Add a host tool to pack R_ARM_RELATIVE relocations in libchrome.<ver>.so.

tools/relocation_packer/src/packer.cc

+// Copyright 2014 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.
+
+// TODO(simonb): Extend for 64-bit target libraries.
+
+#include "packer.h"
+
+#include <string.h>
+#include <string>
+#include <vector>
+
+#include "debug.h"
+#include "leb128.h"
+#include "run_length_encoder.h"
+
+namespace relocation_packer {
+
+// Pack R_ARM_RELATIVE relocations into a run-length encoded packed
+// representation.
+void RelocationPacker::PackRelativeRelocations(
+    const std::vector<Elf32_Rel>& relocations,
+    std::vector<uint8_t>* packed) {
+
+  // Run-length encode.
+  std::vector<Elf32_Word> packed_words;
+  RelocationRunLengthCodec codec;
+  codec.Encode(relocations, &packed_words);
+
+  // If insufficient data to run-length encode, do nothing.
+  if (packed_words.empty())
+    return;
+
+  // LEB128 encode, with "APR1" prefix.
+  Leb128Encoder encoder;
+  encoder.Enqueue('A');
+  encoder.Enqueue('P');
+  encoder.Enqueue('R');
+  encoder.Enqueue('1');
+  encoder.EnqueueAll(packed_words);
+
+  encoder.GetEncoding(packed);
+
+  // Pad packed to a whole number of words.  This padding will decode as
+  // LEB128 zeroes.  Run-length decoding ignores it because encoding
+  // embeds the pairs count in the stream itself.
+  while (packed->size() % sizeof(uint32_t))
+    packed->push_back(0);
+}
+
+// Unpack R_ARM_RELATIVE relocations from a run-length encoded packed
+// representation.
+void RelocationPacker::UnpackRelativeRelocations(
+    const std::vector<uint8_t>& packed,
+    std::vector<Elf32_Rel>* relocations) {
+
+  // LEB128 decode, after checking and stripping "APR1" prefix.
+  std::vector<Elf32_Word> packed_words;
+  Leb128Decoder decoder(packed);
+  CHECK(decoder.Dequeue() == 'A' && decoder.Dequeue() == 'P' &&
+        decoder.Dequeue() == 'R' && decoder.Dequeue() == '1');
+  decoder.DequeueAll(&packed_words);
+
+  // Run-length decode.
+  RelocationRunLengthCodec codec;
+  codec.Decode(packed_words, relocations);
+}
+
+}  // namespace relocation_packer