tools/relocation_packer/src/run_length_encoder.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.

// Run-length encode and decode R_ARM_RELATIVE relocations.
//
// R_ARM_RELATIVE relocations are the bulk of dynamic relocations (the
// .rel.dyn section) in libchrome<version>.so, and the ELF standard
// representation of them is wasteful.
//
// A relocation is 8 bytes (16 bytes on 64 bit plaforms), split into offset
// and info fields.  Offsets strictly increase, and each is commonly a
// few bytes different from its predecessor.  There are long runs where
// the difference does not change.  The info field is always 0x17.  Example,
// from 'readelf -x4 libchrome.<version>.so':
//
//   offset   info     offset   info
//   808fef01 17000000 848fef01 17000000 ................
//   888fef01 17000000 8c8fef01 17000000 ................
//   908fef01 17000000 948fef01 17000000 ................
//
// Run length encoding packs this data more efficiently, by representing it
// as a delta and a count of entries each differing from its predecessor
// by this delta.  The above can be represented as a start address followed
// by an encoded count of 6 and offset difference of 4:
//
//   start    count    diff
//   808fef01 00000006 00000004
//
// Because R_ARM_RELATIVE relocation offsets strictly increase, the complete
// set of R_ARM_RELATIVE relocations in libchrome.<version>.so can be
// represented by a single start address followed by one or more difference
// and count encoded word pairs:
//
//   start    run1 count run1 diff  run2 count run2 diff
//   808fef01 00000006   00000004   00000010   00000008 ...
//
// Decoding regenerates R_ARM_RELATIVE relocations beginning at address
// 'start' and for each encoded run, incrementing the address by 'difference'
// for 'count' iterations and emitting a new R_ARM_RELATIVE relocation.
//
// Once encoded, data is prefixed by a single word count of packed delta and
// count pairs.  A final run-length encoded R_ARM_RELATIVE relocations vector
// might therefore look something like:
//
//   pairs    start    run 1             run 2             ... run 15
//   0000000f 808fef01 00000006 00000004 00000010 00000008 ...
// Interpreted as:
//   pairs=15 start=.. count=6,delta=4   count=16,delta=8

#ifndef TOOLS_RELOCATION_PACKER_SRC_RUN_LENGTH_ENCODER_H_
#define TOOLS_RELOCATION_PACKER_SRC_RUN_LENGTH_ENCODER_H_

#include <stdint.h>
#include <string.h>
#include <vector>

#include "elf.h"

namespace relocation_packer {

// A RelocationRunLengthCodec packs vectors of R_ARM_RELATIVE relocations
// into more compact forms, and unpacks them to reproduce the pre-packed data.
class RelocationRunLengthCodec {
 public:
  // Encode 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 words into which relocations are packed.
  static void Encode(const std::vector<Elf32_Rel>& relocations,
                     std::vector<Elf32_Word>* packed);

  // Decode 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 Decode(const std::vector<Elf32_Word>& packed,
                     std::vector<Elf32_Rel>* relocations);
};

}  // namespace relocation_packer

#endif  // TOOLS_RELOCATION_PACKER_SRC_RUN_LENGTH_ENCODER_H_