Make LayoutBlockFlowRareData to hold NGOffsetMapping for legacy layout

This patch makes |LayoutBlockFlowRareData| to hold |NGOffsetMapping| for legacy
layout to make |NGInlineNode::GetOffsetMapping()| can use |NGOffsetMapping|
once it is computed until next layout for making repeated call of
|Element#innerText| faster.

Before this patch, |NGOffsetMapping| for legacy layout tree is hold by callers
to avoid computing |NGOffsetMapping| in same |LayoutBlockFlow|.

However, this simple cache doesn't work for |Element#innerText| when it is
called for each element:

document.body.innerHTML = '<span>a</span>'.repeat(3000);
[...document.getElementByTagName('*')].forEach(e => e.innerText);

Before this patch, above code fragment takes 4000ms, after this patch it takes
40ms on my local machine.

This patch also movs |LayoutBlockFlowRareData| constructor to ".cc" file to
avoid including "ng_offset_mapping.h" to instantiate
|std::unique_ptr<NGOffsetMapping| in various places.


Note: Following patch[1] will get rid of redundant parameter from
|NGInlineNode::GetOffsetMapping()|.

[1] http://crrev.com/c/1488400 Get rid of redundant parameter from
NGInlineNode::GetOffsetMapping()

Bug: 935237
Change-Id: I5990f53632b06e721cc17eca0c80aea8aac36675
Reviewed-on: https://chromium-review.googlesource.com/c/1488479
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636384}

third_party/blink/renderer/core/layout/layout_block_flow.cc

@@ -55,6 +55,7 @@
 #include "third_party/blink/renderer/core/layout/line/line_width.h"
 #include "third_party/blink/renderer/core/layout/logical_values.h"
 #include "third_party/blink/renderer/core/layout/ng/inline/ng_line_height_metrics.h"
+#include "third_party/blink/renderer/core/layout/ng/inline/ng_offset_mapping.h"
 #include "third_party/blink/renderer/core/layout/ng/layout_ng_block_flow.h"
 #include "third_party/blink/renderer/core/layout/ng/legacy_layout_tree_walking.h"
 #include "third_party/blink/renderer/core/layout/ng/ng_absolute_utils.h"
@@ -446,6 +447,9 @@ void LayoutBlockFlow::UpdateBlockLayout(bool relayout_children) {
   if (RuntimeEnabledFeatures::TrackLayoutPassesPerBlockEnabled())
     IncrementLayoutPassCount();
 
+  if (rare_data_)
+    ClearOffsetMapping();
+
   if (!relayout_children && SimplifiedLayout())
     return;
 
@@ -4906,4 +4910,37 @@ int LayoutBlockFlow::GetLayoutPassCountForTesting() {
   return GetLayoutPassCountMap().find(this)->value;
 }
 
+LayoutBlockFlow::LayoutBlockFlowRareData::LayoutBlockFlowRareData(
+    const LayoutBlockFlow* block)
+    : margins_(PositiveMarginBeforeDefault(block),
+               NegativeMarginBeforeDefault(block),
+               PositiveMarginAfterDefault(block),
+               NegativeMarginAfterDefault(block)),
+      break_before_(static_cast<unsigned>(EBreakBetween::kAuto)),
+      break_after_(static_cast<unsigned>(EBreakBetween::kAuto)),
+      line_break_to_avoid_widow_(-1),
+      did_break_at_line_to_avoid_widow_(false),
+      discard_margin_before_(false),
+      discard_margin_after_(false) {}
+
+LayoutBlockFlow::LayoutBlockFlowRareData::~LayoutBlockFlowRareData() = default;
+
+void LayoutBlockFlow::ClearOffsetMapping() {
+  DCHECK(!IsLayoutNGObject());
+  DCHECK(rare_data_);
+  rare_data_->offset_mapping_.reset();
+}
+
+const NGOffsetMapping* LayoutBlockFlow::GetOffsetMapping() const {
+  DCHECK(!IsLayoutNGObject());
+  return rare_data_ ? rare_data_->offset_mapping_.get() : nullptr;
+}
+
+void LayoutBlockFlow::SetOffsetMapping(
+    std::unique_ptr<NGOffsetMapping> offset_mapping) {
+  DCHECK(!IsLayoutNGObject());
+  DCHECK(offset_mapping);
+  EnsureRareData().offset_mapping_ = std::move(offset_mapping);
+}
+
 }  // namespace blink

third_party/blink/renderer/core/layout/layout_block_flow.h

@@ -63,6 +63,7 @@ class NGBlockBreakToken;
 class NGBreakToken;
 class NGConstraintSpace;
 class NGLayoutResult;
+class NGOffsetMapping;
 class NGPaintFragment;
 class NGPhysicalFragment;
 
@@ -754,18 +755,8 @@ class CORE_EXPORT LayoutBlockFlow : public LayoutBlock {
     USING_FAST_MALLOC(LayoutBlockFlowRareData);
 
    public:
-    LayoutBlockFlowRareData(const LayoutBlockFlow* block)
-        : margins_(PositiveMarginBeforeDefault(block),
-                   NegativeMarginBeforeDefault(block),
-                   PositiveMarginAfterDefault(block),
-                   NegativeMarginAfterDefault(block)),
-          multi_column_flow_thread_(nullptr),
-          break_before_(static_cast<unsigned>(EBreakBetween::kAuto)),
-          break_after_(static_cast<unsigned>(EBreakBetween::kAuto)),
-          line_break_to_avoid_widow_(-1),
-          did_break_at_line_to_avoid_widow_(false),
-          discard_margin_before_(false),
-          discard_margin_after_(false) {}
+    explicit LayoutBlockFlowRareData(const LayoutBlockFlow* block);
+    ~LayoutBlockFlowRareData();
 
     static LayoutUnit PositiveMarginBeforeDefault(
         const LayoutBlockFlow* block) {
@@ -787,7 +778,13 @@ class CORE_EXPORT LayoutBlockFlow : public LayoutBlock {
 
     LayoutUnit first_forced_break_offset_;
 
-    LayoutMultiColumnFlowThread* multi_column_flow_thread_;
+    LayoutMultiColumnFlowThread* multi_column_flow_thread_ = nullptr;
+
+    // |offset_mapping_| is used only for legacy layout tree for caching offset
+    // mapping for |NGInlineNode::GetOffsetMapping()|.
+    // TODO(yosin): Once we have no legacy support, we should get rid of
+    // |offset_mapping_| here.
+    std::unique_ptr<NGOffsetMapping> offset_mapping_;
 
     unsigned break_before_ : 4;
     unsigned break_after_ : 4;
@@ -798,6 +795,10 @@ class CORE_EXPORT LayoutBlockFlow : public LayoutBlock {
     DISALLOW_COPY_AND_ASSIGN(LayoutBlockFlowRareData);
   };
 
+  void ClearOffsetMapping();
+  const NGOffsetMapping* GetOffsetMapping() const;
+  void SetOffsetMapping(std::unique_ptr<NGOffsetMapping>);
+
   const FloatingObjects* GetFloatingObjects() const {
     return floating_objects_.get();
   }

third_party/blink/renderer/core/layout/ng/inline/ng_inline_node.cc

@@ -466,22 +466,20 @@ const NGOffsetMapping* NGInlineNode::GetOffsetMapping(
   // If |layout_block_flow| is LayoutNG, compute from |NGInlineNode|.
   if (layout_block_flow->IsLayoutNGMixin()) {
     NGInlineNode node(layout_block_flow);
-    if (node.IsPrepareLayoutFinished())
-      return node.ComputeOffsetMappingIfNeeded();
-
-    // When this is not laid out yet, compute each time it is requested.
-    // TODO(kojii): We could still keep the result for later uses but it would
-    // add more states. Reconsider if this turned out to be needed.
+    CHECK(node.IsPrepareLayoutFinished());
+    return node.ComputeOffsetMappingIfNeeded();
   }
 
-  // If this is not LayoutNG, compute the offset mapping and store in |storage|.
-  // The caller is responsible to keep |storage| for the life cycle.
+  // If this is not LayoutNG, compute the offset mapping and store into
+  // |LayoutBlockFlowRateData|.
+  if (const NGOffsetMapping* mapping = layout_block_flow->GetOffsetMapping())
+    return mapping;
   DCHECK(storage);
   NGInlineNodeData data;
   ComputeOffsetMapping(layout_block_flow, &data);
-  *storage = std::move(data.offset_mapping);
-  DCHECK(*storage);
-  return storage->get();
+  NGOffsetMapping* const mapping = data.offset_mapping.get();
+  layout_block_flow->SetOffsetMapping(std::move(data.offset_mapping));
+  return mapping;
 }
 
 // Depth-first-scan of all LayoutInline and LayoutText nodes that make up this

third_party/blink/renderer/core/layout/ng/inline/ng_inline_node.h

@@ -84,6 +84,8 @@ class CORE_EXPORT NGInlineNode : public NGLayoutInputNode {
   // Otherwise, this function computes |NGOffsetMapping| and store in |storage|
   // as well as returning the pointer. The caller is responsible for keeping
   // |storage| for the life cycle of the returned |NGOffsetMapping|.
+  // TODO(yosin): We should get rid of |storage| parameter, since it is no
+  // longer used.
   static const NGOffsetMapping* GetOffsetMapping(
       LayoutBlockFlow* layout_block_flow,
       std::unique_ptr<NGOffsetMapping>* storage);

third_party/blink/renderer/core/layout/ng/inline/ng_offset_mapping_test.cc

@@ -1185,7 +1185,6 @@ TEST_P(NGOffsetMappingGetterTest, Get) {
   const NGOffsetMapping* mapping =
       NGInlineNode::GetOffsetMapping(layout_block_flow, &storage);
   EXPECT_TRUE(mapping);
-  EXPECT_EQ(!storage, GetParam());  // |storage| is used only for legacy.
 
   const String& text_content = mapping->GetText();
   EXPECT_EQ(text_content, "Whitespaces in this text should be collapsed.");