servo: Merge #19673 - style: Make GetFlattenedTreeParent more straight-forward (from emilio:flattened-tree-parent); r=smaug

Now that accessing nsIContent slots is not a blob of virtual function calls, we
should be able to unify logic here, and speed up the not-so-rare case for
chrome, while keeping the usual case fast.

Bug: 1427511
Reviewed-by: smaug
MozReview-Commit-ID: 87iY5Cbhx4T
Source-Repo: https://github.com/servo/servo
Source-Revision: 9187c9a093860d9f3c31b5a5f402458aa4a607cb

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 246fcb20110ef1096cf79d73a3b2985ea15aa466

servo/components/style/gecko/generated/bindings.rs

@@ -503,8 +503,6 @@ extern "C" {
  pub fn Gecko_RecordTraversalStatistics ( total : u32 , parallel : u32 , total_t : u32 , parallel_t : u32 , total_s : u32 , parallel_s : u32 , ) ; 
 } extern "C" {
  pub fn Gecko_IsInDocument ( node : RawGeckoNodeBorrowed , ) -> bool ; 
-} extern "C" {
- pub fn Gecko_FlattenedTreeParentIsParent ( node : RawGeckoNodeBorrowed , ) -> bool ; 
 } extern "C" {
  pub fn Gecko_IsSignificantChild ( node : RawGeckoNodeBorrowed , text_is_significant : bool , whitespace_is_significant : bool , ) -> bool ; 
 } extern "C" {
@@ -1599,4 +1597,4 @@ extern "C" {
  pub fn Gecko_GetElementsWithId ( aDocument : * const nsIDocument , aId : * mut nsAtom , ) -> * const nsTArray < * mut Element > ; 
 } extern "C" {
  pub fn Gecko_GetBoolPrefValue ( pref_name : * const :: std :: os :: raw :: c_char , ) -> bool ; 
-}
+}

servo/components/style/gecko/wrapper.rs

@@ -236,13 +236,23 @@ impl<'ln> GeckoNode<'ln> {
 
     #[inline]
     fn flattened_tree_parent(&self) -> Option<Self> {
-        let fast_path = self.flattened_tree_parent_is_parent();
-        debug_assert!(fast_path == unsafe { bindings::Gecko_FlattenedTreeParentIsParent(self.0) });
-        if fast_path {
-            unsafe { self.0.mParent.as_ref().map(GeckoNode) }
-        } else {
-            unsafe { bindings::Gecko_GetFlattenedTreeParentNode(self.0).map(GeckoNode) }
+        // TODO(emilio): Measure and consider not doing this fast-path and take
+        // always the common path, it's only a function call and from profiles
+        // it seems that keeping this fast path makes the compiler not inline
+        // `flattened_tree_parent`.
+        if self.flattened_tree_parent_is_parent() {
+            debug_assert_eq!(
+                unsafe { bindings::Gecko_GetFlattenedTreeParentNode(self.0).map(GeckoNode) },
+                self.parent_node(),
+                "Fast path stopped holding!"
+            );
+
+            return self.parent_node();
         }
+
+        // NOTE(emilio): If this call is too expensive, we could manually
+        // inline more aggressively.
+        unsafe { bindings::Gecko_GetFlattenedTreeParentNode(self.0).map(GeckoNode) }
     }
 
     #[inline]