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Unified Diff: tools/relocation_packer/src/run_length_encoder.cc

Issue 310483003: Add a host tool to pack R_ARM_RELATIVE relocations in libchrome.<ver>.so. (Closed) Base URL: https://chromium.googlesource.com/chromium/src.git@master
Patch Set: Remove binary test data files, dcommit separately. Created 6 years, 6 months ago
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Index: tools/relocation_packer/src/run_length_encoder.cc
diff --git a/tools/relocation_packer/src/run_length_encoder.cc b/tools/relocation_packer/src/run_length_encoder.cc
new file mode 100644
index 0000000000000000000000000000000000000000..78949a9ee646af3b74c8821018a1a8d3ab1c6606
--- /dev/null
+++ b/tools/relocation_packer/src/run_length_encoder.cc
@@ -0,0 +1,139 @@
+// 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
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