Fix alignment issue of ContiguousContainer

cc/base/contiguous_container.cc

Index: cc/base/contiguous_container.cc
diff --git a/cc/base/contiguous_container.cc b/cc/base/contiguous_container.cc
index d6280b43290b8d4d9547f56a1a62b070caf63e66..6e9a3c1139d23c93d75c69a9224c9e20b3839032 100644
--- a/cc/base/contiguous_container.cc
+++ b/cc/base/contiguous_container.cc
@@ -10,9 +10,23 @@
 
 namespace cc {
 
+namespace {
+
 // Default number of max-sized elements to allocate space for, if there is no
 // initial buffer.
-static const unsigned kDefaultInitialBufferSize = 32;
+const unsigned kDefaultInitialBufferSize = 32;
+
+inline char* AlignAddress(char* address, size_t alignment) {
+  char* aligned = reinterpret_cast<char*>(
+      alignment *
+      ((reinterpret_cast<size_t>(address) + alignment - 1) / alignment));
+  DCHECK(reinterpret_cast<size_t>(aligned) % alignment == 0);
+  DCHECK(aligned >= address);
+  DCHECK(aligned < address + alignment);
+  return aligned;
+}
+
+}  // namespace
 
 class ContiguousContainerBase::Buffer {
  public:
@@ -26,25 +40,38 @@ class ContiguousContainerBase::Buffer {
   size_t UnusedCapacity() const { return Capacity() - UsedCapacity(); }
   bool empty() const { return UsedCapacity() == 0; }
 
-  void* Allocate(size_t object_size) {
-    DCHECK_GE(UnusedCapacity(), object_size);
-    void* result = end_;
-    end_ += object_size;
-    return result;
+  void* Allocate(size_t object_size, size_t alignment) {
+    char* aligned_address = AlignAddress(end_, alignment);
+    if (aligned_address - begin() + object_size > Capacity())
+      return nullptr;
+    if (empty())
+      first_object_ = aligned_address;
+    end_ = aligned_address + object_size;
+    return aligned_address;
   }
 
   void DeallocateLastObject(void* object) {
     DCHECK_LE(begin(), object);
     DCHECK_LT(object, end_);
     end_ = static_cast<char*>(object);
+    if (end_ == first_object_) {
+      // We have deallocated the first object in this buffer. Reset en_ so that
+      // empty() can return correct value.
+      end_ = begin();
+    }
+    // Otherwise we may leave a gap between the end of the previous object
+    // (which we don't know) and end_ because of alignment of the deallocated
+    // object.
   }
 
  private:
   char* begin() { return &data_[0]; }
   const char* begin() const { return &data_[0]; }
 
-  // begin() <= end_ <= begin() + capacity_
+  // begin() <= first_object_ <= end_ <= begin() + capacity_
   std::unique_ptr<char[]> data_;
+  // Because of alignment, the first object may be allocated after begin().
+  char* first_object_;
   char* end_;
   size_t capacity_;
 };
@@ -80,26 +107,24 @@ size_t ContiguousContainerBase::MemoryUsageInBytes() const {
          elements_.capacity() * sizeof(elements_[0]);
 }
 
-void* ContiguousContainerBase::Allocate(size_t object_size) {
+void* ContiguousContainerBase::Allocate(size_t object_size, size_t alignment) {
   DCHECK_LE(object_size, max_object_size_);
 
-  Buffer* buffer_for_alloc = nullptr;
-  if (!buffers_.empty()) {
-    Buffer* end_buffer = buffers_[end_index_].get();
-    if (end_buffer->UnusedCapacity() >= object_size)
-      buffer_for_alloc = end_buffer;
-    else if (end_index_ + 1 < buffers_.size())
+  void* element = buffers_.empty() ? nullptr : buffers_[end_index_]->Allocate(
+                                                   object_size, alignment);
+  if (!element) {
+    Buffer* buffer_for_alloc;
+    if (end_index_ + 1 < buffers_.size()) {
       buffer_for_alloc = buffers_[++end_index_].get();
+    } else {
+      size_t new_buffer_size =
+          buffers_.empty() ? kDefaultInitialBufferSize * max_object_size_
+                           : 2 * buffers_.back()->Capacity();
+      buffer_for_alloc = AllocateNewBufferForNextAllocation(new_buffer_size);
+    }
+    element = buffer_for_alloc->Allocate(object_size, alignment);
+    DCHECK(element);
   }
-
-  if (!buffer_for_alloc) {
-    size_t new_buffer_size = buffers_.empty()
-                                 ? kDefaultInitialBufferSize * max_object_size_
-                                 : 2 * buffers_.back()->Capacity();
-    buffer_for_alloc = AllocateNewBufferForNextAllocation(new_buffer_size);
-  }
-
-  void* element = buffer_for_alloc->Allocate(object_size);
   elements_.push_back(element);
   return element;
 }