[LayoutNG] Reduce memory allocations in NGInlineNode::ShapeText

32 can avoid most of memory allocations, by consuming 512
bytes on stack ((4+4+8) * 32) on 64 bits.

Most major sites are up to 8 or so, such as Google SRP,
facebook, twitter, MDN, youtube, etc. Wikipedia is up to 21.

Change-Id: I7f06f0ce01b5d2960becf99e0b453d92723e5253
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1821501
Auto-Submit: Koji Ishii <kojii@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699655}

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

@@ -1158,9 +1158,9 @@ void NGInlineNode::ShapeText(NGInlineItemsData* data,
     // If the text is from multiple items, split the ShapeResult to
     // corresponding items.
     DCHECK_GT(num_text_items, 0u);
-    std::unique_ptr<ShapeResult::ShapeRange[]> text_item_ranges =
-        std::make_unique<ShapeResult::ShapeRange[]>(num_text_items);
-    unsigned range_index = 0;
+    // "32" is heuristic, most major sites are up to 8 or so, wikipedia is 21.
+    Vector<ShapeResult::ShapeRange, 32> text_item_ranges;
+    text_item_ranges.ReserveInitialCapacity(num_text_items);
     for (; index < end_index; index++) {
       NGInlineItem& item = (*items)[index];
       if (item.Type() != NGInlineItem::kText || !item.Length())
@@ -1174,12 +1174,12 @@ void NGInlineNode::ShapeText(NGInlineItemsData* data,
       // item that has its first code unit keeps the glyph.
       scoped_refptr<ShapeResult> item_result =
           ShapeResult::CreateEmpty(*shape_result.get());
-      item.shape_result_ = item_result;
-      text_item_ranges[range_index++] = {item.StartOffset(), item.EndOffset(),
-                                         item_result.get()};
+      text_item_ranges.emplace_back(item.StartOffset(), item.EndOffset(),
+                                    item_result.get());
+      item.shape_result_ = std::move(item_result);
     }
-    DCHECK_EQ(range_index, num_text_items);
-    shape_result->CopyRanges(&text_item_ranges[0], num_text_items);
+    DCHECK_EQ(text_item_ranges.size(), num_text_items);
+    shape_result->CopyRanges(text_item_ranges.data(), text_item_ranges.size());
   }
 
 #if DCHECK_IS_ON()

third_party/blink/renderer/platform/fonts/shaping/shape_result.h

@@ -263,8 +263,9 @@ class PLATFORM_EXPORT ShapeResult : public RefCounted<ShapeResult> {
   void CopyRange(unsigned start, unsigned end, ShapeResult*) const;
 
   struct ShapeRange {
-    // ShapeRange(unsigned start, unsigned end, ShapeResult* target)
-    //    : start(start), end(end), target(target){};
+    ShapeRange(unsigned start, unsigned end, ShapeResult* target)
+        : start(start), end(end), target(target) {}
+
     unsigned start;
     unsigned end;
     ShapeResult* target;