Fix alignment issue of ContiguousContainer

cc/base/contiguous_container.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/base/contiguous_container.h"
 
 #include <stddef.h>
 
 #include <utility>
 
 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, unsigned log2_alignment) {
+  char* aligned = reinterpret_cast<char*>(
+      (((reinterpret_cast<size_t>(address) - 1) >> log2_alignment) + 1)
+      << log2_alignment);
+  DCHECK_EQ(0u, reinterpret_cast<size_t>(aligned) % (1 << log2_alignment));
+  DCHECK(aligned >= address);
+  DCHECK(aligned < address + (1 << log2_alignment));
+  return aligned;
+}
+
+}  // namespace
 
 class ContiguousContainerBase::Buffer {
  public:
   explicit Buffer(size_t buffer_size)
       : data_(new char[buffer_size]), end_(begin()), capacity_(buffer_size) {}
 
   ~Buffer() {}
 
   size_t Capacity() const { return capacity_; }
   size_t UsedCapacity() const { return end_ - begin(); }
   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, unsigned log2_alignment) {
+    char* aligned_address;
+    if (empty()) {
+      // begin()'s alignment must be suitable for all possible types.
+      DCHECK_EQ(begin(), AlignAddress(begin(), log2_alignment));
+      DCHECK(object_size <= capacity_);
+      aligned_address = begin();
+    } else {
+      aligned_address = AlignAddress(end_, log2_alignment);
+      if (aligned_address - begin() + object_size > capacity_)
+        return nullptr;
+    }
+    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);
+    // 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_
   std::unique_ptr<char[]> data_;
   char* end_;
   size_t capacity_;
@@ skipping 23 matching lines @@
   for (const auto& buffer : buffers_)
     used_capacity += buffer->UsedCapacity();
   return used_capacity;
 }
 
 size_t ContiguousContainerBase::MemoryUsageInBytes() const {
   return sizeof(*this) + GetCapacityInBytes() +
          elements_.capacity() * sizeof(elements_[0]);
 }
 
-void* ContiguousContainerBase::Allocate(size_t object_size) {
+void* ContiguousContainerBase::Allocate(size_t object_size,
+                                        unsigned log2_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, log2_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, log2_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;
 }
 
 void ContiguousContainerBase::RemoveLast() {
   void* object = elements_.back();
   elements_.pop_back();
 
   Buffer* end_buffer = buffers_[end_index_].get();
   end_buffer->DeallocateLastObject(object);
@@ skipping 24 matching lines @@
     size_t buffer_size) {
   DCHECK(buffers_.empty() || end_index_ == buffers_.size() - 1);
   std::unique_ptr<Buffer> new_buffer(new Buffer(buffer_size));
   Buffer* buffer_to_return = new_buffer.get();
   buffers_.push_back(std::move(new_buffer));
   end_index_ = buffers_.size() - 1;
   return buffer_to_return;
 }
 
 }  // namespace cc