perf(dom): compact named index strings

Cargo.lock

@@ -2482,6 +2482,7 @@ dependencies = [
  "percent-encoding",
  "selectors",
  "serde_json",
+ "servo_arc",
  "thin-vec",
  "url",
 ]

Cargo.toml

@@ -94,6 +94,7 @@ cookie = { git = "https://github.com/ldm0/cookie-rs", branch = "priority" }
 v8 = { path = "vendor/v8-152.2.0" }
 deno_v8 = { path = "vendor/deno_v8-0.3.0" }
 selectors = { git = "https://github.com/ldm0/stylo", rev = "671d13d31b3b00e7b1e0088ba2b36a4e35c0858c" }
+servo_arc = { git = "https://github.com/ldm0/stylo", rev = "671d13d31b3b00e7b1e0088ba2b36a4e35c0858c" }
 stylo = { git = "https://github.com/ldm0/stylo", rev = "671d13d31b3b00e7b1e0088ba2b36a4e35c0858c" }
 stylo_atoms = { git = "https://github.com/ldm0/stylo", rev = "671d13d31b3b00e7b1e0088ba2b36a4e35c0858c" }
 stylo_dom = { git = "https://github.com/ldm0/stylo", rev = "671d13d31b3b00e7b1e0088ba2b36a4e35c0858c" }

moli-dom/Cargo.toml

@@ -12,6 +12,7 @@ moli-html-input-temporal = { path = "../moli-html-input-temporal" }
 percent-encoding = "2.3"
 selectors = "0.40"
 serde_json = "1.0.145"
+servo_arc = "0.4.3"
 thin-vec = "0.2.14"
 url = "2.5.7"
 

moli-dom/src/native/host.rs

@@ -38,7 +38,7 @@ use self::query_index::ElementQueryIndex;
 use self::types::{
     CachedConnectedShadowRoots, CachedLiveCollection, LiveCollectionCacheKey,
     LiveCollectionCacheKind, MutationScope, NamedElementIndex, ShadowRootState,
-    ShadowSlotNameIndex,
+    ShadowSlotNameIndex, ThinArcStr,
 };
 pub use self::types::{ConnectedShadowRootSnapshot, DomHandle, DomHost};
 pub use self::types::{HostElementSnapshot, ShadowRootBindingSnapshot, ShadowRootInit};

moli-dom/src/native/host/document.rs

@@ -320,13 +320,12 @@ impl DomHost {
             }
         }
         if let Some(current_value) = current_value {
-            let current_value = std::sync::Arc::<str>::from(current_value);
-            index
+            let entry = index
                 .handles_by_value
-                .entry(current_value.clone())
-                .or_default()
-                .insert(handle);
-            index.value_by_handle.insert(handle, current_value);
+                .entry(ThinArcStr::from(current_value));
+            let canonical_value = entry.key().clone();
+            entry.or_default().insert(handle);
+            index.value_by_handle.insert(handle, canonical_value);
         }
     }
 

moli-dom/src/native/host/query_index.rs

@@ -993,14 +993,14 @@ mod tests {
     #[test]
     fn named_candidate_index_shares_values_between_directions() {
         let mut host = test_host();
-        let element = host.create_element("div");
-        assert!(host.set_attribute(element, "id", "shared-identifier"));
-        assert!(host.append_child(host.document_handle(), element));
+        let first = host.create_element("div");
+        let second = host.create_element("span");
+        for element in [first, second] {
+            assert!(host.set_attribute(element, "id", "shared-identifier"));
+            assert!(host.append_child(host.document_handle(), element));
+        }
 
-        assert_eq!(
-            host.element_handle_by_id("shared-identifier"),
-            Some(element)
-        );
+        assert_eq!(host.element_handle_by_id("shared-identifier"), Some(first));
 
         let index = host.id_index.borrow();
         let index = index.as_ref().expect("materialized ID index");
@@ -1009,11 +1009,17 @@ mod tests {
             .keys()
             .find(|value| value.as_ref() == "shared-identifier")
             .expect("forward index key");
-        let reverse = index
-            .value_by_handle
-            .get(&element)
-            .expect("reverse index value");
-        assert!(std::sync::Arc::ptr_eq(forward, reverse));
+        for element in [first, second] {
+            let reverse = index
+                .value_by_handle
+                .get(&element)
+                .expect("reverse index value");
+            assert!(forward.ptr_eq(reverse));
+        }
+        assert_eq!(
+            std::mem::size_of::<ThinArcStr>(),
+            std::mem::size_of::<usize>()
+        );
     }
 
     #[test]

moli-dom/src/native/host/types.rs

@@ -1,5 +1,10 @@
 use super::*;
-use std::sync::Arc;
+use servo_arc::ThinArc;
+use std::{
+    borrow::Borrow,
+    fmt,
+    hash::{Hash, Hasher},
+};
 
 pub type DomHandle = NativeNodeId;
 
@@ -10,8 +15,63 @@ pub struct HostElementSnapshot {
 
 #[derive(Debug, Clone, Default)]
 pub(super) struct NamedElementIndex {
-    pub(super) handles_by_value: HashMap<Arc<str>, IndexSet<DomHandle>>,
-    pub(super) value_by_handle: HashMap<DomHandle, Arc<str>>,
+    pub(super) handles_by_value: HashMap<ThinArcStr, IndexSet<DomHandle>>,
+    pub(super) value_by_handle: HashMap<DomHandle, ThinArcStr>,
+}
+
+#[repr(transparent)]
+#[derive(Clone)]
+pub(super) struct ThinArcStr(ThinArc<(), u8>);
+
+impl ThinArcStr {
+    fn as_str(&self) -> &str {
+        // SAFETY: Construction only accepts UTF-8 `str` bytes, and the shared
+        // backing slice cannot be mutated.
+        unsafe { std::str::from_utf8_unchecked(self.0.slice()) }
+    }
+
+    #[cfg(test)]
+    pub(super) fn ptr_eq(&self, other: &Self) -> bool {
+        servo_arc::Arc::ptr_eq(&self.0, &other.0)
+    }
+}
+
+impl From<&str> for ThinArcStr {
+    fn from(value: &str) -> Self {
+        Self(ThinArc::from_header_and_iter((), value.bytes()))
+    }
+}
+
+impl Borrow<str> for ThinArcStr {
+    fn borrow(&self) -> &str {
+        self.as_str()
+    }
+}
+
+impl AsRef<str> for ThinArcStr {
+    fn as_ref(&self) -> &str {
+        self.as_str()
+    }
+}
+
+impl PartialEq for ThinArcStr {
+    fn eq(&self, other: &Self) -> bool {
+        self.as_str() == other.as_str()
+    }
+}
+
+impl Eq for ThinArcStr {}
+
+impl Hash for ThinArcStr {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.as_str().hash(state);
+    }
+}
+
+impl fmt::Debug for ThinArcStr {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.as_str().fmt(formatter)
+    }
 }
 
 #[derive(Debug, Clone, Default)]