DwarfLineNumberVM::StepResult DwarfLineNumberVM::step()

in folly/experimental/symbolizer/DwarfLineNumberVM.cpp [449:559]

• 93 lines of code
• 23 McCabe index (conditional complexity)

DwarfLineNumberVM::StepResult DwarfLineNumberVM::step(
    folly::StringPiece& program) {
  auto opcode = read<uint8_t>(program);

  if (opcode >= opcodeBase_) { // special opcode
    uint8_t adjustedOpcode = opcode - opcodeBase_;
    uint8_t opAdvance = adjustedOpcode / lineRange_;

    address_ += minLength_ * opAdvance;
    line_ += lineBase_ + adjustedOpcode % lineRange_;

    basicBlock_ = false;
    prologueEnd_ = false;
    epilogueBegin_ = false;
    discriminator_ = 0;
    return COMMIT;
  }

  if (opcode != 0) { // standard opcode
    // Only interpret opcodes that are recognized by the version we're parsing;
    // the others are vendor extensions and we should ignore them.
    switch (opcode) {
      case DW_LNS_copy:
        basicBlock_ = false;
        prologueEnd_ = false;
        epilogueBegin_ = false;
        discriminator_ = 0;
        return COMMIT;
      case DW_LNS_advance_pc:
        address_ += minLength_ * readULEB(program);
        return CONTINUE;
      case DW_LNS_advance_line:
        line_ += readSLEB(program);
        return CONTINUE;
      case DW_LNS_set_file:
        file_ = readULEB(program);
        return CONTINUE;
      case DW_LNS_set_column:
        column_ = readULEB(program);
        return CONTINUE;
      case DW_LNS_negate_stmt:
        isStmt_ = !isStmt_;
        return CONTINUE;
      case DW_LNS_set_basic_block:
        basicBlock_ = true;
        return CONTINUE;
      case DW_LNS_const_add_pc:
        address_ += minLength_ * ((255 - opcodeBase_) / lineRange_);
        return CONTINUE;
      case DW_LNS_fixed_advance_pc:
        address_ += read<uint16_t>(program);
        return CONTINUE;
      case DW_LNS_set_prologue_end:
        if (version_ == 2) {
          break; // not supported in version 2
        }
        prologueEnd_ = true;
        return CONTINUE;
      case DW_LNS_set_epilogue_begin:
        if (version_ == 2) {
          break; // not supported in version 2
        }
        epilogueBegin_ = true;
        return CONTINUE;
      case DW_LNS_set_isa:
        if (version_ == 2) {
          break; // not supported in version 2
        }
        isa_ = readULEB(program);
        return CONTINUE;
    }

    // Unrecognized standard opcode, slurp the appropriate number of LEB
    // arguments.
    uint8_t argCount = standardOpcodeLengths_[opcode - 1];
    while (argCount--) {
      readULEB(program);
    }
    return CONTINUE;
  }

  // Extended opcode
  auto length = readULEB(program);
  // the opcode itself should be included in the length, so length >= 1
  FOLLY_SAFE_CHECK(length != 0, "invalid extended opcode length");
  auto extendedOpcode = read<uint8_t>(program);
  --length;

  switch (extendedOpcode) {
    case DW_LNE_end_sequence:
      return END;
    case DW_LNE_set_address:
      address_ = read<uintptr_t>(program);
      return CONTINUE;
    case DW_LNE_define_file:
      FOLLY_SAFE_CHECK(version_ < 5, "DW_LNE_define_file deprecated in DWARF5");
      // We can't process DW_LNE_define_file here, as it would require us to
      // use unbounded amounts of state (ie. use the heap).  We'll do a second
      // pass (using nextDefineFile()) if necessary.
      break;
#if !defined(__FreeBSD__)
    case DW_LNE_set_discriminator:
      discriminator_ = readULEB(program);
      return CONTINUE;
#endif
  }

  // Unrecognized extended opcode
  program.advance(length);
  return CONTINUE;
}