Auto merge of #12862 - servo:layout-new, r=emilio

added dom obj counting to decide sequential/parallel layout (#10110)

This is a rebased version of #11713

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #10110 (github issue number if applicable).
- [X] There are no tests for these changes because it's an optimization with no visible behavioral changes

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components/script/dom/document.rs

@@ -284,6 +284,12 @@ pub struct Document {
     last_click_info: DOMRefCell<Option<(Instant, Point2D<f32>)>>,
     /// https://html.spec.whatwg.org/multipage/#ignore-destructive-writes-counter
     ignore_destructive_writes_counter: Cell<u32>,
+    /// Track the total number of elements in this DOM's tree.
+    /// This is sent to the layout thread every time a reflow is done;
+    /// layout uses this to determine if the gains from parallel layout will be worth the overhead.
+    ///
+    /// See also: https://github.com/servo/servo/issues/10110
+    dom_count: Cell<u32>,
 }
 
 #[derive(JSTraceable, HeapSizeOf)]
@@ -455,6 +461,22 @@ impl Document {
         self.base_element.set(base.r());
     }
 
+    pub fn dom_count(&self) -> u32 {
+        self.dom_count.get()
+    }
+
+    /// This is called by `bind_to_tree` when a node is added to the DOM.
+    /// The internal count is used by layout to determine whether to be sequential or parallel.
+    /// (it's sequential for small DOMs)
+    pub fn increment_dom_count(&self) {
+        self.dom_count.set(self.dom_count.get() + 1);
+    }
+
+    /// This is called by `unbind_from_tree` when a node is removed from the DOM.
+    pub fn decrement_dom_count(&self) {
+        self.dom_count.set(self.dom_count.get() - 1);
+    }
+
     pub fn quirks_mode(&self) -> QuirksMode {
         self.quirks_mode.get()
     }
@@ -1884,6 +1906,7 @@ impl Document {
             target_element: MutNullableHeap::new(None),
             last_click_info: DOMRefCell::new(None),
             ignore_destructive_writes_counter: Default::default(),
+            dom_count: Cell::new(1),
         }
     }