Chromium Code Reviews| Index: tools/relocation_packer/src/delta_encoder.h |
| diff --git a/tools/relocation_packer/src/delta_encoder.h b/tools/relocation_packer/src/delta_encoder.h |
| new file mode 100644 |
| index 0000000000000000000000000000000000000000..9c5a35fc450d2436d00d5a2f047872a4b7c6daf1 |
| --- /dev/null |
| +++ b/tools/relocation_packer/src/delta_encoder.h |
| @@ -0,0 +1,79 @@ |
| +// 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
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_ |