style: Share CascadeData instances across ShadowRoots

This should be both a memory and speed win for pages using a lot of
Shadow DOM.

In order to make the cache properly work we need to start keying media query
results on the actual StyleSheetContents, as that's what we share on Gecko, but
that should all be fine.

Differential Revision: https://phabricator.services.mozilla.com/D107266

components/style/author_styles.rs

@@ -5,16 +5,16 @@
 //! A set of author stylesheets and their computed representation, such as the
 //! ones used for ShadowRoot.
 
-use crate::context::QuirksMode;
 use crate::dom::TElement;
 #[cfg(feature = "gecko")]
 use crate::gecko_bindings::sugar::ownership::{HasBoxFFI, HasFFI, HasSimpleFFI};
 use crate::invalidation::media_queries::ToMediaListKey;
-use crate::media_queries::Device;
 use crate::shared_lock::SharedRwLockReadGuard;
+use crate::stylist::Stylist;
 use crate::stylesheet_set::AuthorStylesheetSet;
 use crate::stylesheets::StylesheetInDocument;
 use crate::stylist::CascadeData;
+use servo_arc::Arc;
 
 /// A set of author stylesheets and their computed representation, such as the
 /// ones used for ShadowRoot.
@@ -27,7 +27,14 @@ where
     /// and all that stuff.
     pub stylesheets: AuthorStylesheetSet<S>,
     /// The actual cascade data computed from the stylesheets.
-    pub data: CascadeData,
+    #[ignore_malloc_size_of = "Measured as part of the stylist"]
+    pub data: Arc<CascadeData>,
+}
+
+lazy_static! {
+    static ref EMPTY_CASCADE_DATA: Arc<CascadeData> = {
+        Arc::new_leaked(CascadeData::new())
+    };
 }
 
 impl<S> AuthorStyles<S>
@@ -39,7 +46,7 @@ where
     pub fn new() -> Self {
         Self {
             stylesheets: AuthorStylesheetSet::new(),
-            data: CascadeData::new(),
+            data: EMPTY_CASCADE_DATA.clone(),
         }
     }
 
@@ -50,8 +57,7 @@ where
     #[inline]
     pub fn flush<E>(
         &mut self,
-        device: &Device,
-        quirks_mode: QuirksMode,
+        stylist: &mut Stylist,
         guard: &SharedRwLockReadGuard,
     ) where
         E: TElement,
@@ -61,10 +67,10 @@ where
             .stylesheets
             .flush::<E>(/* host = */ None, /* snapshot_map = */ None);
 
-        // Ignore OOM.
-        let _ = self
-            .data
-            .rebuild(device, quirks_mode, flusher.sheets, guard);
+        let result = stylist.rebuild_author_data(&self.data, flusher.sheets, guard);
+        if let Ok(Some(new_data)) = result {
+            self.data = new_data;
+        }
     }
 }