Update from chromium 62675d9fb31fb8cedc40f68e78e8445a74f362e7

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 relative relocations.
+//
+// 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 no addend is 8 bytes on 32 bit platforms and 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 constant.  Example, from 'readelf -x4 libchrome.<version>.so' 32 bit:
+//
+//   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
+//   01ef8f80 00000006 00000004
+//
+// Because relative relocation offsets strictly increase, the complete
+// set of 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
+//   01ef8f80 00000006   00000004   00000010   00000008 ...
+//
+// Decoding regenerates relative relocations beginning at address
+// 'start' and for each encoded run, incrementing the address by 'difference'
+// for 'count' iterations and emitting a new relative relocation.
+//
+// Once encoded, data is prefixed by a single word count of packed delta and
+// count pairs.  A final run-length encoded relative relocations vector
+// might therefore look something like:
+//
+//   pairs    start    run 1             run 2             ... run 15
+//   0000000f 01ef8f80 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 <vector>
+
+#include "elf.h"
+#include "elf_traits.h"
+
+namespace relocation_packer {
+
+// A RelocationRunLengthCodec packs vectors of relative relocations
+// into more compact forms, and unpacks them to reproduce the pre-packed data.
+class RelocationRunLengthCodec {
+ public:
+  // Encode relative relocations into a more compact form.
+  // |relocations| is a vector of relative relocation structs.
+  // |packed| is the vector of packed words into which relocations are packed.
+  static void Encode(const std::vector<ELF::Rel>& relocations,
+                     std::vector<ELF::Xword>* packed);
+
+  // Decode relative relocations from their more compact form.
+  // |packed| is the vector of packed relocations.
+  // |relocations| is a vector of unpacked relative relocation structs.
+  static void Decode(const std::vector<ELF::Xword>& packed,
+                     std::vector<ELF::Rel>* relocations);
+};
+
+}  // namespace relocation_packer
+
+#endif  // TOOLS_RELOCATION_PACKER_SRC_RUN_LENGTH_ENCODER_H_