From 44e4ea3d7aaf9aed6ad09cc9f049b2842e3e0d28 Mon Sep 17 00:00:00 2001
From: Andreas Kling <andreas@ladybird.org>
Date: Sat, 7 Sep 2024 11:45:50 +0200
Subject: [PATCH] LibWeb: Cache the qualified layer name in CSSRule

This makes cascade layer filtering take <2% of CPU time when loading
https://vercel.com instead of 30%.
---
 .../LibWeb/CSS/CSSLayerBlockRule.cpp          |  3 +-
 .../LibWeb/CSS/CSSLayerStatementRule.cpp      |  3 +-
 Userland/Libraries/LibWeb/CSS/CSSRule.cpp     | 54 ++++++++++---------
 Userland/Libraries/LibWeb/CSS/CSSRule.h       |  4 +-
 4 files changed, 33 insertions(+), 31 deletions(-)

diff --git a/Userland/Libraries/LibWeb/CSS/CSSLayerBlockRule.cpp b/Userland/Libraries/LibWeb/CSS/CSSLayerBlockRule.cpp
index 475f91da24e..66265921398 100644
--- a/Userland/Libraries/LibWeb/CSS/CSSLayerBlockRule.cpp
+++ b/Userland/Libraries/LibWeb/CSS/CSSLayerBlockRule.cpp
@@ -71,8 +71,7 @@ String CSSLayerBlockRule::serialized() const
 
 FlyString CSSLayerBlockRule::internal_qualified_name(Badge<StyleComputer>) const
 {
-    // TODO: Cache this?
-    auto parent_name = parent_layer_internal_qualified_name();
+    auto const& parent_name = parent_layer_internal_qualified_name();
     if (parent_name.is_empty())
         return m_name_internal;
     return MUST(String::formatted("{}.{}", parent_name, m_name_internal));
diff --git a/Userland/Libraries/LibWeb/CSS/CSSLayerStatementRule.cpp b/Userland/Libraries/LibWeb/CSS/CSSLayerStatementRule.cpp
index 82c53b268e9..cb9c1cda12f 100644
--- a/Userland/Libraries/LibWeb/CSS/CSSLayerStatementRule.cpp
+++ b/Userland/Libraries/LibWeb/CSS/CSSLayerStatementRule.cpp
@@ -42,10 +42,9 @@ String CSSLayerStatementRule::serialized() const
 
 Vector<FlyString> CSSLayerStatementRule::internal_qualified_name_list(Badge<StyleComputer>) const
 {
-    // TODO: Cache these?
     Vector<FlyString> qualified_layer_names;
 
-    auto qualified_parent_layer_name = parent_layer_internal_qualified_name();
+    auto const& qualified_parent_layer_name = parent_layer_internal_qualified_name();
     if (qualified_parent_layer_name.is_empty())
         return m_name_list;
 
diff --git a/Userland/Libraries/LibWeb/CSS/CSSRule.cpp b/Userland/Libraries/LibWeb/CSS/CSSRule.cpp
index 707f88195cb..16060d8629e 100644
--- a/Userland/Libraries/LibWeb/CSS/CSSRule.cpp
+++ b/Userland/Libraries/LibWeb/CSS/CSSRule.cpp
@@ -48,37 +48,39 @@ void CSSRule::set_parent_style_sheet(CSSStyleSheet* parent_style_sheet)
     m_parent_style_sheet = parent_style_sheet;
 }
 
-String CSSRule::parent_layer_internal_qualified_name() const
+FlyString const& CSSRule::parent_layer_internal_qualified_name() const
 {
-    // TODO: Cache this?
-    Vector<FlyString> layer_names;
-    for (auto* rule = parent_rule(); rule; rule = rule->parent_rule()) {
-        switch (rule->type()) {
-        case CSSRule::Type::Import:
-            // TODO: Handle `layer(foo)` in import rules once we implement that.
-            break;
+    if (!m_cached_layer_name.has_value()) {
+        Vector<FlyString> layer_names;
+        for (auto* rule = parent_rule(); rule; rule = rule->parent_rule()) {
+            switch (rule->type()) {
+            case CSSRule::Type::Import:
+                // TODO: Handle `layer(foo)` in import rules once we implement that.
+                break;
 
-        case CSSRule::Type::LayerBlock: {
-            auto& layer_block = static_cast<CSSLayerBlockRule const&>(*rule);
-            layer_names.append(layer_block.internal_name());
-            break;
+            case CSSRule::Type::LayerBlock: {
+                auto& layer_block = static_cast<CSSLayerBlockRule const&>(*rule);
+                layer_names.append(layer_block.internal_name());
+                break;
+            }
+
+                // Ignore everything else
+                // Note that LayerStatement cannot have child rules so we still ignore it here.
+            case CSSRule::Type::LayerStatement:
+            case CSSRule::Type::Style:
+            case CSSRule::Type::Media:
+            case CSSRule::Type::FontFace:
+            case CSSRule::Type::Keyframes:
+            case CSSRule::Type::Keyframe:
+            case CSSRule::Type::Namespace:
+            case CSSRule::Type::Supports:
+                break;
+            }
         }
 
-            // Ignore everything else
-            // Note that LayerStatement cannot have child rules so we still ignore it here.
-        case CSSRule::Type::LayerStatement:
-        case CSSRule::Type::Style:
-        case CSSRule::Type::Media:
-        case CSSRule::Type::FontFace:
-        case CSSRule::Type::Keyframes:
-        case CSSRule::Type::Keyframe:
-        case CSSRule::Type::Namespace:
-        case CSSRule::Type::Supports:
-            break;
-        }
+        m_cached_layer_name = MUST(String::join("."sv, layer_names.in_reverse()));
     }
-
-    return MUST(String::join("."sv, layer_names.in_reverse()));
+    return m_cached_layer_name.value();
 }
 
 }
diff --git a/Userland/Libraries/LibWeb/CSS/CSSRule.h b/Userland/Libraries/LibWeb/CSS/CSSRule.h
index 32fab795eb0..dcac9ce8689 100644
--- a/Userland/Libraries/LibWeb/CSS/CSSRule.h
+++ b/Userland/Libraries/LibWeb/CSS/CSSRule.h
@@ -58,10 +58,12 @@ protected:
 
     virtual void visit_edges(Cell::Visitor&) override;
 
-    String parent_layer_internal_qualified_name() const;
+    FlyString const& parent_layer_internal_qualified_name() const;
 
     JS::GCPtr<CSSRule> m_parent_rule;
     JS::GCPtr<CSSStyleSheet> m_parent_style_sheet;
+
+    mutable Optional<FlyString> m_cached_layer_name;
 };
 
 }