LibWeb: Cache the root element font metrics when calculating them

The root element font metrics were getting queried again and again
during style computation. Before this change we would do some work to
recalculate them each time.

This patch simply caches them in a StyleComputer member. Since style
updates always start with the root element, we know that it'll be
up-to-date by the time we look at any other element.

Before this change, we were spending ~5% of CPU time on Google Groups
in root_element_font_metrics().

Userland/Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -63,6 +63,8 @@ namespace Web::CSS {
 
 StyleComputer::StyleComputer(DOM::Document& document)
     : m_document(document)
+    , m_default_font_metrics(16, Gfx::FontDatabase::default_font().pixel_metrics(), 16)
+    , m_root_element_font_metrics(m_default_font_metrics)
 {
 }
 
@@ -1053,24 +1055,14 @@ void StyleComputer::compute_defaulted_values(StyleProperties& style, DOM::Elemen
     }
 }
 
-Length::FontMetrics StyleComputer::root_element_font_metrics() const
+Length::FontMetrics StyleComputer::calculate_root_element_font_metrics(StyleProperties const& style) const
 {
-    Length::FontMetrics const default_font_metrics { 16, Gfx::FontDatabase::default_font().pixel_metrics(), 16 };
+    auto root_value = style.property(CSS::PropertyID::FontSize);
 
-    auto const* root_element = m_document->first_child_of_type<HTML::HTMLHtmlElement>();
-    if (!root_element)
-        return default_font_metrics;
-
-    auto const* computed_root_style = root_element->computed_css_values();
-    if (!computed_root_style)
-        return default_font_metrics;
-
-    auto root_value = computed_root_style->property(CSS::PropertyID::FontSize);
-
-    auto font_pixel_metrics = computed_root_style->computed_font().pixel_metrics();
-    Length::FontMetrics font_metrics { default_font_metrics.font_size, font_pixel_metrics, font_pixel_metrics.line_spacing() };
+    auto font_pixel_metrics = style.computed_font().pixel_metrics();
+    Length::FontMetrics font_metrics { m_default_font_metrics.font_size, font_pixel_metrics, font_pixel_metrics.line_spacing() };
     font_metrics.font_size = root_value->to_length().to_px(viewport_rect(), font_metrics, font_metrics);
-    font_metrics.line_height = computed_root_style->line_height(viewport_rect(), font_metrics, font_metrics);
+    font_metrics.line_height = style.line_height(viewport_rect(), font_metrics, font_metrics);
 
     return font_metrics;
 }
@@ -1218,8 +1210,6 @@ void StyleComputer::compute_font(StyleProperties& style, DOM::Element const* ele
         font_size_in_px *= multiplier;
 
     } else {
-        auto root_font_metrics = root_element_font_metrics();
-
         Gfx::FontPixelMetrics font_pixel_metrics;
         if (parent_element && parent_element->computed_css_values())
             font_pixel_metrics = parent_element->computed_css_values()->computed_font().pixel_metrics();
@@ -1235,7 +1225,7 @@ void StyleComputer::compute_font(StyleProperties& style, DOM::Element const* ele
                 auto parent_line_height = parent_or_root_element_line_height(parent_element, {});
                 if (length.is_absolute() || length.is_relative()) {
                     Length::FontMetrics font_metrics { font_size_in_px, font_pixel_metrics, parent_line_height };
-                    return length.to_px(viewport_rect(), font_metrics, root_font_metrics);
+                    return length.to_px(viewport_rect(), font_metrics, m_root_element_font_metrics);
                 }
             }
             return font_size_in_px;
@@ -1256,7 +1246,7 @@ void StyleComputer::compute_font(StyleProperties& style, DOM::Element const* ele
         if (maybe_length.has_value()) {
             auto parent_line_height = parent_or_root_element_line_height(element, pseudo_element);
             Length::FontMetrics font_metrics { parent_font_size(), font_pixel_metrics, parent_line_height };
-            auto px = maybe_length.value().to_px(viewport_rect(), font_metrics, root_font_metrics).value();
+            auto px = maybe_length.value().to_px(viewport_rect(), font_metrics, m_root_element_font_metrics).value();
             if (px != 0)
                 font_size_in_px = px;
         }
@@ -1368,6 +1358,10 @@ void StyleComputer::compute_font(StyleProperties& style, DOM::Element const* ele
     style.set_property(CSS::PropertyID::FontWeight, NumericStyleValue::create_integer(weight).release_value_but_fixme_should_propagate_errors());
 
     style.set_computed_font(found_font.release_nonnull());
+
+    if (element && is<HTML::HTMLHtmlElement>(*element)) {
+        const_cast<StyleComputer&>(*this).m_root_element_font_metrics = calculate_root_element_font_metrics(style);
+    }
 }
 
 Gfx::Font const& StyleComputer::initial_font() const
@@ -1378,20 +1372,17 @@ Gfx::Font const& StyleComputer::initial_font() const
 
 CSSPixels StyleComputer::parent_or_root_element_line_height(DOM::Element const* element, Optional<CSS::Selector::PseudoElement> pseudo_element) const
 {
-    auto root_font_metrics = root_element_font_metrics();
     auto* parent_element = element_to_inherit_style_from(element, pseudo_element);
     if (!parent_element)
-        return root_font_metrics.line_height;
-    auto* computed_values = parent_element->computed_css_values();
-    auto parent_font_pixel_metrics = parent_element->layout_node()
-        ? parent_element->layout_node()->font().pixel_metrics()
-        : Gfx::FontDatabase::default_font().pixel_metrics();
+        return m_root_element_font_metrics.line_height;
+    auto const* computed_values = parent_element->computed_css_values();
+    auto parent_font_pixel_metrics = computed_values->computed_font().pixel_metrics();
     auto parent_font_size = computed_values->property(CSS::PropertyID::FontSize)->to_length();
     // FIXME: Can the parent font size be non-absolute here?
-    auto parent_font_size_value = parent_font_size.is_absolute() ? parent_font_size.absolute_length_to_px() : root_font_metrics.font_size;
+    auto parent_font_size_value = parent_font_size.is_absolute() ? parent_font_size.absolute_length_to_px() : m_root_element_font_metrics.font_size;
     auto parent_parent_line_height = parent_or_root_element_line_height(parent_element, {});
     Length::FontMetrics parent_font_metrics { parent_font_size_value, parent_font_pixel_metrics, parent_parent_line_height };
-    return computed_values->line_height(viewport_rect(), parent_font_metrics, root_font_metrics);
+    return computed_values->line_height(viewport_rect(), parent_font_metrics, m_root_element_font_metrics);
 }
 
 ErrorOr<void> StyleComputer::absolutize_values(StyleProperties& style, DOM::Element const* element, Optional<CSS::Selector::PseudoElement> pseudo_element) const
@@ -1399,11 +1390,10 @@ ErrorOr<void> StyleComputer::absolutize_values(StyleProperties& style, DOM::Elem
     auto parent_or_root_line_height = parent_or_root_element_line_height(element, pseudo_element);
 
     auto font_pixel_metrics = style.computed_font().pixel_metrics();
-    auto root_font_metrics = root_element_font_metrics();
 
-    Length::FontMetrics font_metrics { root_font_metrics.font_size, font_pixel_metrics, parent_or_root_line_height };
+    Length::FontMetrics font_metrics { m_root_element_font_metrics.font_size, font_pixel_metrics, parent_or_root_line_height };
 
-    auto font_size = style.property(CSS::PropertyID::FontSize)->to_length().to_px(viewport_rect(), font_metrics, root_font_metrics);
+    auto font_size = style.property(CSS::PropertyID::FontSize)->to_length().to_px(viewport_rect(), font_metrics, m_root_element_font_metrics);
     font_metrics.font_size = font_size;
 
     // NOTE: Percentage line-height values are relative to the font-size of the element.
@@ -1416,7 +1406,7 @@ ErrorOr<void> StyleComputer::absolutize_values(StyleProperties& style, DOM::Elem
             Length::make_px(font_size * line_height_value_slot->as_percentage().percentage().as_fraction())));
     }
 
-    auto line_height = style.line_height(viewport_rect(), font_metrics, root_font_metrics);
+    auto line_height = style.line_height(viewport_rect(), font_metrics, m_root_element_font_metrics);
     font_metrics.line_height = line_height;
 
     // NOTE: line-height might be using lh which should be resolved against the parent line height (like we did here already)
@@ -1427,7 +1417,7 @@ ErrorOr<void> StyleComputer::absolutize_values(StyleProperties& style, DOM::Elem
         auto& value_slot = style.m_property_values[i];
         if (!value_slot)
             continue;
-        value_slot = TRY(value_slot->absolutized(viewport_rect(), font_metrics, root_font_metrics));
+        value_slot = TRY(value_slot->absolutized(viewport_rect(), font_metrics, m_root_element_font_metrics));
     }
     return {};
 }

Userland/Libraries/LibWeb/CSS/StyleComputer.h

@@ -102,7 +102,7 @@ private:
     void for_each_stylesheet(CascadeOrigin, Callback) const;
 
     CSSPixelRect viewport_rect() const;
-    Length::FontMetrics root_element_font_metrics() const;
+    [[nodiscard]] Length::FontMetrics calculate_root_element_font_metrics(StyleProperties const&) const;
     CSSPixels parent_or_root_element_line_height(DOM::Element const*, Optional<CSS::Selector::PseudoElement>) const;
 
     struct MatchingRuleSet {
@@ -133,6 +133,9 @@ private:
 
     class FontLoader;
     HashMap<String, NonnullOwnPtr<FontLoader>> m_loaded_fonts;
+
+    Length::FontMetrics m_default_font_metrics;
+    Length::FontMetrics m_root_element_font_metrics;
 };
 
 }