Add a host tool to pack R_ARM_RELATIVE relocations in libchrome.<ver>.so.

tools/relocation_packer/src/run_length_encoder.cc

+// 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.
+
+// TODO(simonb): Extend for 64-bit target libraries.
+
+#include "run_length_encoder.h"
+
+#include <string.h>
+#include <string>
+#include <vector>
+
+#include "debug.h"
+
+namespace relocation_packer {
+
+namespace {
+
+// Generate a vector of deltas between the r_offset fields of adjacent
+// R_ARM_RELATIVE relocations.
+void GetDeltas(const std::vector<Elf32_Rel>& relocations,
+               std::vector<Elf32_Addr>* deltas) {
+  CHECK(relocations.size() >= 2);
+
+  for (size_t i = 0; i < relocations.size() - 1; ++i) {
+    const Elf32_Addr first = relocations[i].r_offset;
+    const Elf32_Addr second = relocations[i + 1].r_offset;
+    // Requires that offsets are 'strictly increasing'.  The packing
+    // algorithm fails if this does not hold.
+    CHECK(second > first);
+    deltas->push_back(second - first);
+  }
+}
+
+// Condense a set of r_offset deltas into a run-length encoded packing.
+// Represented as count-delta pairs, where count is the run length and
+// delta the common difference between adjacent r_offsets.
+void Condense(const std::vector<Elf32_Addr>& deltas,
+              std::vector<Elf32_Word>* packed) {
+  CHECK(!deltas.empty());
+  size_t count = 0;
+  Elf32_Addr current = deltas[0];
+
+  // Identify spans of identically valued deltas.
+  for (size_t i = 0; i < deltas.size(); ++i) {
+    const Elf32_Addr delta = deltas[i];
+    if (delta == current) {
+      count++;
+    } else {
+      // We reached the end of a span of identically valued deltas.
+      packed->push_back(count);
+      packed->push_back(current);
+      current = delta;
+      count = 1;
+    }
+  }
+
+  // Write the final span.
+  packed->push_back(count);
+  packed->push_back(current);
+}
+
+// Uncondense a set of r_offset deltas from a run-length encoded packing.
+// The initial address for uncondensing, the start index for the first
+// condensed slot in packed, and the count of pairs are provided.
+void Uncondense(Elf32_Addr addr,
+                const std::vector<Elf32_Word>& packed,
+                size_t start_index,
+                size_t end_index,
+                std::vector<Elf32_Rel>* relocations) {
+  // The first relocation is just one created from the initial address.
+  const Elf32_Rel initial = {addr, R_ARM_RELATIVE};
+  relocations->push_back(initial);
+
+  // Read each count and delta pair, beginning at the start index and
+  // finishing at the end index.
+  for (size_t i = start_index; i < end_index; i += 2) {
+    size_t count = packed[i];
+    const Elf32_Addr delta = packed[i + 1];
+    CHECK(count > 0 && delta > 0);
+
+    // Generate relocations for this count and delta pair.
+    while (count) {
+      addr += delta;
+      const Elf32_Rel relocation = {addr, R_ARM_RELATIVE};
+      relocations->push_back(relocation);
+      count--;
+    }
+  }
+}
+
+}  // namespace
+
+// Encode R_ARM_RELATIVE relocations into a run-length encoded (packed)
+// representation.
+void RelocationRunLengthCodec::Encode(const std::vector<Elf32_Rel>& relocations,
+                                      std::vector<Elf32_Word>* packed) {
+  // If we have zero or one relocation only then there is no packing
+  // possible; a run-length encoding needs a run.
+  if (relocations.size() < 2)
+    return;
+
+  std::vector<Elf32_Addr> deltas;
+  GetDeltas(relocations, &deltas);
+
+  // Reserve space for the element count.
+  packed->push_back(0);
+
+  // Initialize the packed data with the first offset, then follow up with
+  // the condensed deltas vector.
+  packed->push_back(relocations[0].r_offset);
+  Condense(deltas, packed);
+
+  // Fill in the packed pair count.
+  packed->at(0) = (packed->size() - 2) >> 1;
+}
+
+// Decode R_ARM_RELATIVE reloctions from a run-length encoded (packed)
+// representation.
+void RelocationRunLengthCodec::Decode(const std::vector<Elf32_Word>& packed,
+                                      std::vector<Elf32_Rel>* relocations) {
+  // We need at least one packed pair after the packed pair count to be
+  // able to unpack.
+  if (packed.size() < 3)
+    return;
+
+  // Ensure that the packed data offers enough pairs.  There may be zero
+  // padding on it that we ignore.
+  CHECK(packed[0] <= (packed.size() - 2) >> 1);
+
+  // The first packed vector element is the pairs count and the second the
+  // initial address.  Start uncondensing pairs at the third, and finish
+  // at the end of the pairs data.
+  const size_t pairs_count = packed[0];
+  const Elf32_Addr addr = packed[1];
+  Uncondense(addr, packed, 2, 2 + (pairs_count << 1), relocations);
+}
+
+}  // namespace relocation_packer