LibWeb: Add a 1-entry lookup cache to FormattingState

This makes repeated lookups of the state for the same box much faster
by bypassing the HashMap.

Author: awesomekling

Userland/Libraries/LibWeb/Layout/FormattingState.cpp

@@ -12,32 +12,48 @@ namespace Web::Layout {
 
 FormattingState::NodeState& FormattingState::get_mutable(NodeWithStyleAndBoxModelMetrics const& box)
 {
-    if (auto it = nodes.find(&box); it != nodes.end())
-        return *it->value;
-
-    for (auto* ancestor = m_parent; ancestor; ancestor = ancestor->m_parent) {
-        if (auto it = ancestor->nodes.find(&box); it != ancestor->nodes.end()) {
-            auto cow_node_state = adopt_own(*new NodeState(*it->value));
-            auto* cow_node_state_ptr = cow_node_state.ptr();
-            nodes.set(&box, move(cow_node_state));
-            return *cow_node_state_ptr;
+    if (m_lookup_cache.box == &box && m_lookup_cache.is_mutable)
+        return *m_lookup_cache.state;
+
+    auto& node_state = [&]() -> NodeState& {
+        if (auto it = nodes.find(&box); it != nodes.end())
+            return *it->value;
+
+        for (auto const* ancestor = m_parent; ancestor; ancestor = ancestor->m_parent) {
+            if (auto it = ancestor->nodes.find(&box); it != ancestor->nodes.end()) {
+                auto cow_node_state = adopt_own(*new NodeState(*it->value));
+                auto* cow_node_state_ptr = cow_node_state.ptr();
+                nodes.set(&box, move(cow_node_state));
+                return *cow_node_state_ptr;
+            }
         }
-    }
 
-    return *nodes.ensure(&box, [] { return adopt_own(*new NodeState); });
+        return *nodes.ensure(&box, [] { return adopt_own(*new NodeState); });
+    }();
+
+    m_lookup_cache = LookupCache { .box = &box, .state = &node_state, .is_mutable = true };
+
+    return node_state;
 }
 
 FormattingState::NodeState const& FormattingState::get(NodeWithStyleAndBoxModelMetrics const& box) const
 {
-    if (auto it = nodes.find(&box); it != nodes.end())
-        return *it->value;
+    if (m_lookup_cache.box == &box)
+        return *m_lookup_cache.state;
 
-    for (auto* ancestor = m_parent; ancestor; ancestor = ancestor->m_parent) {
-        if (auto it = ancestor->nodes.find(&box); it != ancestor->nodes.end())
+    auto& node_state = [&]() -> NodeState const& {
+        if (auto it = nodes.find(&box); it != nodes.end())
             return *it->value;
-    }
 
-    return *const_cast<FormattingState&>(*this).nodes.ensure(&box, [] { return adopt_own(*new NodeState); });
+        for (auto* ancestor = m_parent; ancestor; ancestor = ancestor->m_parent) {
+            if (auto it = ancestor->nodes.find(&box); it != ancestor->nodes.end())
+                return *it->value;
+        }
+        return *const_cast<FormattingState&>(*this).nodes.ensure(&box, [] { return adopt_own(*new NodeState); });
+    }();
+
+    const_cast<FormattingState*>(this)->m_lookup_cache = LookupCache { .box = &box, .state = const_cast<NodeState*>(&node_state), .is_mutable = false };
+    return node_state;
 }
 
 void FormattingState::commit()

Userland/Libraries/LibWeb/Layout/FormattingState.h

@@ -111,6 +111,13 @@ struct FormattingState {
 
     FormattingState const* m_parent { nullptr };
     FormattingState const& m_root;
+
+    struct LookupCache {
+        NodeWithStyleAndBoxModelMetrics const* box { nullptr };
+        NodeState* state { nullptr };
+        bool is_mutable { false };
+    };
+    LookupCache m_lookup_cache;
 };
 
 Gfx::FloatRect absolute_content_rect(Box const&, FormattingState const&);