Index: tools/relocation_packer/src/delta_encoder.h |
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+// 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 upper four bytes of the eight byte offset and |
+// addend are invariably zeroes. The difference between adjacent offsets |
+// is almost always small, and between adjacent addends is often small. And |
+// info is constant and 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_ |