As part of identifying redundant allocations in Chrome, I found that memory allocated by ContiguousContainerBase::Buffer::Buffer() sometimes is not really used, resulting in 98 KiB of zeroes (14 * 7200), see https://docs.google.com/spreadsheets/d/1HfHKXy4q-ytZ-nwPay8YYyVSn1o-X9I7cWzYlygvtwQ/edit#gid=167958829

Backtrace misses some calls (because of inlining), but here is what happens (Android, 32-bit):

* DisplayItemList::Inputs::Inputs() creates ContiguousContainerBase object with the second argument of 7200 (LargestDisplayItemSize() * kDefaultNumDisplayItemsToReserve = 72 * 100)
* ContiguousContainerBase::ContiguousContainerBase() calls AllocateNewBufferForNextAllocation() with 7200
* AllocateNewBufferForNextAllocation() creates ContiguousContainerBase::Buffer() with 7200
* ContiguousContainerBase::Buffer() allocates 7200 bytes
* In some cases preallocated Buffer object is never written to (via Allocate()), and its data_ remains blob of zeroes

I'm proposing deferring allocation of Buffer::data_ until it's really needed, i.e. until the first Buffer::Allocate() call. The change is simple:

diff --git a/cc/base/contiguous_container.cc b/cc/base/contiguous_container.cc
index d6280b4..6439ceb 100644
--- a/cc/base/contiguous_container.cc
+++ b/cc/base/contiguous_container.cc
@@ -17,7 +17,7 @@ static const unsigned kDefaultInitialBufferSize = 32;
 class ContiguousContainerBase::Buffer {
  public:
   explicit Buffer(size_t buffer_size)
-      : data_(new char[buffer_size]), end_(begin()), capacity_(buffer_size) {}
+      : end_(nullptr), capacity_(buffer_size) {}
 
   ~Buffer() {}
 
@@ -28,6 +28,10 @@ class ContiguousContainerBase::Buffer {
 
   void* Allocate(size_t object_size) {
     DCHECK_GE(UnusedCapacity(), object_size);
+    if (!data_) {
+      data_.reset(new char[capacity_]);
+      end_ = begin();
+    }
     void* result = end_;
     end_ += object_size;
     return result;
@@ -40,8 +44,8 @@ class ContiguousContainerBase::Buffer {
   }
 
  private:
-  char* begin() { return &data_[0]; }
-  const char* begin() const { return &data_[0]; }
+  char* begin() { return data_.get(); }
+  const char* begin() const { return data_.get(); }
 
   // begin() <= end_ <= begin() + capacity_
   std::unique_ptr<char[]> data_;


(More info about the effort to kill redundant allocations is here: https://groups.google.com/a/chromium.org/forum/#!msg/project-trim/EiKXwnbTvbo/0z-Y5Kg1CgAJ)