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LibGfx: Cache ASCII font resolution in FontCascadeList

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font_for_code_point() was the heaviest function in layout profiles
of a YouTube page (216ms CPU out of 2900ms total). Every call walked
the full cascade and ran a virtual contains_glyph() against each
entry, even though the result is the same for most ASCII code points
across a document.

Add a 128-entry direct-mapped cache keyed by code point that stores
the resolved Font pointer on first lookup. Subsequent ASCII lookups
become a null check plus a load.

No invalidation is needed: m_fonts is append-only, and the cascade
returns the first matching font, so once an entry claims a code
point, later appends cannot change the answer.
This commit is contained in:
Andreas Kling
2026-04-24 08:16:13 +02:00
committed by Andreas Kling
parent 6b7fb4b273
commit bfead0cb20
Notes: github-actions[bot] 2026-04-24 10:55:12 +00:00 
Author: https://github.com/awesomekling
Commit: https://github.com/LadybirdBrowser/ladybird/commit/bfead0cb20c
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/9068
2 changed files with 20 additions and 4 deletions
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19
Libraries/LibGfx/FontCascadeList.cpp
View File

@@ -41,27 +41,38 @@ void FontCascadeList::extend(FontCascadeList const& other)
Gfx::Font const& FontCascadeList::font_for_code_point(u32 code_point) const
{
if (code_point < m_ascii_cache.size()) {
if (auto const* cached = m_ascii_cache[code_point])
return *cached;
}
auto cache_and_return = [&](Font const& font) -> Font const& {
if (code_point < m_ascii_cache.size())
m_ascii_cache[code_point] = &font;
return font;
};
for (auto const& entry : m_fonts) {
if (entry.range_data.has_value()) {
if (!entry.range_data->enclosing_range.contains(code_point))
continue;
for (auto const& range : entry.range_data->unicode_ranges) {
if (range.contains(code_point) && entry.font->contains_glyph(code_point))
return entry.font;
return cache_and_return(*entry.font);
}
} else if (entry.font->contains_glyph(code_point)) {
return entry.font;
return cache_and_return(*entry.font);
}
}
if (m_system_font_fallback_callback) {
if (auto fallback = m_system_font_fallback_callback(code_point, first())) {
m_fonts.append({ fallback.release_nonnull(), {} });
return *m_fonts.last().font;
return cache_and_return(*m_fonts.last().font);
}
}
return *m_last_resort_font;
return cache_and_return(*m_last_resort_font);
}
bool FontCascadeList::equals(FontCascadeList const& other) const

5
Libraries/LibGfx/FontCascadeList.h
View File

@@ -6,6 +6,7 @@
#pragma once
#include <AK/Array.h>
#include <AK/Function.h>
#include <LibGfx/Font/Font.h>
#include <LibGfx/Font/UnicodeRange.h>
@@ -67,6 +68,10 @@ private:
RefPtr<Font const> m_last_resort_font;
mutable Vector<Entry> m_fonts;
SystemFontFallbackCallback m_system_font_fallback_callback;
// OPTIMIZATION: Cache of resolved fonts for ASCII code points. Since m_fonts only grows and the cascade returns
// the first matching font, a cached hit can never become stale.
mutable Array<Font const*, 128> m_ascii_cache {};
};
}