[layoutng] Prereserve vector size for the children/offset vectors

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I9eefb234301b9bc8a9c4912d032eaf83a0b80d93
Reviewed-on: https://chromium-review.googlesource.com/c/1253983
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596445}

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

@@ -141,6 +141,10 @@ scoped_refptr<NGLayoutResult> NGLineBoxFragmentBuilder::ToLineBoxFragment() {
     child->PropagateContentsInkOverflow(&contents_ink_overflow, child.Offset());
   }
 
+  // Because this vector will be long-lived, make sure to not waaste space.
+  // (We reserve an initial capacity when adding the first child)
+  if (children_.size())
+    children_.ShrinkToReasonableCapacity();
   scoped_refptr<const NGPhysicalLineBoxFragment> fragment =
       base::AdoptRef(new NGPhysicalLineBoxFragment(
           Style(), style_variant_, physical_size, children_,

third_party/blink/renderer/core/layout/ng/ng_container_fragment_builder.cc

@@ -104,6 +104,10 @@ NGContainerFragmentBuilder& NGContainerFragmentBuilder::AddChild(
         has_last_resort_break_ = true;
     }
   }
+  // Assume that if we have one child, we may have more than one and try to
+  // limit the number of allocations we would do in that case.
+  if (!children_.capacity())
+    children_.ReserveCapacity(16);
   children_.emplace_back(std::move(child), NGPhysicalOffset());
   offsets_.push_back(child_offset);
   return *this;

third_party/blink/renderer/core/layout/ng/ng_container_fragment_builder.h

@@ -31,6 +31,8 @@ class CORE_EXPORT NGContainerFragmentBuilder : public NGBaseFragmentBuilder {
   STACK_ALLOCATED();
 
  public:
+  typedef Vector<NGLogicalOffset, 16> OffsetVector;
+
   ~NGContainerFragmentBuilder() override;
 
   LayoutUnit InlineSize() const { return size_.inline_size; }
@@ -210,12 +212,17 @@ class CORE_EXPORT NGContainerFragmentBuilder : public NGBaseFragmentBuilder {
   // Store NGLinks rather than NGPhysicalOffsets even though we don't have the
   // offsets yet to allow us to move the entire vector to the fragment at
   // construction time.
+  // Unlike OffsetVector, we don't want to keep the inline capacity around
+  // because this vector will be long-lived, and it would be wasteful to reserve
+  // space for 16 children in every fragment. Instead, we reserve some initial
+  // capacity for it when adding the first child  and shrink it down when
+  // creating the fragment.
   Vector<NGLink> children_;
 
   // Logical offsets for the children. Stored as logical offsets as we can't
   // convert to physical offsets until layout of all children has been
   // determined.
-  Vector<NGLogicalOffset> offsets_;
+  OffsetVector offsets_;
 
   NGFloatTypes adjoining_floats_ = kFloatTypeNone;
 

third_party/blink/renderer/core/layout/ng/ng_fragment_builder.cc

@@ -310,6 +310,10 @@ scoped_refptr<NGLayoutResult> NGFragmentBuilder::ToBoxFragment(
     }
   }
 
+  // Because this vector will be long-lived, make sure to not waste space.
+  // (We reserve an initial capacity when adding the first child)
+  if (children_.size())
+    children_.ShrinkToReasonableCapacity();
   scoped_refptr<const NGPhysicalBoxFragment> fragment =
       base::AdoptRef(new NGPhysicalBoxFragment(
           layout_object_, Style(), style_variant_, physical_size, children_,

third_party/blink/renderer/core/layout/ng/ng_fragment_builder.h

@@ -129,8 +129,8 @@ class CORE_EXPORT NGFragmentBuilder final : public NGContainerFragmentBuilder {
   scoped_refptr<NGLayoutResult> Abort(NGLayoutResult::NGLayoutResultStatus);
 
   // A vector of child offsets. Initially set by AddChild().
-  const Vector<NGLogicalOffset>& Offsets() const { return offsets_; }
-  Vector<NGLogicalOffset>& MutableOffsets() { return offsets_; }
+  const OffsetVector& Offsets() const { return offsets_; }
+  OffsetVector& MutableOffsets() { return offsets_; }
 
   NGPhysicalFragment::NGBoxType BoxType() const;
   NGFragmentBuilder& SetBoxType(NGPhysicalFragment::NGBoxType);