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

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_