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

tools/relocation_packer/src/packer.h

+// 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.
+
+// Pack R_ARM_RELATIVE relocations into a more compact form.
+//
+// Applies two packing strategies.  The first is run-length encoding, which
+// turns a large set of R_ARM_RELATIVE relocations into a much smaller set
+// of delta-count pairs, prefixed with a two-word header comprising the
+// count of pairs and the initial relocation offset.  The second is LEB128
+// encoding, which compacts the result of run-length encoding.
+//
+// Once packed, data is prefixed by an identifier that allows for any later
+// versioning of packing strategies.
+//
+// A complete packed stream might look something like:
+//
+//   "APR1"   pairs  init_offset count1 delta1 count2 delta2 ...
+//   41505231 f2b003 b08ac716    e001   04     01     10     ...
+
+#ifndef TOOLS_RELOCATION_PACKER_SRC_PACKER_H_
+#define TOOLS_RELOCATION_PACKER_SRC_PACKER_H_
+
+#include <stdint.h>
+#include <string.h>
+#include <vector>
+
+#include "elf.h"
+
+namespace relocation_packer {
+
+// A RelocationPacker packs vectors of R_ARM_RELATIVE relocations into more
+// compact forms, and unpacks them to reproduce the pre-packed data.
+class RelocationPacker {
+ public:
+  // Pack R_ARM_RELATIVE relocations into a more compact form.
+  // |relocations| is a vector of R_ARM_RELATIVE relocation structs.
+  // |packed| is the vector of packed bytes into which relocations are packed.
+  static void PackRelativeRelocations(const std::vector<Elf32_Rel>& relocations,
+                                      std::vector<uint8_t>* packed);
+
+  // Unpack R_ARM_RELATIVE relocations from their more compact form.
+  // |packed| is the vector of packed relocations.
+  // |relocations| is a vector of unpacked R_ARM_RELATIVE relocation structs.
+  static void UnpackRelativeRelocations(const std::vector<uint8_t>& packed,
+                                        std::vector<Elf32_Rel>* relocations);
+};
+
+}  // namespace relocation_packer
+
+#endif  // TOOLS_RELOCATION_PACKER_SRC_PACKER_H_