,11 @@ macro_rules! sizeof_checker (
 );
 
 // Update the sizes here
-sizeof_checker!(size_event_target, EventTarget, 48);
-sizeof_checker!(size_node, Node, 152);
-sizeof_checker!(size_element, Element, 344);
-sizeof_checker!(size_htmlelement, HTMLElement, 360);
-sizeof_checker!(size_div, HTMLDivElement, 360);
-sizeof_checker!(size_span, HTMLSpanElement, 360);
-sizeof_checker!(size_text, Text, 184);
-sizeof_checker!(size_characterdata, CharacterData, 184);
+sizeof_checker!(size_event_target, EventTarget, 56);
+sizeof_checker!(size_node, Node, 160);
+sizeof_checker!(size_element, Element, 352);
+sizeof_checker!(size_htmlelement, HTMLElement, 368);
+sizeof_checker!(size_div, HTMLDivElement, 368);
+sizeof_checker!(size_span, HTMLSpanElement, 368);
+sizeof_checker!(size_text, Text, 192);
+sizeof_checker!(size_characterdata, CharacterData, 192);