Bug 1663618 - Speed up custom property diffing. r=boris

When entering or leaving fullscreen in youtube, we spend most of the
restyle time diffing custom properties, under IndexMap::eq.

Turns out that IndexMap equality is not order-aware, and thus you
actually need to make a hashmap lookup for each entry in the map, which
is unnecessarily inefficient.

Instead, just compare the iterators.

See indexmap-rs/indexmap#153.

Differential Revision: https://phabricator.services.mozilla.com/D89434

servo/components/style/gecko/restyle_damage.rs

@@ -57,7 +57,7 @@ impl GeckoRestyleDamage {
                 &mut reset_only,
             )
         };
-        if reset_only && old_style.custom_properties() != new_style.custom_properties() {
+        if reset_only && !old_style.custom_properties_equal(new_style) {
             // The Gecko_CalcStyleDifference call only checks the non-custom
             // property structs, so we check the custom properties here. Since
             // they generate no damage themselves, we can skip this check if we

servo/components/style/properties/properties.mako.rs

@@ -2975,6 +2975,27 @@ impl ComputedValues {
         self.custom_properties.as_ref()
     }
 
+    /// Returns whether we have the same custom properties as another style.
+    ///
+    /// This should effectively be just:
+    ///
+    ///   self.custom_properties() == other.custom_properties()
+    ///
+    /// But that's not really the case because IndexMap equality doesn't
+    /// consider ordering, which we have to account for. Also, for the same
+    /// reason, IndexMap equality comparisons are slower than needed.
+    ///
+    /// See https://github.com/bluss/indexmap/issues/153
+    pub fn custom_properties_equal(&self, other: &Self) -> bool {
+        match (self.custom_properties(), other.custom_properties()) {
+            (Some(l), Some(r)) => {
+                l.len() == r.len() && l.iter().zip(r.iter()).all(|((k1, v1), (k2, v2))| k1 == k2 && v1 == v2)
+            },
+            (None, None) => true,
+            _ => false,
+        }
+    }
+
 % for prop in data.longhands:
     /// Gets the computed value of a given property.
     #[inline(always)]

servo/components/style/servo/restyle_damage.rs

@@ -254,7 +254,7 @@ fn compute_damage(old: &ComputedValues, new: &ComputedValues) -> ServoRestyleDam
 
     // Paint worklets may depend on custom properties,
     // so if they have changed we should repaint.
-    if old.custom_properties() != new.custom_properties() {
+    if !old.custom_properties_equal(new) {
         damage.insert(ServoRestyleDamage::REPAINT);
     }