[swiftBasic] Introduce an exponentially growing appending binary stream

It is more efficient than llvm::AppendingBinaryByteStream if a lot of
small data gets appended to it because it doesn't need to resize its
buffer on each write.

lib/Basic/ExponentialGrowthAppendingBinaryByteStream.cpp

@@ -0,0 +1,69 @@
+//===-------- ExponentialGrowthAppendingBinaryByteStream.cpp --------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#include "swift/Basic/ExponentialGrowthAppendingBinaryByteStream.h"
+
+using namespace llvm;
+using namespace swift;
+
+Error ExponentialGrowthAppendingBinaryByteStream::readBytes(
+    uint32_t Offset, uint32_t Size, ArrayRef<uint8_t> &Buffer) {
+  if (Offset > this->Size)
+    return make_error<BinaryStreamError>(stream_error_code::invalid_offset);
+
+  if (Offset + Size > this->Size)
+    return make_error<BinaryStreamError>(stream_error_code::stream_too_short);
+
+  Buffer = ArrayRef<uint8_t>(Data + Offset, Size);
+  return Error::success();
+}
+
+Error ExponentialGrowthAppendingBinaryByteStream::readLongestContiguousChunk(
+    uint32_t Offset, ArrayRef<uint8_t> &Buffer) {
+  if (Offset > this->Size)
+    return make_error<BinaryStreamError>(stream_error_code::invalid_offset);
+
+  Buffer = ArrayRef<uint8_t>(Data + Offset, Size - Offset);
+  return Error::success();
+}
+
+void ExponentialGrowthAppendingBinaryByteStream::reserve(size_t Size) {
+  if (Size <= ReservedBufferSize) {
+    // The buffer is already large enough. Nothing to do.
+    return;
+  }
+  Data = (uint8_t *)realloc(Data, Size);
+  ReservedBufferSize = Size;
+}
+
+Error ExponentialGrowthAppendingBinaryByteStream::writeBytes(
+    uint32_t Offset, ArrayRef<uint8_t> Buffer) {
+  if (Buffer.empty())
+    return Error::success();
+
+  if (Offset > Size)
+    return make_error<BinaryStreamError>(stream_error_code::invalid_offset);
+
+  // Resize the internal buffer if needed.
+  uint32_t RequiredSize = Offset + Buffer.size();
+  uint32_t NewBufferSize = ReservedBufferSize;
+  // Double the buffer size until its size is sufficient to hold the new data.
+  while (RequiredSize > NewBufferSize) {
+    NewBufferSize *= 2;
+  }
+  reserve(NewBufferSize);
+
+  Size += Buffer.size();
+  ::memcpy(Data + Offset, Buffer.data(), Buffer.size());
+
+  return Error::success();
+}