From 8d81421526488c3f6bc72c8ea4a5f9470bab972b Mon Sep 17 00:00:00 2001
From: Tim Ledbetter <tim.ledbetter@ladybird.org>
Date: Tue, 4 Nov 2025 23:25:15 +0000
Subject: [PATCH] LibWeb: Cache font matching algorithm results

The majority of time in `compute_font()` was spent in
`font_matching_algorithm()` repeatedly computing the same values. We
now cache these values to avoid unnecessary work.
---
 Libraries/LibWeb/CSS/StyleComputer.cpp | 23 ++++++++++++++++++++++-
 Libraries/LibWeb/CSS/StyleComputer.h   | 12 ++++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/Libraries/LibWeb/CSS/StyleComputer.cpp b/Libraries/LibWeb/CSS/StyleComputer.cpp
index 643592cac7c..e7138af3072 100644
--- a/Libraries/LibWeb/CSS/StyleComputer.cpp
+++ b/Libraries/LibWeb/CSS/StyleComputer.cpp
@@ -131,6 +131,18 @@ struct Traits<Web::CSS::OwnFontFaceKey> : public DefaultTraits<Web::CSS::OwnFont
     static unsigned hash(Web::CSS::OwnFontFaceKey const& key) { return pair_int_hash(key.family_name.hash(), pair_int_hash(key.weight, key.slope)); }
 };
 
+template<>
+struct Traits<Web::CSS::FontMatchingAlgorithmCacheKey> : public DefaultTraits<Web::CSS::FontMatchingAlgorithmCacheKey> {
+    static unsigned hash(Web::CSS::FontMatchingAlgorithmCacheKey const& key)
+    {
+        auto hash = key.family_name.hash();
+        hash = pair_int_hash(hash, key.weight);
+        hash = pair_int_hash(hash, key.slope);
+        hash = pair_int_hash(hash, Traits<float>::hash(key.font_size_in_pt));
+        return hash;
+    }
+};
+
 }
 
 namespace Web::CSS {
@@ -1694,9 +1706,17 @@ RefPtr<Gfx::FontCascadeList const> StyleComputer::find_matching_font_weight_desc
     return {};
 }
 
+RefPtr<Gfx::FontCascadeList const> StyleComputer::font_matching_algorithm(FlyString const& family_name, int weight, int slope, float font_size_in_pt) const
+{
+    FontMatchingAlgorithmCacheKey key { family_name, weight, slope, font_size_in_pt };
+    return m_font_matching_algorithm_cache.ensure(key, [&] {
+        return font_matching_algorithm_impl(family_name, weight, slope, font_size_in_pt);
+    });
+}
+
 // Partial implementation of the font-matching algorithm: https://www.w3.org/TR/css-fonts-4/#font-matching-algorithm
 // FIXME: This should be replaced by the full CSS font selection algorithm.
-RefPtr<Gfx::FontCascadeList const> StyleComputer::font_matching_algorithm(FlyString const& family_name, int weight, int slope, float font_size_in_pt) const
+RefPtr<Gfx::FontCascadeList const> StyleComputer::font_matching_algorithm_impl(FlyString const& family_name, int weight, int slope, float font_size_in_pt) const
 {
     // If a font family match occurs, the user agent assembles the set of font faces in that family and then
     // narrows the set to a single face using other font properties in the order given below.
@@ -2948,6 +2968,7 @@ void StyleComputer::invalidate_rule_cache()
 
 void StyleComputer::did_load_font(FlyString const&)
 {
+    m_font_matching_algorithm_cache = {};
     document().invalidate_style(DOM::StyleInvalidationReason::CSSFontLoaded);
 }
 
diff --git a/Libraries/LibWeb/CSS/StyleComputer.h b/Libraries/LibWeb/CSS/StyleComputer.h
index 7b33345c904..e52aa01c9f4 100644
--- a/Libraries/LibWeb/CSS/StyleComputer.h
+++ b/Libraries/LibWeb/CSS/StyleComputer.h
@@ -111,6 +111,15 @@ struct OwnFontFaceKey {
     int slope { 0 };
 };
 
+struct FontMatchingAlgorithmCacheKey {
+    FlyString family_name;
+    int weight;
+    int slope;
+    float font_size_in_pt;
+
+    [[nodiscard]] bool operator==(FontMatchingAlgorithmCacheKey const& other) const = default;
+};
+
 struct RuleCache {
     HashMap<FlyString, Vector<MatchingRule>> rules_by_id;
     HashMap<FlyString, Vector<MatchingRule>> rules_by_class;
@@ -241,6 +250,7 @@ private:
     static RefPtr<Gfx::FontCascadeList const> find_matching_font_weight_ascending(Vector<MatchingFontCandidate> const& candidates, int target_weight, float font_size_in_pt, Gfx::FontVariationSettings const& variations, bool inclusive);
     static RefPtr<Gfx::FontCascadeList const> find_matching_font_weight_descending(Vector<MatchingFontCandidate> const& candidates, int target_weight, float font_size_in_pt, Gfx::FontVariationSettings const& variations, bool inclusive);
     RefPtr<Gfx::FontCascadeList const> font_matching_algorithm(FlyString const& family_name, int weight, int slope, float font_size_in_pt) const;
+    RefPtr<Gfx::FontCascadeList const> font_matching_algorithm_impl(FlyString const& family_name, int weight, int slope, float font_size_in_pt) const;
     void compute_custom_properties(ComputedProperties&, DOM::AbstractElement) const;
     void compute_math_depth(ComputedProperties&, Optional<DOM::AbstractElement>) const;
     void start_needed_transitions(ComputedProperties const& old_style, ComputedProperties& new_style, DOM::AbstractElement) const;
@@ -309,6 +319,8 @@ private:
     CSSPixelRect m_viewport_rect;
 
     OwnPtr<CountingBloomFilter<u8, 14>> m_ancestor_filter;
+
+    mutable HashMap<FontMatchingAlgorithmCacheKey, RefPtr<Gfx::FontCascadeList const>> m_font_matching_algorithm_cache;
 };
 
 class FontLoader final : public GC::Cell {