Extend relocation packing to cover arm64.

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 ARM relative relocations.
+// Run-length encode and decode relative relocations.
 //
-// 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.
+// 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 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':
+// 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
-//   808fef01 00000006 00000004
+//   01ef8f80 00000006 00000004
 //
-// Because ARM relative relocation offsets strictly increase, the complete
-// set of ARM relative relocations in libchrome.<version>.so can be
+// 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
-//   808fef01 00000006   00000004   00000010   00000008 ...
+//   01ef8f80 00000006   00000004   00000010   00000008 ...
 //
-// Decoding regenerates ARM relative relocations beginning at address
+// 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 ARM relative relocation.
+// 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 ARM relative relocations vector
+// count pairs.  A final run-length encoded relative relocations vector
 // might therefore look something like:
 //
 //   pairs    start    run 1             run 2             ... run 15
-//   0000000f 808fef01 00000006 00000004 00000010 00000008 ...
+//   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 ARM relative relocations
+// A RelocationRunLengthCodec packs vectors of relative relocations
 // into more compact forms, and unpacks them to reproduce the pre-packed data.
 class RelocationRunLengthCodec {
  public:
-  // Encode ARM relative relocations into a more compact form.
-  // |relocations| is a vector of ARM relative relocation structs.
+  // 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 ARM relative relocations from their more compact form.
+  // Decode relative relocations from their more compact form.
   // |packed| is the vector of packed relocations.
-  // |relocations| is a vector of unpacked ARM relative relocation structs.
+  // |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_