Extend relocation packing to cover arm64.

tools/relocation_packer/src/delta_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.
+
+// Delta encode and decode relative relocations with addends.
+//
+// Relative relocations are the bulk of dynamic relocations (the
+// .rel.dyn or .rela.dyn sections) in libchrome<version>.so, and the ELF
+// standard representation of them is wasteful.  .rel.dyn contains
+// relocations without addends, .rela.dyn relocations with addends.
+//
+// A relocation with an addend is 12 bytes on 32 bit platforms and 24 bytes
+// on 64 bit plaforms.  It is split into offset, info, and addend fields.
+// Offsets strictly increase, and each is commonly a few bytes different
+// from its predecessor.  Addends are less well behaved.  The info field is
+// constant.  Example, from 'readelf -x4 libchrome.<version>.so' 64 bit:
+//
+//   offset            info
+//   80949303 00000000 03040000 00000000 ................
+//   addend            offset
+//   fc015b00 00000000 88949303 00000000 ..[.............
+//   info              addend
+//   03040000 00000000 24025b00 00000000 ........$.[.....
+//   offset            info
+//   90949303 00000000 03040000 00000000 ................
+//   addend            offset
+//   3c025b00 00000000 98949303 00000000 <.[.............
+//   info              addend
+//   03040000 00000000 50025b00 00000000 ........P.[.....
+//
+// The offset strictly increases, but the addend is unpredictable, so run
+// length encoding will not work well with this data.  We can however pack
+// with delta encoding.  The difference between offsets is small, between
+// addends is sometimes small, four of eight bytes are almost always

[rmcilroy, 2014/07/28 10:08:44]

"small - four of the eight bytes are almost always unused - and the..."

[simonb (inactive), 2014/07/28 12:20:56]

Reworded for clarity.

+// unused, and the info is constant and so can be eliminated.
+//
+// Delta encoding reduces the size of the data modestly, so that the first
+// three relocations above can be represented as:
+//
+// initial offset    initial addend    offset delta      addend delta
+// 00000000 03939480 00000000 005b01fc 00000000 00000008 00000000 00000028
+// offset delta      addend delta      ...
+// 00000000 00000008 00000000 0000009f
+//
+// The addend delta can be negative as well as positive, but overall the
+// deltas have a much smaller range than the input data.  When encoded as
+// signed LEB128 the total data reduction becomes useful.
+
+#ifndef TOOLS_RELOCATION_PACKER_SRC_DELTA_ENCODER_H_
+#define TOOLS_RELOCATION_PACKER_SRC_DELTA_ENCODER_H_
+
+#include <vector>
+
+#include "elf.h"
+#include "elf_traits.h"
+
+namespace relocation_packer {
+
+// A RelocationDeltaCodec packs vectors of relative relocations with
+// addends into more compact forms, and unpacks them to reproduce the
+// pre-packed data.
+class RelocationDeltaCodec {
+ public:
+  // Encode relative relocations with addends into a more compact form.
+  // |relocations| is a vector of relative relocation with addend structs.
+  // |packed| is the vector of packed words into which relocations are packed.
+  static void Encode(const std::vector<ELF::Rela>& relocations,
+                     std::vector<ELF::Sxword>* packed);
+
+  // Decode relative relocations with addends from their more compact form.
+  // |packed| is the vector of packed relocations.
+  // |relocations| is a vector of unpacked relative relocations.
+  static void Decode(const std::vector<ELF::Sxword>& packed,
+                     std::vector<ELF::Rela>* relocations);
+};
+
+}  // namespace relocation_packer
+
+#endif  // TOOLS_RELOCATION_PACKER_SRC_DELTA_ENCODER_H_