Subtree caching implementation in blink-core

Source/core/paint/DisplayItemListPaintTest.cpp

Index: Source/core/paint/DisplayItemListPaintTest.cpp
diff --git a/Source/core/paint/DisplayItemListPaintTest.cpp b/Source/core/paint/DisplayItemListPaintTest.cpp
index 7a161d5da682c29ef42e148539d6f54c457ca88a..f22747ad6d1ea93740d7fb1c25f361be0783c544 100644
--- a/Source/core/paint/DisplayItemListPaintTest.cpp
+++ b/Source/core/paint/DisplayItemListPaintTest.cpp
@@ -67,6 +67,10 @@ protected:
     DisplayItemList& rootDisplayItemList() { return *layoutView().layer()->graphicsLayerBacking()->displayItemList(); }
     const DisplayItems& newPaintListBeforeUpdate() { return rootDisplayItemList().m_newDisplayItems; }
 
+    // Expose some document lifecycle steps.
+    void updateLifecyclePhasesToPaintForSlimmingPaintV2Clean() { document().view()->updateLifecyclePhasesInternal(FrameView::OnlyUpToPaintForSlimmingPaintV2Clean); }
+    void compositeForSlimmingPaintV2() { document().view()->compositeForSlimmingPaintV2(); }
+
 private:
     void SetUp() override
     {
@@ -255,4 +259,41 @@ TEST_F(DisplayItemListPaintTestForSlimmingPaintV2, InlineRelayout)
         TestDisplayItem(divBlock, DisplayItem::paintPhaseToEndSubtreeType(PaintPhaseForeground)));
 }
 
+TEST_F(DisplayItemListPaintTestForSlimmingPaintV2, SubtreeCache)
+{
+    setBodyInnerHTML(
+        "<div id='container1' style='width: 200px; height: 200px; background-color: blue'>"
+        "  <div id='content1a'>AAA</div>"
+        "  <div id='content1b' style='width: 100px; height: 100px; background-color: red'>BBB</div>"
+        "</div>"
+        "<div id='container2' style='width: 200px; height: 200px; background-color: blue'>"
+        "  <div id='content2a'>CCC</div>"
+        "  <div id='content2b' style='width: 100px; height: 100px; background-color: green'>DDD</div>"
+        "</div>");
+    document().view()->updateAllLifecyclePhases();
+
+    size_t displayItemCount = rootDisplayItemList().displayItems().size();
+
+    Element& content1b = *document().getElementById("content1b");
+    content1b.setAttribute(HTMLNames::styleAttr, "width: 100px; height: 100px; background-color: green");
+
+    updateLifecyclePhasesToPaintForSlimmingPaintV2Clean();
+    const LayoutObject* container2 = document().getElementById("container2")->layoutObject();
+    int cachedSubtreeCount = 0;
+    for (auto& item : newPaintListBeforeUpdate()) {
+        if (item.isCachedSubtree()) {
+            ++cachedSubtreeCount;
+            EXPECT_EQ(container2->displayItemClient(), item.client());
+            if (cachedSubtreeCount == 1)
+                EXPECT_EQ(item.type(), DisplayItem::paintPhaseToCachedSubtreeType(PaintPhaseChildBlockBackground));
+            else if (cachedSubtreeCount == 2)
+                EXPECT_EQ(item.type(), DisplayItem::paintPhaseToCachedSubtreeType(PaintPhaseForeground));
+        }
+    }
+    EXPECT_EQ(2, cachedSubtreeCount);
+
+    compositeForSlimmingPaintV2();
+    EXPECT_EQ(displayItemCount, rootDisplayItemList().displayItems().size());
+}
+
 } // namespace blink