Remove redundant reading and writing of data about SharedBuffer.

third_party/WebKit/Source/platform/SharedBuffer.cpp

Index: third_party/WebKit/Source/platform/SharedBuffer.cpp
diff --git a/third_party/WebKit/Source/platform/SharedBuffer.cpp b/third_party/WebKit/Source/platform/SharedBuffer.cpp
index add05a7b0e6dc5f8669f456b1237af1db5fb9cce..a696a696f89486ac84913c79c6b5b52234ba4fb7 100644
--- a/third_party/WebKit/Source/platform/SharedBuffer.cpp
+++ b/third_party/WebKit/Source/platform/SharedBuffer.cpp
@@ -32,32 +32,21 @@
 
 namespace blink {
 
-static inline size_t SegmentIndex(size_t position) {
-  return position / SharedBuffer::kSegmentSize;
+SharedBuffer::SharedBuffer() {
+  current_step_ = SharedBufferStep::Create();
 }
 
-static inline size_t OffsetInSegment(size_t position) {
-  return position % SharedBuffer::kSegmentSize;
+SharedBuffer::SharedBuffer(size_t size) {
+  current_step_ = SharedBufferStep::Create(size);
 }
 
-static inline char* AllocateSegment() {
-  return static_cast<char*>(WTF::Partitions::FastMalloc(
-      SharedBuffer::kSegmentSize, "blink::SharedBuffer"));
-}
-
-static inline void FreeSegment(char* p) {
-  WTF::Partitions::FastFree(p);
-}
-
-SharedBuffer::SharedBuffer() : size_(0) {}
-
-SharedBuffer::SharedBuffer(size_t size) : size_(size), buffer_(size) {}
-
-SharedBuffer::SharedBuffer(const char* data, size_t size) : size_(0) {
+SharedBuffer::SharedBuffer(const char* data, size_t size) {
+  current_step_ = SharedBufferStep::Create();
   AppendInternal(data, size);
 }
 
-SharedBuffer::SharedBuffer(const unsigned char* data, size_t size) : size_(0) {
+SharedBuffer::SharedBuffer(const unsigned char* data, size_t size) {
+  current_step_ = SharedBufferStep::Create();
   AppendInternal(reinterpret_cast<const char*>(data), size);
 }
 
@@ -67,18 +56,157 @@ SharedBuffer::~SharedBuffer() {
 
 PassRefPtr<SharedBuffer> SharedBuffer::AdoptVector(Vector<char>& vector) {
   RefPtr<SharedBuffer> buffer = Create();
-  buffer->buffer_.swap(vector);
-  buffer->size_ = buffer->buffer_.size();
+  buffer->current_step_ = SharedBufferStep::AdoptVector(vector);
   return buffer.Release();
 }
 
+SharedBuffer::ThreadSafeStepper::Steps::Steps() {
+  for (size_t index = 0; index < kStepArraySize; ++index) {
+    step_holder_arr[index].self_keep = false;
+    step_holder_arr[index].keep_ref = false;
+    step_holder_arr[index].keep_count = 0;
+  }
+  next = nullptr;
+}
+
+SharedBuffer::ThreadSafeStepper::ThreadSafeStepper(
+    PassRefPtr<SharedBufferStep> step) {
+  DCHECK(step);
+  steps_head_ = steps_tail_ = new Steps();
+  Steps::StepHolder* step_holder_to_write = steps_head_->step_holder_arr;
+  step_holder_to_write->step = std::move(step);
+  step_holder_to_write->self_keep = true;
+  step_holder_to_write->keep_ref = true;
+  std::atomic_store_explicit(&step_holder_to_write->keep_count,
+                             static_cast<size_t>(1), std::memory_order_relaxed);
+  steps_to_write_ = steps_head_;
+  to_write_index_ = 1;
+  step_holder_to_read_ = step_holder_to_write;
+}
+
+SharedBuffer::ThreadSafeStepper::~ThreadSafeStepper() {
+  for (Steps* steps = steps_head_; steps;) {
+    Steps* del_steps = steps;
+    steps = steps->next;
+    delete del_steps;
+  }
+}
+
+void SharedBuffer::ThreadSafeStepper::ClearStepHolderSelfKeep(
+    SharedBuffer::ThreadSafeStepper::Steps::StepHolder* step_holder) {
+  if (std::atomic_fetch_sub_explicit(&step_holder->keep_count,
+                                     static_cast<size_t>(1),
+                                     std::memory_order_relaxed) == 0) {
+    ClearStepHolder(step_holder);
+  }
+  step_holder->self_keep = false;
+}
+
+void SharedBuffer::ThreadSafeStepper::ClearStepHolder(
+    SharedBuffer::ThreadSafeStepper::Steps::StepHolder* step_holder) {
+  DCHECK_EQ(step_holder->keep_ref, true);
+  DCHECK_EQ(step_holder->keep_count, static_cast<size_t>(0));
+  step_holder->step.Clear();
+  step_holder->keep_ref = false;
+}
+
+PassRefPtr<SharedBufferStep> SharedBuffer::ThreadSafeStepper::current_step() {
+  while (true) {
+    if (!step_holder_to_read_->self_keep || !step_holder_to_read_->keep_ref) {
+      continue;
+    }
+    size_t keep_count = std::atomic_load_explicit(
+        &step_holder_to_read_->keep_count, std::memory_order_relaxed);
+    if (keep_count == 0) {
+      continue;
+    }
+    if (!std::atomic_compare_exchange_weak_explicit(
+            &step_holder_to_read_->keep_count, &keep_count, keep_count + 1,
+            std::memory_order_relaxed, std::memory_order_relaxed)) {
+      continue;
+    }
+
+    RefPtr<SharedBufferStep> step = step_holder_to_read_->step;
+    if (std::atomic_fetch_sub_explicit(&step_holder_to_read_->keep_count,
+                                       static_cast<size_t>(1),
+                                       std::memory_order_relaxed) == 0) {
+      ClearStepHolder(step_holder_to_read_);
+    }
+
+    return step.Release();
+  };
+
+  NOTREACHED();
+  return nullptr;
+}
+
+void SharedBuffer::ThreadSafeStepper::set_current_step(
+    PassRefPtr<SharedBufferStep> step) {
+  Steps::StepHolder* step_holder_to_write = nullptr;
+  if (to_write_index_ < Steps::kStepArraySize) {
+    step_holder_to_write = steps_to_write_->step_holder_arr + to_write_index_;
+  } else if (steps_to_write_->next) {
+    steps_to_write_ = steps_to_write_->next;
+    to_write_index_ = 0;
+    step_holder_to_write = steps_to_write_->step_holder_arr;
+  } else {
+    steps_to_write_ = steps_head_;
+    to_write_index_ = 0;
+    step_holder_to_write = steps_to_write_->step_holder_arr;
+  }
+
+  if (!step_holder_to_write || step_holder_to_write->keep_ref) {
+    steps_tail_->next = new Steps();
+    steps_tail_ = steps_tail_->next;
+    steps_to_write_ = steps_tail_;
+    to_write_index_ = 0;
+    step_holder_to_write = steps_to_write_->step_holder_arr;
+  }
+
+  step_holder_to_write->step = std::move(step);
+  step_holder_to_write->self_keep = true;
+  step_holder_to_write->keep_ref = true;
+  std::atomic_store_explicit(&step_holder_to_write->keep_count,
+                             static_cast<size_t>(1), std::memory_order_relaxed);
+  to_write_index_ += 1;
+
+  Steps::StepHolder* step_holder_to_clear = step_holder_to_read_;
+  step_holder_to_read_ = step_holder_to_write;
+  ClearStepHolderSelfKeep(step_holder_to_clear);
+}
+
+PassRefPtr<SharedBuffer::ThreadSafeStepper> SharedBuffer::thread_safe_stepper()
+    const {
+  if (thread_safe_stepper_) {
+    return thread_safe_stepper_;
+  }
+
+  thread_safe_stepper_ = ThreadSafeStepper::Create(current_step_);
+  return thread_safe_stepper_;
+}
+
+void SharedBuffer::SynchronizeStepWithThreadStepper() const {
+  if (!thread_safe_stepper_) {
+    return;
+  }
+
+  if (current_step_ != thread_safe_stepper_->current_step()) {
+    thread_safe_stepper_->set_current_step(current_step_);
+  }
+}
+
 size_t SharedBuffer::size() const {
-  return size_;
+  return current_step_->size();
 }
 
 const char* SharedBuffer::Data() const {
-  MergeSegmentsIntoBuffer();
-  return buffer_.data();
+  if (forward_step_if_needed()) {
+    current_step_ = current_step_->MergeSegmentsIntoBuffer(true);
+    SynchronizeStepWithThreadStepper();
+  } else {
+    current_step_->MergeSegmentsIntoBuffer(false);
+  }
+  return current_step_->buffer()->data();
 }
 
 void SharedBuffer::Append(PassRefPtr<SharedBuffer> data) {
@@ -91,39 +219,11 @@ void SharedBuffer::Append(PassRefPtr<SharedBuffer> data) {
 }
 
 void SharedBuffer::AppendInternal(const char* data, size_t length) {
-  if (!length)
-    return;
-
-  DCHECK_GE(size_, buffer_.size());
-  size_t position_in_segment = OffsetInSegment(size_ - buffer_.size());
-  size_ += length;
-
-  if (size_ <= kSegmentSize) {
-    // No need to use segments for small resource data.
-    buffer_.Append(data, length);
-    return;
-  }
-
-  char* segment;
-  if (!position_in_segment) {
-    segment = AllocateSegment();
-    segments_.push_back(segment);
-  } else
-    segment = segments_.back() + position_in_segment;
-
-  size_t segment_free_space = kSegmentSize - position_in_segment;
-  size_t bytes_to_copy = std::min(length, segment_free_space);
-
-  for (;;) {
-    memcpy(segment, data, bytes_to_copy);
-    if (length == bytes_to_copy)
-      break;
-
-    length -= bytes_to_copy;
-    data += bytes_to_copy;
-    segment = AllocateSegment();
-    segments_.push_back(segment);
-    bytes_to_copy = std::min(length, static_cast<size_t>(kSegmentSize));
+  if (forward_step_if_needed()) {
+    current_step_ = current_step_->Append(data, length, true);
+    SynchronizeStepWithThreadStepper();
+  } else {
+    current_step_->Append(data, length, false);
   }
 }
 
@@ -132,122 +232,46 @@ void SharedBuffer::Append(const Vector<char>& data) {
 }
 
 void SharedBuffer::Clear() {
-  for (size_t i = 0; i < segments_.size(); ++i)
-    FreeSegment(segments_[i]);
-
-  segments_.clear();
-  size_ = 0;
-  buffer_.clear();
+  if (forward_step_if_needed()) {
+    current_step_ = current_step_->Clear(true);
+    thread_safe_stepper_.Clear();
+  } else {
+    current_step_->Clear(false);
+  }
 }
 
 Vector<char> SharedBuffer::Copy() const {
   Vector<char> buffer;
-  buffer.ReserveInitialCapacity(size_);
+  buffer.ReserveInitialCapacity(current_step_->size());
 
   ForEachSegment([&buffer](const char* segment, size_t segment_size,
                            size_t segment_offset) {
     buffer.Append(segment, segment_size);
   });
 
-  DCHECK_EQ(buffer.size(), size_);
+  DCHECK_EQ(buffer.size(), current_step_->size());
   return buffer;
 }
 
-void SharedBuffer::MergeSegmentsIntoBuffer() const {
-  size_t buffer_size = buffer_.size();
-  if (size_ > buffer_size) {
-    size_t bytes_left = size_ - buffer_size;
-    for (size_t i = 0; i < segments_.size(); ++i) {
-      size_t bytes_to_copy =
-          std::min(bytes_left, static_cast<size_t>(kSegmentSize));
-      buffer_.Append(segments_[i], bytes_to_copy);
-      bytes_left -= bytes_to_copy;
-      FreeSegment(segments_[i]);
-    }
-    segments_.clear();
-  }
-}
-
 size_t SharedBuffer::GetSomeDataInternal(const char*& some_data,
                                          size_t position) const {
-  size_t total_size = size();
-  if (position >= total_size) {
-    some_data = 0;
-    return 0;
-  }
-
-  SECURITY_DCHECK(position < size_);
-  size_t consecutive_size = buffer_.size();
-  if (position < consecutive_size) {
-    some_data = buffer_.data() + position;
-    return consecutive_size - position;
-  }
-
-  position -= consecutive_size;
-  size_t segments = segments_.size();
-  size_t max_segmented_size = segments * kSegmentSize;
-  size_t segment = SegmentIndex(position);
-  if (segment < segments) {
-    size_t bytes_left = total_size - consecutive_size;
-    size_t segmented_size = std::min(max_segmented_size, bytes_left);
-
-    size_t position_in_segment = OffsetInSegment(position);
-    some_data = segments_[segment] + position_in_segment;
-    return segment == segments - 1 ? segmented_size - position
-                                   : kSegmentSize - position_in_segment;
-  }
-  NOTREACHED();
-  return 0;
+  return current_step_->GetSomeData(some_data, position);
 }
 
 bool SharedBuffer::GetBytesInternal(void* dest, size_t byte_length) const {
-  if (!dest)
-    return false;
-
-  const char* segment = nullptr;
-  size_t load_position = 0;
-  size_t write_position = 0;
-  while (byte_length > 0) {
-    size_t load_size = GetSomeDataInternal(segment, load_position);
-    if (load_size == 0)
-      break;
-
-    if (byte_length < load_size)
-      load_size = byte_length;
-    memcpy(static_cast<char*>(dest) + write_position, segment, load_size);
-    load_position += load_size;
-    write_position += load_size;
-    byte_length -= load_size;
-  }
-
-  return byte_length == 0;
+  return current_step_->GetBytes(dest, byte_length);
 }
 
 sk_sp<SkData> SharedBuffer::GetAsSkData() const {
-  size_t buffer_length = size();
-  sk_sp<SkData> data = SkData::MakeUninitialized(buffer_length);
-  char* buffer = static_cast<char*>(data->writable_data());
-  const char* segment = 0;
-  size_t position = 0;
-  while (size_t segment_size = GetSomeDataInternal(segment, position)) {
-    memcpy(buffer + position, segment, segment_size);
-    position += segment_size;
-  }
-
-  if (position != buffer_length) {
-    NOTREACHED();
-    // Don't return the incomplete SkData.
-    return nullptr;
-  }
-  return data;
+  return current_step_->GetAsSkData();
 }
 
 void SharedBuffer::OnMemoryDump(const String& dump_prefix,
                                 WebProcessMemoryDump* memory_dump) const {
-  if (buffer_.size()) {
+  if (current_step_->buffer()->size()) {
     WebMemoryAllocatorDump* dump =
         memory_dump->CreateMemoryAllocatorDump(dump_prefix + "/shared_buffer");
-    dump->AddScalar("size", "bytes", buffer_.size());
+    dump->AddScalar("size", "bytes", current_step_->buffer()->size());
     memory_dump->AddSuballocation(
         dump->Guid(), String(WTF::Partitions::kAllocatedObjectPoolName));
   } else {
@@ -255,7 +279,7 @@ void SharedBuffer::OnMemoryDump(const String& dump_prefix,
     // using fastMalloc.
     const String data_dump_name = dump_prefix + "/segments";
     auto dump = memory_dump->CreateMemoryAllocatorDump(data_dump_name);
-    dump->AddScalar("size", "bytes", size_);
+    dump->AddScalar("size", "bytes", size());
     memory_dump->AddSuballocation(
         dump->Guid(), String(WTF::Partitions::kAllocatedObjectPoolName));
   }