LibWeb: Cache CSS::Selector's pseudo element at construction time

Computing the pseudo element of a CSS::Selector was very hot when
mousing around on GitHub in Browser. A profile had it at ~10%.
After these changes, it's totally gone from the profile. :^)

2 files changed, 12 insertions, 14 deletions

diff --git a/Userland/Libraries/LibWeb/CSS/Selector.cpp b/Userland/Libraries/LibWeb/CSS/Selector.cpp
index a81edfe313..14bac1c4e3 100644
--- a/Userland/Libraries/LibWeb/CSS/Selector.cpp
+++ b/Userland/Libraries/LibWeb/CSS/Selector.cpp
@@ -13,25 +13,22 @@ namespace Web::CSS {
 Selector::Selector(Vector<CompoundSelector>&& compound_selectors)
     : m_compound_selectors(move(compound_selectors))
 {
+    // Note: This assumes that only one pseudo-element is allowed in a selector, and that it appears at the end.
+    //       This is true currently, and there are no current proposals to change this, but you never know!
+    if (!m_compound_selectors.is_empty()) {
+        for (auto const& simple_selector : m_compound_selectors.last().simple_selectors) {
+            if (simple_selector.type == SimpleSelector::Type::PseudoElement) {
+                m_pseudo_element = simple_selector.pseudo_element;
+                break;
+            }
+        }
+    }
 }
 
 Selector::~Selector()
 {
 }
 
-Optional<Selector::PseudoElement> Selector::pseudo_element() const
-{
-    // Note: This assumes that only one pseudo-element is allowed in a selector, and that it appears at the end.
-    //       This is true currently, and there are no current proposals to change this, but you never know!
-    if (compound_selectors().is_empty())
-        return {};
-    for (auto const& simple_selector : compound_selectors().last().simple_selectors) {
-        if (simple_selector.type == SimpleSelector::Type::PseudoElement)
-            return simple_selector.pseudo_element;
-    }
-    return {};
-}
-
 // https://www.w3.org/TR/selectors-4/#specificity-rules
 u32 Selector::specificity() const
 {
diff --git a/Userland/Libraries/LibWeb/CSS/Selector.h b/Userland/Libraries/LibWeb/CSS/Selector.h
index 9ee035730f..8d05b6e312 100644
--- a/Userland/Libraries/LibWeb/CSS/Selector.h
+++ b/Userland/Libraries/LibWeb/CSS/Selector.h
@@ -135,7 +135,7 @@ public:
     ~Selector();
 
     Vector<CompoundSelector> const& compound_selectors() const { return m_compound_selectors; }
-    Optional<PseudoElement> pseudo_element() const;
+    Optional<PseudoElement> pseudo_element() const { return m_pseudo_element; }
     u32 specificity() const;
     String serialize() const;
 
@@ -144,6 +144,7 @@ private:
 
     Vector<CompoundSelector> m_compound_selectors;
     mutable Optional<u32> m_specificity;
+    Optional<Selector::PseudoElement> m_pseudo_element;
 };
 
 constexpr StringView pseudo_element_name(Selector::PseudoElement pseudo_element)