From 2665bdd6d29ec2515946ac629002c04c23a50bfa Mon Sep 17 00:00:00 2001
From: "Mark D. Roth" <roth@google.com>
Date: Thu, 18 Aug 2016 08:02:24 -0700
Subject: [PATCH] Reduce allocations associated with
 src/cpp/util/byte_buffer.cc.

---
 include/grpc++/support/byte_buffer.h |  3 +++
 src/cpp/util/byte_buffer.cc          | 25 +++++++++++++++++++------
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/include/grpc++/support/byte_buffer.h b/include/grpc++/support/byte_buffer.h
index 20bd407109127..01249a0b88bd0 100644
--- a/include/grpc++/support/byte_buffer.h
+++ b/include/grpc++/support/byte_buffer.h
@@ -72,6 +72,9 @@ class ByteBuffer GRPC_FINAL {
   /// Buffer size in bytes.
   size_t Length() const;
 
+  /// Swap the state of *this and *other.
+  void Swap(ByteBuffer* other);
+
  private:
   friend class SerializationTraits<ByteBuffer, void>;
 
diff --git a/src/cpp/util/byte_buffer.cc b/src/cpp/util/byte_buffer.cc
index c2cd20ee07f0a..4c4772a92b30d 100644
--- a/src/cpp/util/byte_buffer.cc
+++ b/src/cpp/util/byte_buffer.cc
@@ -37,12 +37,19 @@
 namespace grpc {
 
 ByteBuffer::ByteBuffer(const Slice* slices, size_t nslices) {
-  // TODO(yangg) maybe expose some core API to simplify this
-  std::vector<gpr_slice> c_slices(nslices);
-  for (size_t i = 0; i < nslices; i++) {
-    c_slices[i] = slices[i].slice_;
-  }
-  buffer_ = grpc_raw_byte_buffer_create(c_slices.data(), nslices);
+  // The following assertions check that the representation of a grpc::Slice is
+  // identical to that of a gpr_slice:  it has a gpr_slice field, and nothing
+  // else.
+  static_assert(std::is_same<decltype(slices[0].slice_), gpr_slice>::value,
+                "Slice must have same representation as gpr_slice");
+  static_assert(sizeof(Slice) == sizeof(gpr_slice),
+                "Slice must have same representation as gpr_slice");
+  // The const_cast is legal if grpc_raw_byte_buffer_create() does no more
+  // than its advertised side effect of increasing the reference count of the
+  // slices it processes, and such an increase does not affect the semantics
+  // seen by the caller of this constructor.
+  buffer_ = grpc_raw_byte_buffer_create(
+      reinterpret_cast<gpr_slice*>(const_cast<Slice*>(slices)), nslices);
 }
 
 ByteBuffer::~ByteBuffer() {
@@ -95,4 +102,10 @@ ByteBuffer& ByteBuffer::operator=(const ByteBuffer& buf) {
   return *this;
 }
 
+void ByteBuffer::Swap(ByteBuffer* other) {
+  grpc_byte_buffer* tmp = other->buffer_;
+  other->buffer_ = this->buffer_;
+  this->buffer_ = tmp;
+}
+
 }  // namespace grpc