Reduce number of vector buffer reallocs in Regions::unite .

Source/platform/geometry/Region.cpp

Index: Source/platform/geometry/Region.cpp
diff --git a/Source/platform/geometry/Region.cpp b/Source/platform/geometry/Region.cpp
index e755a8b11680dd3f5ecb5ff72de35b373958d9a2..ff86e137a5118b5573612efdb1627657f449d2fb 100644
--- a/Source/platform/geometry/Region.cpp
+++ b/Source/platform/geometry/Region.cpp
@@ -207,6 +207,12 @@ bool Region::Shape::compareShapes(const Shape& aShape, const Shape& bShape)
     return result;
 }
 
+void Region::Shape::trimCapacities()
+{
+    m_segments.shrinkToReasonableCapacity();
+    m_spans.shrinkToReasonableCapacity();
+}
+
 struct Region::Shape::CompareContainsOperation {
     const static bool defaultResult = true;
     inline static bool aOutsideB(bool& /* result */) { return false; }
@@ -233,6 +239,12 @@ Region::Shape::Shape(const IntRect& rect)
     appendSpan(rect.maxY());
 }
 
+Region::Shape::Shape(size_t segmentsCapacity, size_t spansCapacity)
+{
+    m_segments.reserveCapacity(segmentsCapacity);
+    m_spans.reserveCapacity(spansCapacity);
+}
+
 void Region::Shape::appendSpan(int y)
 {
     m_spans.append(Span(y, m_segments.size()));
@@ -393,7 +405,9 @@ Region::Shape Region::Shape::shapeOperation(const Shape& shape1, const Shape& sh
     COMPILE_ASSERT(!(!Operation::shouldAddRemainingSegmentsFromSpan1 && Operation::shouldAddRemainingSegmentsFromSpan2), invalid_segment_combination);
     COMPILE_ASSERT(!(!Operation::shouldAddRemainingSpansFromShape1 && Operation::shouldAddRemainingSpansFromShape2), invalid_span_combination);
 
-    Shape result;
+    size_t segmentsCapacity = shape1.segmentsSize() + shape2.segmentsSize();
+    size_t spansCapacity = shape1.spansSize() + shape2.spansSize();
+    Shape result(segmentsCapacity, spansCapacity);
     if (Operation::trySimpleOperation(shape1, shape2, result))
         return result;
 
@@ -409,6 +423,9 @@ Region::Shape Region::Shape::shapeOperation(const Shape& shape1, const Shape& sh
     SegmentIterator segments2 = 0;
     SegmentIterator segments2End = 0;
 
+    Vector<int, 32> segments;
+    segments.reserveCapacity(std::max(shape1.segmentsSize(), shape2.segmentsSize()));
+
     // Iterate over all spans.
     while (spans1 != spans1End && spans2 != spans2End) {
         int y = 0;
@@ -435,7 +452,9 @@ Region::Shape Region::Shape::shapeOperation(const Shape& shape1, const Shape& sh
         SegmentIterator s1 = segments1;
         SegmentIterator s2 = segments2;
 
-        Vector<int, 32> segments;
+        // Clear vector without dropping capacity.
+        segments.resize(0);
+        ASSERT(segments.capacity());
 
         // Now iterate over the segments in each span and construct a new vector of segments.
         while (s1 != segments1End && s2 != segments2End) {
@@ -476,6 +495,8 @@ Region::Shape Region::Shape::shapeOperation(const Shape& shape1, const Shape& sh
     else if (Operation::shouldAddRemainingSpansFromShape2 && spans2 != spans2End)
         result.appendSpans(shape2, spans2, spans2End);
 
+    result.trimCapacities();
+
     return result;
 }