From cb722ca18b6128dfee6f257bbb454cfdbac6b417 Mon Sep 17 00:00:00 2001
From: Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
Date: Tue, 1 Apr 2025 00:06:01 +0200
Subject: [PATCH] LibWeb: Allow working on N+1 frame while N is rasterizing

This change allows us to overlap rasterization and rendering work across
threads: while the rasterization thread processes frame N, the main
thread can simultaneously work on producing the display list for frame
N+1.
---
 Services/WebContent/BackingStoreManager.h | 17 ++++++++++++++---
 Services/WebContent/PageClient.cpp        | 23 ++++++++++++-----------
 Services/WebContent/PageClient.h          |  7 +------
 3 files changed, 27 insertions(+), 20 deletions(-)

diff --git a/Services/WebContent/BackingStoreManager.h b/Services/WebContent/BackingStoreManager.h
index 7c39c0d8134..6b02a9af5d3 100644
--- a/Services/WebContent/BackingStoreManager.h
+++ b/Services/WebContent/BackingStoreManager.h
@@ -25,14 +25,25 @@ public:
     void reallocate_backing_stores(Gfx::IntSize);
     void restart_resize_timer();
 
-    Web::Painting::BackingStore* back_store() { return m_back_store.ptr(); }
-    i32 front_id() const { return m_front_bitmap_id; }
+    struct BackingStore {
+        i32 bitmap_id { -1 };
+        Web::Painting::BackingStore* store { nullptr };
+    };
 
-    void swap_back_and_front();
+    BackingStore acquire_store_for_next_frame()
+    {
+        BackingStore backing_store;
+        backing_store.bitmap_id = m_back_bitmap_id;
+        backing_store.store = m_back_store.ptr();
+        swap_back_and_front();
+        return backing_store;
+    }
 
     BackingStoreManager(PageClient&);
 
 private:
+    void swap_back_and_front();
+
     // FIXME: We should come up with an ownership model for this class that makes the GC-checker happy
     IGNORE_GC PageClient& m_page_client;
 
diff --git a/Services/WebContent/PageClient.cpp b/Services/WebContent/PageClient.cpp
index 64bcc3174a3..ff99b62d789 100644
--- a/Services/WebContent/PageClient.cpp
+++ b/Services/WebContent/PageClient.cpp
@@ -81,12 +81,7 @@ PageClient::~PageClient() = default;
 
 bool PageClient::is_ready_to_paint() const
 {
-    return m_paint_state == PaintState::Ready;
-}
-
-void PageClient::ready_to_paint()
-{
-    m_paint_state = PaintState::Ready;
+    return m_number_of_queued_rasterization_tasks <= 1;
 }
 
 void PageClient::visit_edges(JS::Cell::Visitor& visitor)
@@ -220,18 +215,24 @@ void PageClient::process_screenshot_requests()
     }
 }
 
+void PageClient::ready_to_paint()
+{
+    m_number_of_queued_rasterization_tasks--;
+    VERIFY(m_number_of_queued_rasterization_tasks >= 0 && m_number_of_queued_rasterization_tasks < 2);
+}
+
 void PageClient::paint_next_frame()
 {
-    auto back_store = m_backing_store_manager.back_store();
+    auto [backing_store_id, back_store] = m_backing_store_manager.acquire_store_for_next_frame();
     if (!back_store)
         return;
 
-    m_paint_state = PaintState::WaitingForClient;
+    VERIFY(m_number_of_queued_rasterization_tasks <= 1);
+    m_number_of_queued_rasterization_tasks++;
 
     auto viewport_rect = page().css_to_device_rect(page().top_level_traversable()->viewport_rect());
-    start_display_list_rendering(viewport_rect, *back_store, {}, [this, viewport_rect]() {
-        m_backing_store_manager.swap_back_and_front();
-        client().async_did_paint(m_id, viewport_rect.to_type<int>(), m_backing_store_manager.front_id());
+    start_display_list_rendering(viewport_rect, *back_store, {}, [this, viewport_rect, backing_store_id] {
+        client().async_did_paint(m_id, viewport_rect.to_type<int>(), backing_store_id);
     });
 }
 
diff --git a/Services/WebContent/PageClient.h b/Services/WebContent/PageClient.h
index f96da84e314..e89450dfb87 100644
--- a/Services/WebContent/PageClient.h
+++ b/Services/WebContent/PageClient.h
@@ -190,12 +190,7 @@ private:
     bool m_should_show_line_box_borders { false };
     bool m_has_focus { false };
 
-    enum class PaintState {
-        Ready,
-        WaitingForClient,
-    };
-
-    PaintState m_paint_state { PaintState::Ready };
+    i32 m_number_of_queued_rasterization_tasks { 0 };
 
     struct ScreenshotTask {
         Optional<Web::UniqueNodeID> node_id;