runtime/vm/dwarf.cc

// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

#include "vm/dwarf.h"

#include "vm/code_descriptors.h"
#include "vm/elf.h"
#include "vm/image_snapshot.h"
#include "vm/object_store.h"

namespace dart {

#if defined(DART_PRECOMPILER)

class DwarfPosition {
 public:
  // The DWARF standard uses 0 to denote missing line or column information.
  DwarfPosition(intptr_t line, intptr_t column)
      : line_(line > 0 ? line : 0), column_(column > 0 ? column : 0) {
    // Should only have no line information if also no column information.
    ASSERT(line_ > 0 || column_ == 0);
  }
  explicit DwarfPosition(intptr_t line) : DwarfPosition(line, 0) {}
  constexpr DwarfPosition() : line_(0), column_(0) {}

  intptr_t line() const { return line_; }
  intptr_t column() const { return column_; }

 private:
  intptr_t line_;
  intptr_t column_;
};

static constexpr auto kNoDwarfPositionInfo = DwarfPosition();

class InliningNode : public ZoneAllocated {
 public:
  InliningNode(const Function& function,
               const DwarfPosition& position,
               int32_t start_pc_offset)
      : function(function),
        position(position),
        start_pc_offset(start_pc_offset),
        end_pc_offset(-1),
        children_head(NULL),
        children_tail(NULL),
        children_next(NULL) {
    RELEASE_ASSERT(!function.IsNull());
    RELEASE_ASSERT(function.IsNotTemporaryScopedHandle());
  }

  void AppendChild(InliningNode* child) {
    if (children_tail == NULL) {
      children_head = children_tail = child;
    } else {
      children_tail->children_next = child;
      children_tail = child;
    }
  }

  const Function& function;
  DwarfPosition position;
  int32_t start_pc_offset;
  int32_t end_pc_offset;
  InliningNode* children_head;
  InliningNode* children_tail;
  InliningNode* children_next;
};

template <typename T>
Trie<T>* Trie<T>::AddString(Zone* zone,
                            Trie<T>* trie,
                            const char* key,
                            const T* value) {
  ASSERT(key != nullptr);
  if (trie == nullptr) {
    trie = new (zone) Trie<T>();
  }
  if (*key == '\0') {
    ASSERT(trie->value_ == nullptr);
    trie->value_ = value;
  } else {
    auto const index = ChildIndex(*key);
    ASSERT(index >= 0 && index < kNumValidChars);
    trie->children_[index] =
        AddString(zone, trie->children_[index], key + 1, value);
  }

  return trie;
}

template <typename T>
const T* Trie<T>::Lookup(const Trie<T>* trie, const char* key, intptr_t* end) {
  intptr_t i = 0;
  for (; key[i] != '\0'; i++) {
    auto const index = ChildIndex(key[i]);
    ASSERT(index < kNumValidChars);
    if (index < 0) {
      if (end == nullptr) return nullptr;
      break;
    }
    // Still find the longest valid trie prefix when no stored value.
    if (trie == nullptr) continue;
    trie = trie->children_[index];
  }
  if (end != nullptr) {
    *end = i;
  }
  if (trie == nullptr) return nullptr;
  return trie->value_;
}

Dwarf::Dwarf(Zone* zone)
    : zone_(zone),
      reverse_obfuscation_trie_(CreateReverseObfuscationTrie(zone)),
      codes_(zone, 1024),
      code_to_address_(zone),
      functions_(zone, 1024),
      function_to_index_(zone),
      scripts_(zone, 1024),
      script_to_index_(zone),
      temp_(0) {}

SegmentRelativeOffset Dwarf::CodeAddress(const Code& code) const {
  const auto& pair = code_to_address_.LookupValue(&code);
  // This is only used by Elf::Finalize(), and the image writers always give a
  // text offset when calling AddCode() for an Elf object's Dwarf object. Thus,
  // we should have known code offsets for each code object in the map.
  ASSERT(pair.offset != SegmentRelativeOffset::kUnknownOffset);
  return pair;
}

intptr_t Dwarf::AddCode(const Code& orig_code,
                        const SegmentRelativeOffset& offset) {
  ASSERT(!orig_code.IsNull());
  // We should never get the no-argument constructed version here.
  ASSERT(offset.offset != SegmentRelativeOffset::kInvalidOffset);
  // Generate an appropriately zoned ZoneHandle for storing.
  const auto& code = Code::ZoneHandle(zone_, orig_code.raw());

  // For now, we assume one of two flows for a given code object:
  // ELF: Calls to AddCode(code, vm, offset), vm and offset are the same over
  //      all calls.
  // Assembly: An initial call to AddCode(code, vm) (assembly), possibly
  //     followed by a later call to AddCode(code, vm, offset)
  //     (separate debugging info ELF)
  if (offset.offset == SegmentRelativeOffset::kUnknownOffset) {
    // A call without an address should always come before any calls with
    // addresses.
    ASSERT(code_to_address_.Lookup(&code) == nullptr);
    // Insert a marker so on later calls, we know we've already added to codes_.
    code_to_address_.Insert(CodeAddressPair(&code, offset));
  } else {
    const auto& old_value = code_to_address_.LookupValue(&code);
    // ELF does not need to know the index. If we've already added this Code
    // object to codes_ in a previous call, don't bother scanning codes_ to find
    // the corresponding index, just return -1 instead.
    switch (old_value.offset) {
      case SegmentRelativeOffset::kInvalidOffset:
        code_to_address_.Insert(CodeAddressPair(&code, offset));
        break;  // Still need to add to codes_.
      case SegmentRelativeOffset::kUnknownOffset:
        // Code objects should only be associated with either the VM or isolate.
        ASSERT_EQUAL(old_value.vm, offset.vm);
        code_to_address_.Update(CodeAddressPair(&code, offset));
        return -1;
      default:
        // The information for the code object shouldn't have changed since the
        // previous update.
        ASSERT(old_value == offset);
        return -1;
    }
  }

  const intptr_t index = codes_.length();
  codes_.Add(&code);
  if (code.IsFunctionCode()) {
    const Function& function = Function::Handle(zone_, code.function());
    AddFunction(function);
  }
  const Array& inline_functions =
      Array::Handle(zone_, code.inlined_id_to_function());
  if (!inline_functions.IsNull()) {
    Function& function = Function::Handle(zone_);
    for (intptr_t i = 0; i < inline_functions.Length(); i++) {
      function ^= inline_functions.At(i);
      AddFunction(function);
    }
  }
  return index;
}

intptr_t Dwarf::AddFunction(const Function& function) {
  RELEASE_ASSERT(!function.IsNull());
  FunctionIndexPair* pair = function_to_index_.Lookup(&function);
  if (pair != NULL) {
    return pair->index_;
  }
  intptr_t index = functions_.length();
  const Function& zone_func = Function::ZoneHandle(zone_, function.raw());
  function_to_index_.Insert(FunctionIndexPair(&zone_func, index));
  functions_.Add(&zone_func);
  const Script& script = Script::Handle(zone_, function.script());
  AddScript(script);
  return index;
}

intptr_t Dwarf::AddScript(const Script& script) {
  RELEASE_ASSERT(!script.IsNull());
  ScriptIndexPair* pair = script_to_index_.Lookup(&script);
  if (pair != NULL) {
    return pair->index_;
  }
  // DWARF file numbers start from 1.
  intptr_t index = scripts_.length() + 1;
  const Script& zone_script = Script::ZoneHandle(zone_, script.raw());
  script_to_index_.Insert(ScriptIndexPair(&zone_script, index));
  scripts_.Add(&zone_script);
  return index;
}

intptr_t Dwarf::LookupFunction(const Function& function) {
  RELEASE_ASSERT(!function.IsNull());
  FunctionIndexPair* pair = function_to_index_.Lookup(&function);
  if (pair == NULL) {
    FATAL1("Function detected too late during DWARF generation: %s",
           function.ToCString());
  }
  return pair->index_;
}

intptr_t Dwarf::LookupScript(const Script& script) {
  RELEASE_ASSERT(!script.IsNull());
  ScriptIndexPair* pair = script_to_index_.Lookup(&script);
  if (pair == NULL) {
    FATAL1("Script detected too late during DWARF generation: %s",
           script.ToCString());
  }
  return pair->index_;
}

void Dwarf::WriteAbbreviations(DwarfWriteStream* stream) {
  // Dwarf data mostly takes the form of a tree, whose nodes are called
  // DIEs. Each DIE begins with an abbreviation code, and the abbreviation
  // describes the attributes of that DIE and their representation.

  stream->uleb128(kCompilationUnit);     // Abbrev code.
  stream->uleb128(DW_TAG_compile_unit);  // Type.
  stream->u1(DW_CHILDREN_yes);
  stream->uleb128(DW_AT_name);  // Start of attributes.
  stream->uleb128(DW_FORM_string);
  stream->uleb128(DW_AT_producer);
  stream->uleb128(DW_FORM_string);
  stream->uleb128(DW_AT_comp_dir);
  stream->uleb128(DW_FORM_string);
  stream->uleb128(DW_AT_low_pc);
  stream->uleb128(DW_FORM_addr);
  stream->uleb128(DW_AT_high_pc);
  stream->uleb128(DW_FORM_addr);
  stream->uleb128(DW_AT_stmt_list);
  stream->uleb128(DW_FORM_sec_offset);
  stream->uleb128(0);
  stream->uleb128(0);  // End of attributes.

  stream->uleb128(kAbstractFunction);  // Abbrev code.
  stream->uleb128(DW_TAG_subprogram);  // Type.
  stream->u1(DW_CHILDREN_yes);
  stream->uleb128(DW_AT_name);  // Start of attributes.
  stream->uleb128(DW_FORM_string);
  stream->uleb128(DW_AT_decl_file);
  stream->uleb128(DW_FORM_udata);
  stream->uleb128(DW_AT_inline);
  stream->uleb128(DW_FORM_udata);
  stream->uleb128(0);
  stream->uleb128(0);  // End of attributes.

  stream->uleb128(kConcreteFunction);  // Abbrev code.
  stream->uleb128(DW_TAG_subprogram);  // Type.
  stream->u1(DW_CHILDREN_yes);
  stream->uleb128(DW_AT_abstract_origin);  // Start of attributes.
  stream->uleb128(DW_FORM_ref4);
  stream->uleb128(DW_AT_low_pc);
  stream->uleb128(DW_FORM_addr);
  stream->uleb128(DW_AT_high_pc);
  stream->uleb128(DW_FORM_addr);
  stream->uleb128(0);
  stream->uleb128(0);  // End of attributes.

  stream->uleb128(kInlinedFunction);           // Abbrev code.
  stream->uleb128(DW_TAG_inlined_subroutine);  // Type.
  stream->u1(DW_CHILDREN_yes);
  stream->uleb128(DW_AT_abstract_origin);  // Start of attributes.
  stream->uleb128(DW_FORM_ref4);
  stream->uleb128(DW_AT_low_pc);
  stream->uleb128(DW_FORM_addr);
  stream->uleb128(DW_AT_high_pc);
  stream->uleb128(DW_FORM_addr);
  stream->uleb128(DW_AT_call_file);
  stream->uleb128(DW_FORM_udata);
  stream->uleb128(DW_AT_call_line);
  stream->uleb128(DW_FORM_udata);
  stream->uleb128(DW_AT_call_column);
  stream->uleb128(DW_FORM_udata);
  stream->uleb128(0);
  stream->uleb128(0);  // End of attributes.

  stream->uleb128(0);  // End of abbreviations.
}

void Dwarf::WriteDebugInfo(DwarfWriteStream* stream) {
  SnapshotTextObjectNamer namer(zone_);

  // 7.5.1.1 Compilation Unit Header

  // Unit length.
  auto const cu_prefix = "cu";
  intptr_t cu_start;
  intptr_t cu_size_fixup = stream->ReserveSize(cu_prefix, &cu_start);

  stream->u2(2);                            // DWARF version 2
  stream->u4(0);                            // debug_abbrev_offset
  stream->u1(compiler::target::kWordSize);  // address_size

  // Compilation Unit DIE. We describe the entire Dart program as a single
  // compilation unit. Note we write attributes in the same order we declared
  // them in our abbreviation above in WriteAbbreviations.
  stream->uleb128(kCompilationUnit);
  const Library& root_library = Library::Handle(
      zone_, Isolate::Current()->object_store()->root_library());
  const String& root_uri = String::Handle(zone_, root_library.url());
  stream->string(root_uri.ToCString());  // DW_AT_name
  stream->string("Dart VM");             // DW_AT_producer
  stream->string("");                    // DW_AT_comp_dir

  // DW_AT_low_pc
  // The lowest instruction address in this object file that is part of our
  // compilation unit. Dwarf consumers use this to quickly decide which
  // compilation unit DIE to consult for a given pc.
  stream->OffsetFromSymbol(kIsolateSnapshotInstructionsAsmSymbol, 0);

  // DW_AT_high_pc
  // The highest instruction address in this object file that is part of our
  // compilation unit. Dwarf consumers use this to quickly decide which
  // compilation unit DIE to consult for a given pc.
  intptr_t last_code_index = codes_.length() - 1;
  const Code& last_code = *(codes_[last_code_index]);
  auto const last_code_name = namer.SnapshotNameFor(last_code_index, last_code);
  stream->OffsetFromSymbol(last_code_name, last_code.Size());

  // DW_AT_stmt_list (offset into .debug_line)
  // Indicates which line number program is associated with this compilation
  // unit. We only emit a single line number program.
  stream->u4(0);

  WriteAbstractFunctions(stream);
  WriteConcreteFunctions(stream);

  stream->uleb128(0);  // End of children.

  stream->uleb128(0);  // End of entries.
  stream->SetSize(cu_size_fixup, cu_prefix, cu_start);
}

void Dwarf::WriteAbstractFunctions(DwarfWriteStream* stream) {
  Script& script = Script::Handle(zone_);
  String& name = String::Handle(zone_);
  stream->InitializeAbstractOrigins(functions_.length());
  // By the point we're creating DWARF information, scripts have already lost
  // their token stream and we can't look up their line number or column
  // information, hence the lack of DW_AT_decl_line and DW_AT_decl_column.
  for (intptr_t i = 0; i < functions_.length(); i++) {
    const Function& function = *(functions_[i]);
    name = function.QualifiedUserVisibleName();
    script = function.script();
    const intptr_t file = LookupScript(script);
    auto const name_cstr = Deobfuscate(name.ToCString());

    stream->RegisterAbstractOrigin(i);
    stream->uleb128(kAbstractFunction);
    stream->string(name_cstr);        // DW_AT_name
    stream->uleb128(file);            // DW_AT_decl_file
    stream->uleb128(DW_INL_inlined);  // DW_AT_inline
    stream->uleb128(0);               // End of children.
  }
}

void Dwarf::WriteConcreteFunctions(DwarfWriteStream* stream) {
  Function& function = Function::Handle(zone_);
  Script& script = Script::Handle(zone_);
  SnapshotTextObjectNamer namer(zone_);
  for (intptr_t i = 0; i < codes_.length(); i++) {
    const Code& code = *(codes_[i]);
    RELEASE_ASSERT(!code.IsNull());
    if (!code.IsFunctionCode()) {
      continue;
    }

    function = code.function();
    intptr_t function_index = LookupFunction(function);
    script = function.script();
    const char* asm_name = namer.SnapshotNameFor(i, code);

    stream->uleb128(kConcreteFunction);
    // DW_AT_abstract_origin
    // References a node written above in WriteAbstractFunctions.
    stream->AbstractOrigin(function_index);

    // DW_AT_low_pc
    stream->OffsetFromSymbol(asm_name, 0);
    // DW_AT_high_pc
    stream->OffsetFromSymbol(asm_name, code.Size());

    InliningNode* node = ExpandInliningTree(code);
    if (node != NULL) {
      for (InliningNode* child = node->children_head; child != NULL;
           child = child->children_next) {
        WriteInliningNode(stream, child, asm_name, script, &namer);
      }
    }

    stream->uleb128(0);  // End of children.
  }
}

static DwarfPosition ReadPosition(ReadStream* stream) {
  const intptr_t line = stream->Read<int32_t>();
  if (!FLAG_dwarf_stack_traces_mode) {
    return DwarfPosition(line);
  }
  const intptr_t column = stream->Read<int32_t>();
  return DwarfPosition(line, column);
}

// Our state machine encodes position metadata such that we don't know the
// end pc for an inlined function until it is popped, but DWARF DIEs encode
// it where the function is pushed. We expand the state transitions into
// an in-memory tree to do the conversion.
InliningNode* Dwarf::ExpandInliningTree(const Code& code) {
  const CodeSourceMap& map =
      CodeSourceMap::Handle(zone_, code.code_source_map());
  if (map.IsNull()) {
    return NULL;
  }
  const Array& functions = Array::Handle(zone_, code.inlined_id_to_function());
  const Function& root_function = Function::ZoneHandle(zone_, code.function());
  if (root_function.IsNull()) {
    FATAL1("Wherefore art thou functionless code, %s?\n", code.ToCString());
  }

  GrowableArray<InliningNode*> node_stack(zone_, 4);
  GrowableArray<DwarfPosition> token_positions(zone_, 4);

  NoSafepointScope no_safepoint;
  ReadStream stream(map.Data(), map.Length());

  int32_t current_pc_offset = 0;
  token_positions.Add(kNoDwarfPositionInfo);
  InliningNode* root_node =
      new (zone_) InliningNode(root_function, token_positions.Last(), 0);
  root_node->end_pc_offset = code.Size();
  node_stack.Add(root_node);

  while (stream.PendingBytes() > 0) {
    uint8_t opcode = stream.Read<uint8_t>();
    switch (opcode) {
      case CodeSourceMapBuilder::kChangePosition: {
        token_positions[token_positions.length() - 1] = ReadPosition(&stream);
        break;
      }
      case CodeSourceMapBuilder::kAdvancePC: {
        int32_t delta = stream.Read<int32_t>();
        current_pc_offset += delta;
        break;
      }
      case CodeSourceMapBuilder::kPushFunction: {
        int32_t func = stream.Read<int32_t>();
        const Function& child_func =
            Function::ZoneHandle(zone_, Function::RawCast(functions.At(func)));
        InliningNode* child_node = new (zone_)
            InliningNode(child_func, token_positions.Last(), current_pc_offset);
        node_stack.Last()->AppendChild(child_node);
        node_stack.Add(child_node);
        token_positions.Add(kNoDwarfPositionInfo);
        break;
      }
      case CodeSourceMapBuilder::kPopFunction: {
        // We never pop the root function.
        ASSERT(node_stack.length() > 1);
        ASSERT(token_positions.length() > 1);
        node_stack.Last()->end_pc_offset = current_pc_offset;
        node_stack.RemoveLast();
        token_positions.RemoveLast();
        break;
      }
      case CodeSourceMapBuilder::kNullCheck: {
        stream.Read<int32_t>();
        break;
      }
      default:
        UNREACHABLE();
    }
  }

  while (node_stack.length() > 1) {
    node_stack.Last()->end_pc_offset = current_pc_offset;
    node_stack.RemoveLast();
    token_positions.RemoveLast();
  }
  ASSERT(node_stack[0] == root_node);
  return root_node;
}

void Dwarf::WriteInliningNode(DwarfWriteStream* stream,
                              InliningNode* node,
                              const char* root_asm_name,
                              const Script& parent_script,
                              SnapshotTextObjectNamer* namer) {
  intptr_t file = LookupScript(parent_script);
  intptr_t function_index = LookupFunction(node->function);
  const Script& script = Script::Handle(zone_, node->function.script());

  stream->uleb128(kInlinedFunction);
  // DW_AT_abstract_origin
  // References a node written above in WriteAbstractFunctions.
  stream->AbstractOrigin(function_index);

  // DW_AT_low_pc
  stream->OffsetFromSymbol(root_asm_name, node->start_pc_offset);
  // DW_AT_high_pc
  stream->OffsetFromSymbol(root_asm_name, node->end_pc_offset);
  // DW_AT_call_file
  stream->uleb128(file);
  // DW_AT_call_line
  stream->uleb128(node->position.line());
  // DW_at_call_column
  stream->uleb128(node->position.column());

  for (InliningNode* child = node->children_head; child != NULL;
       child = child->children_next) {
    WriteInliningNode(stream, child, root_asm_name, script, namer);
  }

  stream->uleb128(0);  // End of children.
}

void Dwarf::WriteLineNumberProgram(DwarfWriteStream* stream) {
  // 6.2.4 The Line Number Program Header

  // 1. unit_length. This encoding implies 32-bit DWARF.
  auto const line_prefix = "line";
  intptr_t line_start;
  intptr_t line_size_fixup = stream->ReserveSize(line_prefix, &line_start);

  stream->u2(2);  // 2. DWARF version 2

  // 3. header_length
  auto const lineheader_prefix = "lineheader";
  intptr_t lineheader_start;
  intptr_t lineheader_size_fixup =
      stream->ReserveSize(lineheader_prefix, &lineheader_start);

  stream->u1(1);   // 4. minimum_instruction_length
  stream->u1(1);   // 5. default_is_stmt (true for compatibility with dsymutil).
  stream->u1(0);   // 6. line_base
  stream->u1(1);   // 7. line_range
  stream->u1(13);  // 8. opcode_base (12 standard opcodes in Dwarf 2)

  // 9. standard_opcode_lengths
  stream->u1(0);  // DW_LNS_copy, 0 operands
  stream->u1(1);  // DW_LNS_advance_pc, 1 operands
  stream->u1(1);  // DW_LNS_advance_list, 1 operands
  stream->u1(1);  // DW_LNS_set_file, 1 operands
  stream->u1(1);  // DW_LNS_set_column, 1 operands
  stream->u1(0);  // DW_LNS_negate_stmt, 0 operands
  stream->u1(0);  // DW_LNS_set_basic_block, 0 operands
  stream->u1(0);  // DW_LNS_const_add_pc, 0 operands
  stream->u1(1);  // DW_LNS_fixed_advance_pc, 1 operands
  stream->u1(0);  // DW_LNS_set_prolog_end, 0 operands
  stream->u1(0);  // DW_LNS_set_epligoue_begin, 0 operands
  stream->u1(1);  // DW_LNS_set_isa, 1 operands

  // 10. include_directories (sequence of path names)
  // We don't emit any because we use full paths below.
  stream->u1(0);

  // 11. file_names (sequence of file entries)
  String& uri = String::Handle(zone_);
  for (intptr_t i = 0; i < scripts_.length(); i++) {
    const Script& script = *(scripts_[i]);
    uri = script.url();
    auto const uri_cstr = Deobfuscate(uri.ToCString());
    RELEASE_ASSERT(strlen(uri_cstr) != 0);

    stream->string(uri_cstr);  // NOLINT
    stream->uleb128(0);        // Include directory index.
    stream->uleb128(0);        // File modification time.
    stream->uleb128(0);        // File length.
  }
  stream->u1(0);  // End of file names.
  stream->SetSize(lineheader_size_fixup, lineheader_prefix, lineheader_start);

  // 6.2.5 The Line Number Program

  // The initial values for the line number program state machine registers
  // according to the DWARF standard.
  intptr_t previous_pc_offset = 0;
  intptr_t previous_file = 1;
  intptr_t previous_line = 1;
  intptr_t previous_column = 0;
  // Other info not stored in the state machine registers.
  const char* previous_asm_name = nullptr;

  Function& root_function = Function::Handle(zone_);
  Script& script = Script::Handle(zone_);
  CodeSourceMap& map = CodeSourceMap::Handle(zone_);
  Array& functions = Array::Handle(zone_);
  GrowableArray<const Function*> function_stack(zone_, 8);
  GrowableArray<DwarfPosition> token_positions(zone_, 8);
  SnapshotTextObjectNamer namer(zone_);

  for (intptr_t i = 0; i < codes_.length(); i++) {
    const Code& code = *(codes_[i]);
    auto const asm_name = namer.SnapshotNameFor(i, code);

    map = code.code_source_map();
    if (map.IsNull()) {
      continue;
    }
    root_function = code.function();
    functions = code.inlined_id_to_function();

    NoSafepointScope no_safepoint;
    ReadStream code_map_stream(map.Data(), map.Length());

    function_stack.Clear();
    token_positions.Clear();

    int32_t current_pc_offset = 0;
    function_stack.Add(&root_function);
    token_positions.Add(kNoDwarfPositionInfo);

    while (code_map_stream.PendingBytes() > 0) {
      uint8_t opcode = code_map_stream.Read<uint8_t>();
      switch (opcode) {
        case CodeSourceMapBuilder::kChangePosition: {
          token_positions[token_positions.length() - 1] =
              ReadPosition(&code_map_stream);
          break;
        }
        case CodeSourceMapBuilder::kAdvancePC: {
          int32_t delta = code_map_stream.Read<int32_t>();
          current_pc_offset += delta;

          const Function& function = *(function_stack.Last());
          script = function.script();
          intptr_t file = LookupScript(script);

          // 1. Update LNP file.
          if (file != previous_file) {
            stream->u1(DW_LNS_set_file);
            stream->uleb128(file);
            previous_file = file;
          }

          // 2. Update LNP line.
          const intptr_t line = token_positions.Last().line();
          const intptr_t column = token_positions.Last().column();
          if (line != previous_line) {
            stream->u1(DW_LNS_advance_line);
            stream->sleb128(line - previous_line);
            previous_line = line;
          }
          if (column != previous_column) {
            stream->u1(DW_LNS_set_column);
            stream->uleb128(column);
            previous_column = column;
          }

          // 3. Emit LNP row if the address register has been updated to a
          // non-zero value (dartbug.com/41756).
          if (previous_asm_name != nullptr) {
            stream->u1(DW_LNS_copy);
          }

          // 4. Update LNP pc.
          if (previous_asm_name == nullptr) {
            auto const instr_size = 1 + compiler::target::kWordSize;
            stream->u1(0);           // This is an extended opcode
            stream->u1(instr_size);  // that is 5 or 9 bytes long
            stream->u1(DW_LNE_set_address);
            stream->OffsetFromSymbol(asm_name, current_pc_offset);
          } else {
            stream->u1(DW_LNS_advance_pc);
            stream->DistanceBetweenSymbolOffsets(asm_name, current_pc_offset,
                                                 previous_asm_name,
                                                 previous_pc_offset);
          }
          previous_asm_name = asm_name;
          previous_pc_offset = current_pc_offset;
          break;
        }
        case CodeSourceMapBuilder::kPushFunction: {
          int32_t func_index = code_map_stream.Read<int32_t>();
          const Function& child_func = Function::Handle(
              zone_, Function::RawCast(functions.At(func_index)));
          function_stack.Add(&child_func);
          token_positions.Add(kNoDwarfPositionInfo);
          break;
        }
        case CodeSourceMapBuilder::kPopFunction: {
          // We never pop the root function.
          ASSERT(function_stack.length() > 1);
          ASSERT(token_positions.length() > 1);
          function_stack.RemoveLast();
          token_positions.RemoveLast();
          break;
        }
        case CodeSourceMapBuilder::kNullCheck: {
          code_map_stream.Read<int32_t>();
          break;
        }
        default:
          UNREACHABLE();
      }
    }
  }

  // Advance pc to end of the compilation unit if not already there.
  if (codes_.length() != 0) {
    const intptr_t last_code_index = codes_.length() - 1;
    const Code& last_code = *(codes_[last_code_index]);
    const intptr_t last_pc_offset = last_code.Size();
    const char* last_asm_name =
        namer.SnapshotNameFor(last_code_index, last_code);

    stream->u1(DW_LNS_advance_pc);
    if (previous_asm_name != nullptr) {
      stream->DistanceBetweenSymbolOffsets(
          last_asm_name, last_pc_offset, previous_asm_name, previous_pc_offset);
    } else {
      // No LNP entries (e.g., only stub code).
      ASSERT(previous_pc_offset == 0);
      stream->uleb128(last_pc_offset);
    }
  }

  // End of contiguous machine code.
  stream->u1(0);  // This is an extended opcode
  stream->u1(1);  // that is 1 byte long
  stream->u1(DW_LNE_end_sequence);
  stream->SetSize(line_size_fixup, line_prefix, line_start);
}

const char* Dwarf::Deobfuscate(const char* cstr) {
  if (reverse_obfuscation_trie_ == nullptr) return cstr;
  TextBuffer buffer(256);
  // Used to avoid Zone-allocating strings if no deobfuscation was performed.
  bool changed = false;
  intptr_t i = 0;
  while (cstr[i] != '\0') {
    intptr_t offset;
    auto const value = reverse_obfuscation_trie_->Lookup(cstr + i, &offset);
    if (offset == 0) {
      // The first character was an invalid key element (that isn't the null
      // terminator due to the while condition), copy it and skip to the next.
      buffer.AddChar(cstr[i++]);
    } else if (value != nullptr) {
      changed = true;
      buffer.AddString(value);
    } else {
      buffer.AddRaw(reinterpret_cast<const uint8_t*>(cstr + i), offset);
    }
    i += offset;
  }
  if (!changed) return cstr;
  return OS::SCreate(zone_, "%s", buffer.buffer());
}

Trie<const char>* Dwarf::CreateReverseObfuscationTrie(Zone* zone) {
  auto const I = Thread::Current()->isolate();
  auto const map_array = I->obfuscation_map();
  if (map_array == nullptr) return nullptr;

  Trie<const char>* trie = nullptr;
  for (intptr_t i = 0; map_array[i] != nullptr; i += 2) {
    auto const key = map_array[i];
    auto const value = map_array[i + 1];
    ASSERT(value != nullptr);
    // Don't include identity mappings.
    if (strcmp(key, value) == 0) continue;
    // Otherwise, any value in the obfuscation map should be a valid key.
    ASSERT(Trie<const char>::IsValidKey(value));
    trie = Trie<const char>::AddString(zone, trie, value, key);
  }
  return trie;
}

#endif  // DART_PRECOMPILER

}  // namespace dart