text: // Copyright 2015 the V8 project authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file.
  
  #include "src/wasm/module-decoder.h"
  
  #include "src/base/functional.h"
  #include "src/base/platform/platform.h"
  #include "src/base/platform/wrappers.h"
  #include "src/flags/flags.h"
  #include "src/init/v8.h"
  #include "src/logging/counters.h"
  #include "src/logging/metrics.h"
  #include "src/objects/objects-inl.h"
  #include "src/utils/ostreams.h"
  #include "src/wasm/decoder.h"
  #include "src/wasm/function-body-decoder-impl.h"
  #include "src/wasm/init-expr-interface.h"
  #include "src/wasm/struct-types.h"
  #include "src/wasm/wasm-constants.h"
  #include "src/wasm/wasm-engine.h"
  #include "src/wasm/wasm-limits.h"
  #include "src/wasm/wasm-opcodes-inl.h"
  
  namespace v8 {
  namespace internal {
  namespace wasm {
  
  #define TRACE(...)                                    \
    do {                                                \
      if (FLAG_trace_wasm_decoder) PrintF(__VA_ARGS__); \
    } while (false)
  
  namespace {
  
  constexpr char kNameString[] = "name";
  constexpr char kSourceMappingURLString[] = "sourceMappingURL";
  constexpr char kCompilationHintsString[] = "compilationHints";
  constexpr char kBranchHintsString[] = "branchHints";
  constexpr char kDebugInfoString[] = ".debug_info";
  constexpr char kExternalDebugInfoString[] = "external_debug_info";
  
  const char* ExternalKindName(ImportExportKindCode kind) {
    switch (kind) {
      case kExternalFunction:
        return "function";
      case kExternalTable:
        return "table";
      case kExternalMemory:
        return "memory";
      case kExternalGlobal:
        return "global";
      case kExternalTag:
        return "tag";
    }
    return "unknown";
  }
  
  }  // namespace
  
  const char* SectionName(SectionCode code) {
    switch (code) {
      case kUnknownSectionCode:
        return "Unknown";
      case kTypeSectionCode:
        return "Type";
      case kImportSectionCode:
        return "Import";
      case kFunctionSectionCode:
        return "Function";
      case kTableSectionCode:
        return "Table";
      case kMemorySectionCode:
        return "Memory";
      case kGlobalSectionCode:
        return "Global";
      case kExportSectionCode:
        return "Export";
      case kStartSectionCode:
        return "Start";
      case kCodeSectionCode:
        return "Code";
      case kElementSectionCode:
        return "Element";
      case kDataSectionCode:
        return "Data";
      case kTagSectionCode:
        return "Tag";
      case kDataCountSectionCode:
        return "DataCount";
      case kNameSectionCode:
        return kNameString;
      case kSourceMappingURLSectionCode:
        return kSourceMappingURLString;
      case kDebugInfoSectionCode:
        return kDebugInfoString;
      case kExternalDebugInfoSectionCode:
        return kExternalDebugInfoString;
      case kCompilationHintsSectionCode:
        return kCompilationHintsString;
      case kBranchHintsSectionCode:
        return kBranchHintsString;
      default:
        return "<unknown>";
    }
  }
  
  namespace {
  
  bool validate_utf8(Decoder* decoder, WireBytesRef string) {
    return unibrow::Utf8::ValidateEncoding(
        decoder->start() + decoder->GetBufferRelativeOffset(string.offset()),
        string.length());
  }
  
  // Reads a length-prefixed string, checking that it is within bounds. Returns
  // the offset of the string, and the length as an out parameter.
  WireBytesRef consume_string(Decoder* decoder, bool validate_utf8,
                              const char* name) {
    uint32_t length = decoder->consume_u32v("string length");
    uint32_t offset = decoder->pc_offset();
    const byte* string_start = decoder->pc();
    // Consume bytes before validation to guarantee that the string is not oob.
    if (length > 0) {
      decoder->consume_bytes(length, name);
      if (decoder->ok() && validate_utf8 &&
          !unibrow::Utf8::ValidateEncoding(string_start, length)) {
        decoder->errorf(string_start, "%s: no valid UTF-8 string", name);
      }
    }
    return {offset, decoder->failed() ? 0 : length};
  }
  
  namespace {
  SectionCode IdentifyUnknownSectionInternal(Decoder* decoder) {
    WireBytesRef string = consume_string(decoder, true, "section name");
    if (decoder->failed()) {
      return kUnknownSectionCode;
    }
    const byte* section_name_start =
        decoder->start() + decoder->GetBufferRelativeOffset(string.offset());
  
    TRACE("  +%d  section name        : \"%.*s\"\n",
          static_cast<int>(section_name_start - decoder->start()),
          string.length() < 20 ? string.length() : 20, section_name_start);
  
    using SpecialSectionPair = std::pair<base::Vector<const char>, SectionCode>;
    static constexpr SpecialSectionPair kSpecialSections[]{
        {base::StaticCharVector(kNameString), kNameSectionCode},
        {base::StaticCharVector(kSourceMappingURLString),
         kSourceMappingURLSectionCode},
        {base::StaticCharVector(kCompilationHintsString),
         kCompilationHintsSectionCode},
        {base::StaticCharVector(kBranchHintsString), kBranchHintsSectionCode},
        {base::StaticCharVector(kDebugInfoString), kDebugInfoSectionCode},
        {base::StaticCharVector(kExternalDebugInfoString),
         kExternalDebugInfoSectionCode}};
  
    auto name_vec = base::Vector<const char>::cast(
        base::VectorOf(section_name_start, string.length()));
    for (auto& special_section : kSpecialSections) {
      if (name_vec == special_section.first) return special_section.second;
    }
  
    return kUnknownSectionCode;
  }
  }  // namespace
  
  // An iterator over the sections in a wasm binary module.
  // Automatically skips all unknown sections.
  class WasmSectionIterator {
   public:
    explicit WasmSectionIterator(Decoder* decoder)
        : decoder_(decoder),
          section_code_(kUnknownSectionCode),
          section_start_(decoder->pc()),
          section_end_(decoder->pc()) {
      next();
    }
  
    bool more() const { return decoder_->ok() && decoder_->more(); }
  
    SectionCode section_code() const { return section_code_; }
  
    const byte* section_start() const { return section_start_; }
  
    uint32_t section_length() const {
      return static_cast<uint32_t>(section_end_ - section_start_);
    }
  
    base::Vector<const uint8_t> payload() const {
      return {payload_start_, payload_length()};
    }
  
    const byte* payload_start() const { return payload_start_; }
  
    uint32_t payload_length() const {
      return static_cast<uint32_t>(section_end_ - payload_start_);
    }
  
    const byte* section_end() const { return section_end_; }
  
    // Advances to the next section, checking that decoding the current section
    // stopped at {section_end_}.
    void advance(bool move_to_section_end = false) {
      if (move_to_section_end && decoder_->pc() < section_end_) {
        decoder_->consume_bytes(
            static_cast<uint32_t>(section_end_ - decoder_->pc()));
      }
      if (decoder_->pc() != section_end_) {
        const char* msg = decoder_->pc() < section_end_ ? "shorter" : "longer";
        decoder_->errorf(decoder_->pc(),
                         "section was %s than expected size "
                         "(%u bytes expected, %zu decoded)",
                         msg, section_length(),
                         static_cast<size_t>(decoder_->pc() - section_start_));
      }
      next();
    }
  
   private:
    Decoder* decoder_;
    SectionCode section_code_;
    const byte* section_start_;
    const byte* payload_start_;
    const byte* section_end_;
  
    // Reads the section code/name at the current position and sets up
    // the embedder fields.
    void next() {
      if (!decoder_->more()) {
        section_code_ = kUnknownSectionCode;
        return;
      }
      section_start_ = decoder_->pc();
      uint8_t section_code = decoder_->consume_u8("section code");
      // Read and check the section size.
      uint32_t section_length = decoder_->consume_u32v("section length");
  
      payload_start_ = decoder_->pc();
      if (decoder_->checkAvailable(section_length)) {
        // Get the limit of the section within the module.
        section_end_ = payload_start_ + section_length;
      } else {
        // The section would extend beyond the end of the module.
        section_end_ = payload_start_;
      }
  
      if (section_code == kUnknownSectionCode) {
        // Check for the known "name", "sourceMappingURL", or "compilationHints"
        // section.
        // To identify the unknown section we set the end of the decoder bytes to
        // the end of the custom section, so that we do not read the section name
        // beyond the end of the section.
        const byte* module_end = decoder_->end();
        decoder_->set_end(section_end_);
        section_code = IdentifyUnknownSectionInternal(decoder_);
        if (decoder_->ok()) decoder_->set_end(module_end);
        // As a side effect, the above function will forward the decoder to after
        // the identifier string.
        payload_start_ = decoder_->pc();
      } else if (!IsValidSectionCode(section_code)) {
        decoder_->errorf(decoder_->pc(), "unknown section code #0x%02x",
                         section_code);
        section_code = kUnknownSectionCode;
      }
      section_code_ = decoder_->failed() ? kUnknownSectionCode
                                         : static_cast<SectionCode>(section_code);
  
      if (section_code_ == kUnknownSectionCode && section_end_ > decoder_->pc()) {
        // skip to the end of the unknown section.
        uint32_t remaining = static_cast<uint32_t>(section_end_ - decoder_->pc());
        decoder_->consume_bytes(remaining, "section payload");
      }
    }
  };
  
  }  // namespace
  
  // The main logic for decoding the bytes of a module.
  class ModuleDecoderImpl : public Decoder {
   public:
    explicit ModuleDecoderImpl(const WasmFeatures& enabled, ModuleOrigin origin)
        : Decoder(nullptr, nullptr),
          enabled_features_(enabled),
          origin_(origin) {}
  
    ModuleDecoderImpl(const WasmFeatures& enabled, const byte* module_start,
                      const byte* module_end, ModuleOrigin origin)
        : Decoder(module_start, module_end),
          enabled_features_(enabled),
          module_start_(module_start),
          module_end_(module_end),
          origin_(origin) {
      if (end_ < start_) {
        error(start_, "end is less than start");
        end_ = start_;
      }
    }
  
    void onFirstError() override {
      pc_ = end_;  // On error, terminate section decoding loop.
    }
  
    void DumpModule(const base::Vector<const byte> module_bytes) {
      std::string path;
      if (FLAG_dump_wasm_module_path) {
        path = FLAG_dump_wasm_module_path;
        if (path.size() &&
            !base::OS::isDirectorySeparator(path[path.size() - 1])) {
          path += base::OS::DirectorySeparator();
        }
      }
      // File are named `HASH.{ok,failed}.wasm`.
      size_t hash = base::hash_range(module_bytes.begin(), module_bytes.end());
      base::EmbeddedVector<char, 32> buf;
      SNPrintF(buf, "%016zx.%s.wasm", hash, ok() ? "ok" : "failed");
      path += buf.begin();
      size_t rv = 0;
      if (FILE* file = base::OS::FOpen(path.c_str(), "wb")) {
        rv = fwrite(module_bytes.begin(), module_bytes.length(), 1, file);
        base::Fclose(file);
      }
      if (rv != 1) {
        OFStream os(stderr);
        os << "Error while dumping wasm file to " << path << std::endl;
      }
    }
  
    void StartDecoding(Counters* counters, AccountingAllocator* allocator) {
      CHECK_NULL(module_);
      SetCounters(counters);
      module_.reset(
          new WasmModule(std::make_unique<Zone>(allocator, "signatures")));
      module_->initial_pages = 0;
      module_->maximum_pages = 0;
      module_->mem_export = false;
      module_->origin = origin_;
    }
  
    void DecodeModuleHeader(base::Vector<const uint8_t> bytes, uint8_t offset) {
      if (failed()) return;
      Reset(bytes, offset);
  
      const byte* pos = pc_;
      uint32_t magic_word = consume_u32("wasm magic");
  #define BYTES(x) (x & 0xFF), (x >> 8) & 0xFF, (x >> 16) & 0xFF, (x >> 24) & 0xFF
      if (magic_word != kWasmMagic) {
        errorf(pos,
               "expected magic word %02x %02x %02x %02x, "
               "found %02x %02x %02x %02x",
               BYTES(kWasmMagic), BYTES(magic_word));
      }
  
      pos = pc_;
      {
        uint32_t magic_version = consume_u32("wasm version");
        if (magic_version != kWasmVersion) {
          errorf(pos,
                 "expected version %02x %02x %02x %02x, "
                 "found %02x %02x %02x %02x",
                 BYTES(kWasmVersion), BYTES(magic_version));
        }
      }
  #undef BYTES
    }
  
    bool CheckSectionOrder(SectionCode section_code,
                           SectionCode prev_section_code,
                           SectionCode next_section_code) {
      if (next_ordered_section_ > next_section_code) {
        errorf(pc(), "The %s section must appear before the %s section",
               SectionName(section_code), SectionName(next_section_code));
        return false;
      }
      if (next_ordered_section_ <= prev_section_code) {
        next_ordered_section_ = prev_section_code + 1;
      }
      return true;
    }
  
    bool CheckUnorderedSection(SectionCode section_code) {
      if (has_seen_unordered_section(section_code)) {
        errorf(pc(), "Multiple %s sections not allowed",
               SectionName(section_code));
        return false;
      }
      set_seen_unordered_section(section_code);
      return true;
    }
  
    void DecodeSection(SectionCode section_code,
                       base::Vector<const uint8_t> bytes, uint32_t offset,
                       bool verify_functions = true) {
      if (failed()) return;
      Reset(bytes, offset);
      TRACE("Section: %s\n", SectionName(section_code));
      TRACE("Decode Section %p - %p\n", bytes.begin(), bytes.end());
  
      // Check if the section is out-of-order.
      if (section_code < next_ordered_section_ &&
          section_code < kFirstUnorderedSection) {
        errorf(pc(), "unexpected section <%s>", SectionName(section_code));
        return;
      }
  
      switch (section_code) {
        case kUnknownSectionCode:
          break;
        case kDataCountSectionCode:
          if (!CheckUnorderedSection(section_code)) return;
          if (!CheckSectionOrder(section_code, kElementSectionCode,
                                 kCodeSectionCode))
            return;
          break;
        case kTagSectionCode:
          if (!CheckUnorderedSection(section_code)) return;
          if (!CheckSectionOrder(section_code, kMemorySectionCode,
                                 kGlobalSectionCode))
            return;
          break;
        case kNameSectionCode:
          // TODO(titzer): report out of place name section as a warning.
          // Be lenient with placement of name section. All except first
          // occurrence are ignored.
        case kSourceMappingURLSectionCode:
          // sourceMappingURL is a custom section and currently can occur anywhere
          // in the module. In case of multiple sourceMappingURL sections, all
          // except the first occurrence are ignored.
        case kDebugInfoSectionCode:
          // .debug_info is a custom section containing core DWARF information
          // if produced by compiler. Its presence likely means that Wasm was
          // built in a debug mode.
        case kExternalDebugInfoSectionCode:
          // external_debug_info is a custom section containing a reference to an
          // external symbol file.
        case kCompilationHintsSectionCode:
          // TODO(frgossen): report out of place compilation hints section as a
          // warning.
          // Be lenient with placement of compilation hints section. All except
          // first occurrence after function section and before code section are
          // ignored.
          break;
        case kBranchHintsSectionCode:
          // TODO(yuri): report out of place branch hints section as a
          // warning.
          // Be lenient with placement of compilation hints section. All except
          // first occurrence after function section and before code section are
          // ignored.
          break;
        default:
          next_ordered_section_ = section_code + 1;
          break;
      }
  
      switch (section_code) {
        case kUnknownSectionCode:
          break;
        case kTypeSectionCode:
          DecodeTypeSection();
          break;
        case kImportSectionCode:
          DecodeImportSection();
          break;
        case kFunctionSectionCode:
          DecodeFunctionSection();
          break;
        case kTableSectionCode:
          DecodeTableSection();
          break;
        case kMemorySectionCode:
          DecodeMemorySection();
          break;
        case kGlobalSectionCode:
          DecodeGlobalSection();
          break;
        case kExportSectionCode:
          DecodeExportSection();
          break;
        case kStartSectionCode:
          DecodeStartSection();
          break;
        case kCodeSectionCode:
          DecodeCodeSection(verify_functions);
          break;
        case kElementSectionCode:
          DecodeElementSection();
          break;
        case kDataSectionCode:
          DecodeDataSection();
          break;
        case kNameSectionCode:
          DecodeNameSection();
          break;
        case kSourceMappingURLSectionCode:
          DecodeSourceMappingURLSection();
          break;
        case kDebugInfoSectionCode:
          // If there is an explicit source map, prefer it over DWARF info.
          if (module_->debug_symbols.type == WasmDebugSymbols::Type::None) {
            module_->debug_symbols = {WasmDebugSymbols::Type::EmbeddedDWARF, {}};
          }
          consume_bytes(static_cast<uint32_t>(end_ - start_), ".debug_info");
          break;
        case kExternalDebugInfoSectionCode:
          DecodeExternalDebugInfoSection();
          break;
        case kCompilationHintsSectionCode:
          if (enabled_features_.has_compilation_hints()) {
            DecodeCompilationHintsSection();
          } else {
            // Ignore this section when feature was disabled. It is an optional
            // custom section anyways.
            consume_bytes(static_cast<uint32_t>(end_ - start_), nullptr);
          }
          break;
        case kBranchHintsSectionCode:
          if (enabled_features_.has_branch_hinting()) {
            DecodeBranchHintsSection();
          } else {
            // Ignore this section when feature was disabled. It is an optional
            // custom section anyways.
            consume_bytes(static_cast<uint32_t>(end_ - start_), nullptr);
          }
          break;
        case kDataCountSectionCode:
          DecodeDataCountSection();
          break;
        case kTagSectionCode:
          if (enabled_features_.has_eh()) {
            DecodeTagSection();
          } else {
            errorf(pc(),
                   "unexpected section <%s> (enable with --experimental-wasm-eh)",
                   SectionName(section_code));
          }
          break;
        default:
          errorf(pc(), "unexpected section <%s>", SectionName(section_code));
          return;
      }
  
      if (pc() != bytes.end()) {
        const char* msg = pc() < bytes.end() ? "shorter" : "longer";
        errorf(pc(),
               "section was %s than expected size "
               "(%zu bytes expected, %zu decoded)",
               msg, bytes.size(), static_cast<size_t>(pc() - bytes.begin()));
      }
    }
  
    void DecodeTypeSection() {
      uint32_t signatures_count = consume_count("types count", kV8MaxWasmTypes);
      module_->types.reserve(signatures_count);
      for (uint32_t i = 0; ok() && i < signatures_count; ++i) {
        TRACE("DecodeSignature[%d] module+%d\n", i,
              static_cast<int>(pc_ - start_));
        uint8_t kind = consume_u8("type kind");
        switch (kind) {
          case kWasmFunctionTypeCode: {
            const FunctionSig* s = consume_sig(module_->signature_zone.get());
            module_->add_signature(s);
            break;
          }
          case kWasmFunctionExtendingTypeCode: {
            if (!enabled_features_.has_gc()) {
              errorf(pc(),
                     "invalid function type definition, enable with "
                     "--experimental-wasm-gc");
              break;
            }
            const FunctionSig* s = consume_sig(module_->signature_zone.get());
            module_->add_signature(s);
            uint32_t super_index = consume_u32v("supertype");
            if (!module_->has_signature(super_index)) {
              errorf(pc(), "invalid function supertype index: %d", super_index);
              break;
            }
            break;
          }
          case kWasmStructTypeCode: {
            if (!enabled_features_.has_gc()) {
              errorf(pc(),
                     "invalid struct type definition, enable with "
                     "--experimental-wasm-gc");
              break;
            }
            const StructType* s = consume_struct(module_->signature_zone.get());
            module_->add_struct_type(s);
            // TODO(7748): Should we canonicalize struct types, like
            // {signature_map} does for function signatures?
            break;
          }
          case kWasmStructExtendingTypeCode: {
            if (!enabled_features_.has_gc()) {
              errorf(pc(),
                     "invalid struct type definition, enable with "
                     "--experimental-wasm-gc");
              break;
            }
            const StructType* s = consume_struct(module_->signature_zone.get());
            module_->add_struct_type(s);
            uint32_t super_index = consume_u32v("supertype");
            if (!module_->has_struct(super_index)) {
              errorf(pc(), "invalid struct supertype: %d", super_index);
              break;
            }
            break;
          }
          case kWasmArrayTypeCode: {
            if (!enabled_features_.has_gc()) {
              errorf(pc(),
                     "invalid array type definition, enable with "
                     "--experimental-wasm-gc");
              break;
            }
            const ArrayType* type = consume_array(module_->signature_zone.get());
            module_->add_array_type(type);
            break;
          }
          case kWasmArrayExtendingTypeCode: {
            if (!enabled_features_.has_gc()) {
              errorf(pc(),
                     "invalid array type definition, enable with "
                     "--experimental-wasm-gc");
              break;
            }
            const ArrayType* type = consume_array(module_->signature_zone.get());
            module_->add_array_type(type);
            uint32_t super_index = consume_u32v("supertype");
            if (!module_->has_array(super_index)) {
              errorf(pc(), "invalid array supertype: %d", super_index);
              break;
            }
            break;
          }
          default:
            errorf(pc(), "unknown type form: %d", kind);
            break;
        }
      }
      module_->signature_map.Freeze();
    }
  
    void DecodeImportSection() {
      uint32_t import_table_count =
          consume_count("imports count", kV8MaxWasmImports);
      module_->import_table.reserve(import_table_count);
      for (uint32_t i = 0; ok() && i < import_table_count; ++i) {
        TRACE("DecodeImportTable[%d] module+%d\n", i,
              static_cast<int>(pc_ - start_));
  
        module_->import_table.push_back({
            {0, 0},             // module_name
            {0, 0},             // field_name
            kExternalFunction,  // kind
            0                   // index
        });
        WasmImport* import = &module_->import_table.back();
        const byte* pos = pc_;
        import->module_name = consume_string(this, true, "module name");
        import->field_name = consume_string(this, true, "field name");
        import->kind =
            static_cast<ImportExportKindCode>(consume_u8("import kind"));
        switch (import->kind) {
          case kExternalFunction: {
            // ===== Imported function ===========================================
            import->index = static_cast<uint32_t>(module_->functions.size());
            module_->num_imported_functions++;
            module_->functions.push_back({nullptr,        // sig
                                          import->index,  // func_index
                                          0,              // sig_index
                                          {0, 0},         // code
                                          true,           // imported
                                          false,          // exported
                                          false});        // declared
            WasmFunction* function = &module_->functions.back();
            function->sig_index =
                consume_sig_index(module_.get(), &function->sig);
            break;
          }
          case kExternalTable: {
            // ===== Imported table ==============================================
            if (!AddTable(module_.get())) break;
            import->index = static_cast<uint32_t>(module_->tables.size());
            module_->num_imported_tables++;
            module_->tables.emplace_back();
            WasmTable* table = &module_->tables.back();
            table->imported = true;
            const byte* type_position = pc();
            ValueType type = consume_reference_type();
            if (!WasmTable::IsValidTableType(type, module_.get())) {
              error(
                  type_position,
                  "Currently, only externref and function references are allowed "
                  "as table types");
              break;
            }
            table->type = type;
            uint8_t flags = validate_table_flags("element count");
            consume_resizable_limits(
                "element count", "elements", std::numeric_limits<uint32_t>::max(),
                &table->initial_size, &table->has_maximum_size,
                std::numeric_limits<uint32_t>::max(), &table->maximum_size,
                flags);
            break;
          }
          case kExternalMemory: {
            // ===== Imported memory =============================================
            if (!AddMemory(module_.get())) break;
            uint8_t flags = validate_memory_flags(&module_->has_shared_memory,
                                                  &module_->is_memory64);
            consume_resizable_limits(
                "memory", "pages", kSpecMaxMemoryPages, &module_->initial_pages,
                &module_->has_maximum_pages, kSpecMaxMemoryPages,
                &module_->maximum_pages, flags);
            break;
          }
          case kExternalGlobal: {
            // ===== Imported global =============================================
            import->index = static_cast<uint32_t>(module_->globals.size());
            module_->globals.push_back({kWasmVoid, false, {}, {0}, true, false});
            WasmGlobal* global = &module_->globals.back();
            global->type = consume_value_type();
            global->mutability = consume_mutability();
            if (global->mutability) {
              module_->num_imported_mutable_globals++;
            }
            break;
          }
          case kExternalTag: {
            // ===== Imported tag ================================================
            if (!enabled_features_.has_eh()) {
              errorf(pos, "unknown import kind 0x%02x", import->kind);
              break;
            }
            import->index = static_cast<uint32_t>(module_->tags.size());
            const WasmTagSig* tag_sig = nullptr;
            consume_exception_attribute();  // Attribute ignored for now.
            consume_tag_sig_index(module_.get(), &tag_sig);
            module_->tags.emplace_back(tag_sig);
            break;
          }
          default:
            errorf(pos, "unknown import kind 0x%02x", import->kind);
            break;
        }
      }
    }
  
    void DecodeFunctionSection() {
      uint32_t functions_count =
          consume_count("functions count", kV8MaxWasmFunctions);
      auto counter =
          SELECT_WASM_COUNTER(GetCounters(), origin_, wasm_functions_per, module);
      counter->AddSample(static_cast<int>(functions_count));
      DCHECK_EQ(module_->functions.size(), module_->num_imported_functions);
      uint32_t total_function_count =
          module_->num_imported_functions + functions_count;
      module_->functions.reserve(total_function_count);
      module_->num_declared_functions = functions_count;
      for (uint32_t i = 0; i < functions_count; ++i) {
        uint32_t func_index = static_cast<uint32_t>(module_->functions.size());
        module_->functions.push_back({nullptr,     // sig
                                      func_index,  // func_index
                                      0,           // sig_index
                                      {0, 0},      // code
                                      false,       // imported
                                      false,       // exported
                                      false});     // declared
        WasmFunction* function = &module_->functions.back();
        function->sig_index = consume_sig_index(module_.get(), &function->sig);
        if (!ok()) return;
      }
      DCHECK_EQ(module_->functions.size(), total_function_count);
    }
  
    void DecodeTableSection() {
      // TODO(ahaas): Set the correct limit to {kV8MaxWasmTables} once the
      // implementation of ExternRef landed.
      uint32_t max_count =
          enabled_features_.has_reftypes() ? 100000 : kV8MaxWasmTables;
      uint32_t table_count = consume_count("table count", max_count);
  
      for (uint32_t i = 0; ok() && i < table_count; i++) {
        if (!AddTable(module_.get())) break;
        module_->tables.emplace_back();
        WasmTable* table = &module_->tables.back();
        const byte* type_position = pc();
        ValueType table_type = consume_reference_type();
        if (!WasmTable::IsValidTableType(table_type, module_.get())) {
          error(type_position,
                "Currently, only externref and function references are allowed "
                "as table types");
          continue;
        }
        table->type = table_type;
        uint8_t flags = validate_table_flags("table elements");
        consume_resizable_limits(
            "table elements", "elements", std::numeric_limits<uint32_t>::max(),
            &table->initial_size, &table->has_maximum_size,
            std::numeric_limits<uint32_t>::max(), &table->maximum_size, flags);
        if (!table_type.is_defaultable()) {
          table->initial_value = consume_init_expr(module_.get(), table_type);
        }
      }
    }
  
    void DecodeMemorySection() {
      uint32_t memory_count = consume_count("memory count", kV8MaxWasmMemories);
  
      for (uint32_t i = 0; ok() && i < memory_count; i++) {
        if (!AddMemory(module_.get())) break;
        uint8_t flags = validate_memory_flags(&module_->has_shared_memory,
                                              &module_->is_memory64);
        consume_resizable_limits("memory", "pages", kSpecMaxMemoryPages,
                                 &module_->initial_pages,
                                 &module_->has_maximum_pages, kSpecMaxMemoryPages,
                                 &module_->maximum_pages, flags);
      }
    }
  
    void DecodeGlobalSection() {
      uint32_t globals_count = consume_count("globals count", kV8MaxWasmGlobals);
      uint32_t imported_globals = static_cast<uint32_t>(module_->globals.size());
      module_->globals.reserve(imported_globals + globals_count);
      for (uint32_t i = 0; ok() && i < globals_count; ++i) {
        TRACE("DecodeGlobal[%d] module+%d\n", i, static_cast<int>(pc_ - start_));
        ValueType type = consume_value_type();
        bool mutability = consume_mutability();
        if (failed()) break;
        WireBytesRef init = consume_init_expr(module_.get(), type);
        module_->globals.push_back({type, mutability, init, {0}, false, false});
      }
      if (ok()) CalculateGlobalOffsets(module_.get());
    }
  
    void DecodeExportSection() {
      uint32_t export_table_count =
          consume_count("exports count", kV8MaxWasmExports);
      module_->export_table.reserve(export_table_count);
      for (uint32_t i = 0; ok() && i < export_table_count; ++i) {
        TRACE("DecodeExportTable[%d] module+%d\n", i,
              static_cast<int>(pc_ - start_));
  
        module_->export_table.push_back({
            {0, 0},             // name
            kExternalFunction,  // kind
            0                   // index
        });
        WasmExport* exp = &module_->export_table.back();
  
        exp->name = consume_string(this, true, "field name");
  
        const byte* pos = pc();
        exp->kind = static_cast<ImportExportKindCode>(consume_u8("export kind"));
        switch (exp->kind) {
          case kExternalFunction: {
            WasmFunction* func = nullptr;
            exp->index =
                consume_func_index(module_.get(), &func, "export function index");
  
            if (failed()) break;
            DCHECK_NOT_NULL(func);
  
            module_->num_exported_functions++;
            func->exported = true;
            // Exported functions are considered "declared".
            func->declared = true;
            break;
          }
          case kExternalTable: {
            WasmTable* table = nullptr;
            exp->index = consume_table_index(module_.get(), &table);
            if (table) table->exported = true;
            break;
          }
          case kExternalMemory: {
            uint32_t index = consume_u32v("memory index");
            // TODO(titzer): This should become more regular
            // once we support multiple memories.
            if (!module_->has_memory || index != 0) {
              error("invalid memory index != 0");
            }
            module_->mem_export = true;
            break;
          }
          case kExternalGlobal: {
            WasmGlobal* global = nullptr;
            exp->index = consume_global_index(module_.get(), &global);
            if (global) {
              global->exported = true;
            }
            break;
          }
          case kExternalTag: {
            if (!enabled_features_.has_eh()) {
              errorf(pos, "invalid export kind 0x%02x", exp->kind);
              break;
            }
            WasmTag* tag = nullptr;
            exp->index = consume_tag_index(module_.get(), &tag);
            break;
          }
          default:
            errorf(pos, "invalid export kind 0x%02x", exp->kind);
            break;
        }
      }
      // Check for duplicate exports (except for asm.js).
      if (ok() && origin_ == kWasmOrigin && module_->export_table.size() > 1) {
        std::vector<WasmExport> sorted_exports(module_->export_table);
  
        auto cmp_less = [this](const WasmExport& a, const WasmExport& b) {
          // Return true if a < b.
          if (a.name.length() != b.name.length()) {
            return a.name.length() < b.name.length();
          }
          const byte* left = start() + GetBufferRelativeOffset(a.name.offset());
          const byte* right = start() + GetBufferRelativeOffset(b.name.offset());
          return memcmp(left, right, a.name.length()) < 0;
        };
        std::stable_sort(sorted_exports.begin(), sorted_exports.end(), cmp_less);
  
        auto it = sorted_exports.begin();
        WasmExport* last = &*it++;
        for (auto end = sorted_exports.end(); it != end; last = &*it++) {
          DCHECK(!cmp_less(*it, *last));  // Vector must be sorted.
          if (!cmp_less(*last, *it)) {
            const byte* pc = start() + GetBufferRelativeOffset(it->name.offset());
            TruncatedUserString<> name(pc, it->name.length());
            errorf(pc, "Duplicate export name '%.*s' for %s %d and %s %d",
                   name.length(), name.start(), ExternalKindName(last->kind),
                   last->index, ExternalKindName(it->kind), it->index);
            break;
          }
        }
      }
    }
  
    void DecodeStartSection() {
      WasmFunction* func;
      const byte* pos = pc_;
      module_->start_function_index =
          consume_func_index(module_.get(), &func, "start function index");
      if (func &&
          (func->sig->parameter_count() > 0 || func->sig->return_count() > 0)) {
        error(pos, "invalid start function: non-zero parameter or return count");
      }
    }
  
    void DecodeElementSection() {
      uint32_t element_count =
          consume_count("element count", FLAG_wasm_max_table_size);
  
      for (uint32_t i = 0; i < element_count; ++i) {
        bool expressions_as_elements;
        WasmElemSegment segment =
            consume_element_segment_header(&expressions_as_elements);
        if (failed()) return;
        DCHECK_NE(segment.type, kWasmBottom);
  
        uint32_t num_elem =
            consume_count("number of elements", max_table_init_entries());
  
        for (uint32_t j = 0; j < num_elem; j++) {
          WasmElemSegment::Entry init =
              expressions_as_elements
                  ? consume_element_expr()
                  : WasmElemSegment::Entry(WasmElemSegment::Entry::kRefFuncEntry,
                                           consume_element_func_index());
          if (failed()) return;
          if (!IsSubtypeOf(TypeOf(init), segment.type, module_.get())) {
            errorf(pc_,
                   "Invalid type in the init expression. The expected type is "
                   "'%s', but the actual type is '%s'.",
                   segment.type.name().c_str(), TypeOf(init).name().c_str());
            return;
          }
          segment.entries.push_back(init);
        }
        module_->elem_segments.push_back(std::move(segment));
      }
    }
  
    void DecodeCodeSection(bool verify_functions) {
      StartCodeSection();
      uint32_t code_section_start = pc_offset();
      uint32_t functions_count = consume_u32v("functions count");
      CheckFunctionsCount(functions_count, code_section_start);
      for (uint32_t i = 0; ok() && i < functions_count; ++i) {
        const byte* pos = pc();
        uint32_t size = consume_u32v("body size");
        if (size > kV8MaxWasmFunctionSize) {
          errorf(pos, "size %u > maximum function size %zu", size,
                 kV8MaxWasmFunctionSize);
          return;
        }
        uint32_t offset = pc_offset();
        consume_bytes(size, "function body");
        if (failed()) break;
        DecodeFunctionBody(i, size, offset, verify_functions);
      }
      DCHECK_GE(pc_offset(), code_section_start);
      set_code_section(code_section_start, pc_offset() - code_section_start);
    }
  
    void StartCodeSection() {
      if (ok()) {
        // Make sure global offset were calculated before they get accessed during
        // function compilation.
        CalculateGlobalOffsets(module_.get());
      }
    }
  
    bool CheckFunctionsCount(uint32_t functions_count, uint32_t error_offset) {
      if (functions_count != module_->num_declared_functions) {
        errorf(error_offset, "function body count %u mismatch (%u expected)",
               functions_count, module_->num_declared_functions);
        return false;
      }
      return true;
    }
  
    void DecodeFunctionBody(uint32_t index, uint32_t length, uint32_t offset,
                            bool verify_functions) {
      WasmFunction* function =
          &module_->functions[index + module_->num_imported_functions];
      function->code = {offset, length};
      if (verify_functions) {
        ModuleWireBytes bytes(module_start_, module_end_);
        VerifyFunctionBody(module_->signature_zone->allocator(),
                           index + module_->num_imported_functions, bytes,
                           module_.get(), function);
      }
    }
  
    bool CheckDataSegmentsCount(uint32_t data_segments_count) {
      if (has_seen_unordered_section(kDataCountSectionCode) &&
          data_segments_count != module_->num_declared_data_segments) {
        errorf(pc(), "data segments count %u mismatch (%u expected)",
               data_segments_count, module_->num_declared_data_segments);
        return false;
      }
      return true;
    }
  
    void DecodeDataSection() {
      uint32_t data_segments_count =
          consume_count("data segments count", kV8MaxWasmDataSegments);
      if (!CheckDataSegmentsCount(data_segments_count)) return;
  
      module_->data_segments.reserve(data_segments_count);
      for (uint32_t i = 0; ok() && i < data_segments_count; ++i) {
        const byte* pos = pc();
        TRACE("DecodeDataSegment[%d] module+%d\n", i,
              static_cast<int>(pc_ - start_));
  
        bool is_active;
        uint32_t memory_index;
        WireBytesRef dest_addr;
        consume_data_segment_header(&is_active, &memory_index, &dest_addr);
        if (failed()) break;
  
        if (is_active) {
          if (!module_->has_memory) {
            error("cannot load data without memory");
            break;
          }
          if (memory_index != 0) {
            errorf(pos, "illegal memory index %u != 0", memory_index);
            break;
          }
        }
  
        uint32_t source_length = consume_u32v("source size");
        uint32_t source_offset = pc_offset();
  
        if (is_active) {
          module_->data_segments.emplace_back(std::move(dest_addr));
        } else {
          module_->data_segments.emplace_back();
        }
  
        WasmDataSegment* segment = &module_->data_segments.back();
  
        consume_bytes(source_length, "segment data");
        if (failed()) break;
  
        segment->source = {source_offset, source_length};
      }
    }
  
    void DecodeNameSection() {
      // TODO(titzer): find a way to report name errors as warnings.
      // Ignore all but the first occurrence of name section.
      if (!has_seen_unordered_section(kNameSectionCode)) {
        set_seen_unordered_section(kNameSectionCode);
        // Use an inner decoder so that errors don't fail the outer decoder.
        Decoder inner(start_, pc_, end_, buffer_offset_);
        // Decode all name subsections.
        // Be lenient with their order.
        while (inner.ok() && inner.more()) {
          uint8_t name_type = inner.consume_u8("name type");
          if (name_type & 0x80) inner.error("name type if not varuint7");
  
          uint32_t name_payload_len = inner.consume_u32v("name payload length");
          if (!inner.checkAvailable(name_payload_len)) break;
  
          // Decode module name, ignore the rest.
          // Function and local names will be decoded when needed.
          if (name_type == NameSectionKindCode::kModule) {
            WireBytesRef name = consume_string(&inner, false, "module name");
            if (inner.ok() && validate_utf8(&inner, name)) {
              module_->name = name;
            }
          } else {
            inner.consume_bytes(name_payload_len, "name subsection payload");
          }
        }
      }
      // Skip the whole names section in the outer decoder.
      consume_bytes(static_cast<uint32_t>(end_ - start_), nullptr);
    }
  
    void DecodeSourceMappingURLSection() {
      Decoder inner(start_, pc_, end_, buffer_offset_);
      WireBytesRef url = wasm::consume_string(&inner, true, "module name");
      if (inner.ok() &&
          module_->debug_symbols.type != WasmDebugSymbols::Type::SourceMap) {
        module_->debug_symbols = {WasmDebugSymbols::Type::SourceMap, url};
      }
      set_seen_unordered_section(kSourceMappingURLSectionCode);
      consume_bytes(static_cast<uint32_t>(end_ - start_), nullptr);
    }
  
    void DecodeExternalDebugInfoSection() {
      Decoder inner(start_, pc_, end_, buffer_offset_);
      WireBytesRef url =
          wasm::consume_string(&inner, true, "external symbol file");
      // If there is an explicit source map, prefer it over DWARF info.
      if (inner.ok() &&
          module_->debug_symbols.type != WasmDebugSymbols::Type::SourceMap) {
        module_->debug_symbols = {WasmDebugSymbols::Type::ExternalDWARF, url};
        set_seen_unordered_section(kExternalDebugInfoSectionCode);
      }
      consume_bytes(static_cast<uint32_t>(end_ - start_), nullptr);
    }
  
    void DecodeCompilationHintsSection() {
      TRACE("DecodeCompilationHints module+%d\n", static_cast<int>(pc_ - start_));
  
      // TODO(frgossen): Find a way to report compilation hint errors as warnings.
      // All except first occurrence after function section and before code
      // section are ignored.
      const bool before_function_section =
          next_ordered_section_ <= kFunctionSectionCode;
      const bool after_code_section = next_ordered_section_ > kCodeSectionCode;
      if (before_function_section || after_code_section ||
          has_seen_unordered_section(kCompilationHintsSectionCode)) {
        return;
      }
      set_seen_unordered_section(kCompilationHintsSectionCode);
  
      // TODO(frgossen) Propagate errors to outer decoder in experimental phase.
      // We should use an inner decoder later and propagate its errors as
      // warnings.
      Decoder& decoder = *this;
      // Decoder decoder(start_, pc_, end_, buffer_offset_);
  
      // Ensure exactly one compilation hint per function.
      uint32_t hint_count = decoder.consume_u32v("compilation hint count");
      if (hint_count != module_->num_declared_functions) {
        decoder.errorf(decoder.pc(), "Expected %u compilation hints (%u found)",
                       module_->num_declared_functions, hint_count);
      }
  
      // Decode sequence of compilation hints.
      if (decoder.ok()) {
        module_->compilation_hints.reserve(hint_count);
      }
      for (uint32_t i = 0; decoder.ok() && i < hint_count; i++) {
        TRACE("DecodeCompilationHints[%d] module+%d\n", i,
              static_cast<int>(pc_ - start_));
  
        // Compilation hints are encoded in one byte each.
        // +-------+----------+---------------+----------+
        // | 2 bit | 2 bit    | 2 bit         | 2 bit    |
        // | ...   | Top tier | Baseline tier | Strategy |
        // +-------+----------+---------------+----------+
        uint8_t hint_byte = decoder.consume_u8("compilation hint");
        if (!decoder.ok()) break;
  
        // Decode compilation hint.
        WasmCompilationHint hint;
        hint.strategy =
            static_cast<WasmCompilationHintStrategy>(hint_byte & 0x03);
        hint.baseline_tier =
            static_cast<WasmCompilationHintTier>(hint_byte >> 2 & 0x3);
        hint.top_tier =
            static_cast<WasmCompilationHintTier>(hint_byte >> 4 & 0x3);
  
        // Ensure that the top tier never downgrades a compilation result.
        // If baseline and top tier are the same compilation will be invoked only
        // once.
        if (hint.top_tier < hint.baseline_tier &&
            hint.top_tier != WasmCompilationHintTier::kDefault) {
          decoder.errorf(decoder.pc(),
                         "Invalid compilation hint %#x (forbidden downgrade)",
                         hint_byte);
        }
  
        // Happily accept compilation hint.
        if (decoder.ok()) {
          module_->compilation_hints.push_back(std::move(hint));
        }
      }
  
      // If section was invalid reset compilation hints.
      if (decoder.failed()) {
        module_->compilation_hints.clear();
      }
  
      // @TODO(frgossen) Skip the whole compilation hints section in the outer
      // decoder if inner decoder was used.
      // consume_bytes(static_cast<uint32_t>(end_ - start_), nullptr);
    }
  
    void DecodeBranchHintsSection() {
      TRACE("DecodeBranchHints module+%d\n", static_cast<int>(pc_ - start_));
      if (!has_seen_unordered_section(kBranchHintsSectionCode)) {
        set_seen_unordered_section(kBranchHintsSectionCode);
        // Use an inner decoder so that errors don't fail the outer decoder.
        Decoder inner(start_, pc_, end_, buffer_offset_);
        BranchHintInfo branch_hints;
  
        uint32_t func_count = inner.consume_u32v("number of functions");
        // Keep track of the previous function index to validate the ordering
        int64_t last_func_idx = -1;
        for (uint32_t i = 0; i < func_count; i++) {
          uint32_t func_idx = inner.consume_u32v("function index");
          if (int64_t(func_idx) <= last_func_idx) {
            inner.errorf("Invalid function index: %d", func_idx);
            break;
          }
          last_func_idx = func_idx;
          uint8_t reserved = inner.consume_u8("reserved byte");
          if (reserved != 0x0) {
            inner.errorf("Invalid reserved byte: %#x", reserved);
            break;
          }
          uint32_t num_hints = inner.consume_u32v("number of hints");
          BranchHintMap func_branch_hints;
          TRACE("DecodeBranchHints[%d] module+%d\n", func_idx,
                static_cast<int>(inner.pc() - inner.start()));
          // Keep track of the previous branch offset to validate the ordering
          int64_t last_br_off = -1;
          for (uint32_t j = 0; j < num_hints; ++j) {
            uint32_t br_dir = inner.consume_u32v("branch direction");
            uint32_t br_off = inner.consume_u32v("branch instruction offset");
            if (int64_t(br_off) <= last_br_off) {
              inner.errorf("Invalid branch offset: %d", br_off);
              break;
            }
            last_br_off = br_off;
            TRACE("DecodeBranchHints[%d][%d] module+%d\n", func_idx, br_off,
                  static_cast<int>(inner.pc() - inner.start()));
            WasmBranchHint hint;
            switch (br_dir) {
              case 0:
                hint = WasmBranchHint::kUnlikely;
                break;
              case 1:
                hint = WasmBranchHint::kLikely;
                break;
              default:
                hint = WasmBranchHint::kNoHint;
                inner.errorf(inner.pc(), "Invalid branch hint %#x", br_dir);
                break;
            }
            if (!inner.ok()) {
              break;
            }
            func_branch_hints.insert(br_off, hint);
          }
          if (!inner.ok()) {
            break;
          }
          branch_hints.emplace(func_idx, std::move(func_branch_hints));
        }
        // Extra unexpected bytes are an error.
        if (inner.more()) {
          inner.errorf("Unexpected extra bytes: %d\n",
                       static_cast<int>(inner.pc() - inner.start()));
        }
        // If everything went well, accept the hints for the module.
        if (inner.ok()) {
          module_->branch_hints = std::move(branch_hints);
        }
      }
      // Skip the whole branch hints section in the outer decoder.
      consume_bytes(static_cast<uint32_t>(end_ - start_), nullptr);
    }
  
    void DecodeDataCountSection() {
      module_->num_declared_data_segments =
          consume_count("data segments count", kV8MaxWasmDataSegments);
    }
  
    void DecodeTagSection() {
      uint32_t tag_count = consume_count("tag count", kV8MaxWasmTags);
      for (uint32_t i = 0; ok() && i < tag_count; ++i) {
        TRACE("DecodeTag[%d] module+%d\n", i, static_cast<int>(pc_ - start_));
        const WasmTagSig* tag_sig = nullptr;
        consume_exception_attribute();  // Attribute ignored for now.
        consume_tag_sig_index(module_.get(), &tag_sig);
        module_->tags.emplace_back(tag_sig);
      }
    }
  
    bool CheckMismatchedCounts() {
      // The declared vs. defined function count is normally checked when
      // decoding the code section, but we have to check it here too in case the
      // code section is absent.
      if (module_->num_declared_functions != 0) {
        DCHECK_LT(module_->num_imported_functions, module_->functions.size());
        // We know that the code section has been decoded if the first
        // non-imported function has its code set.
        if (!module_->functions[module_->num_imported_functions].code.is_set()) {
          errorf(pc(), "function count is %u, but code section is absent",
                 module_->num_declared_functions);
          return false;
        }
      }
      // Perform a similar check for the DataCount and Data sections, where data
      // segments are declared but the Data section is absent.
      if (!CheckDataSegmentsCount(
              static_cast<uint32_t>(module_->data_segments.size()))) {
        return false;
      }
      return true;
    }
  
    ModuleResult FinishDecoding(bool verify_functions = true) {
      if (ok() && CheckMismatchedCounts()) {
        // We calculate the global offsets here, because there may not be a global
        // section and code section that would have triggered the calculation
        // before. Even without the globals section the calculation is needed
        // because globals can also be defined in the import section.
        CalculateGlobalOffsets(module_.get());
      }
  
      ModuleResult result = toResult(std::move(module_));
      if (verify_functions && result.ok() && intermediate_error_.has_error()) {
        // Copy error message and location.
        return ModuleResult{std::move(intermediate_error_)};
      }
      return result;
    }
  
    void set_code_section(uint32_t offset, uint32_t size) {
      module_->code = {offset, size};
    }
  
    // Decodes an entire module.
    ModuleResult DecodeModule(Counters* counters, AccountingAllocator* allocator,
                              bool verify_functions = true) {
      StartDecoding(counters, allocator);
      uint32_t offset = 0;
      base::Vector<const byte> orig_bytes(start(), end() - start());
      DecodeModuleHeader(base::VectorOf(start(), end() - start()), offset);
      if (failed()) {
        return FinishDecoding(verify_functions);
      }
      // Size of the module header.
      offset += 8;
      Decoder decoder(start_ + offset, end_, offset);
  
      WasmSectionIterator section_iter(&decoder);
  
      while (ok()) {
        // Shift the offset by the section header length
        offset += section_iter.payload_start() - section_iter.section_start();
        if (section_iter.section_code() != SectionCode::kUnknownSectionCode) {
          DecodeSection(section_iter.section_code(), section_iter.payload(),
                        offset, verify_functions);
        }
        // Shift the offset by the remaining section payload
        offset += section_iter.payload_length();
        if (!section_iter.more()) break;
        section_iter.advance(true);
      }
  
      if (FLAG_dump_wasm_module) DumpModule(orig_bytes);
  
      if (decoder.failed()) {
        return decoder.toResult<std::unique_ptr<WasmModule>>(nullptr);
      }
  
      return FinishDecoding(verify_functions);
    }
  
    // Decodes a single anonymous function starting at {start_}.
    FunctionResult DecodeSingleFunction(Zone* zone,
                                        const ModuleWireBytes& wire_bytes,
                                        const WasmModule* module,
                                        std::unique_ptr<WasmFunction> function) {
      pc_ = start_;
      expect_u8("type form", kWasmFunctionTypeCode);
      if (!ok()) return FunctionResult{std::move(intermediate_error_)};
      function->sig = consume_sig(zone);
      function->code = {off(pc_), static_cast<uint32_t>(end_ - pc_)};
  
      if (ok())
        VerifyFunctionBody(zone->allocator(), 0, wire_bytes, module,
                           function.get());
  
      if (intermediate_error_.has_error()) {
        return FunctionResult{std::move(intermediate_error_)};
      }
  
      return FunctionResult(std::move(function));
    }
  
    // Decodes a single function signature at {start}.
    const FunctionSig* DecodeFunctionSignature(Zone* zone, const byte* start) {
      pc_ = start;
      if (!expect_u8("type form", kWasmFunctionTypeCode)) return nullptr;
      const FunctionSig* result = consume_sig(zone);
      return ok() ? result : nullptr;
    }
  
    WireBytesRef DecodeInitExprForTesting(ValueType expected) {
      return consume_init_expr(module_.get(), expected);
    }
  
    const std::shared_ptr<WasmModule>& shared_module() const { return module_; }
  
    Counters* GetCounters() const {
      DCHECK_NOT_NULL(counters_);
      return counters_;
    }
  
    void SetCounters(Counters* counters) {
      DCHECK_NULL(counters_);
      counters_ = counters;
    }
  
   private:
    const WasmFeatures enabled_features_;
    std::shared_ptr<WasmModule> module_;
    const byte* module_start_ = nullptr;
    const byte* module_end_ = nullptr;
    Counters* counters_ = nullptr;
    // The type section is the first section in a module.
    uint8_t next_ordered_section_ = kFirstSectionInModule;
    // We store next_ordered_section_ as uint8_t instead of SectionCode so that
    // we can increment it. This static_assert should make sure that SectionCode
    // does not get bigger than uint8_t accidentially.
    static_assert(sizeof(ModuleDecoderImpl::next_ordered_section_) ==
                      sizeof(SectionCode),
                  "type mismatch");
    uint32_t seen_unordered_sections_ = 0;
    static_assert(kBitsPerByte *
                          sizeof(ModuleDecoderImpl::seen_unordered_sections_) >
                      kLastKnownModuleSection,
                  "not enough bits");
    WasmError intermediate_error_;
    ModuleOrigin origin_;
    AccountingAllocator allocator_;
    Zone init_expr_zone_{&allocator_, "initializer expression zone"};
  
    ValueType TypeOf(WasmElemSegment::Entry entry) {
      switch (entry.kind) {
        case WasmElemSegment::Entry::kGlobalGetEntry:
          return module_->globals[entry.index].type;
        case WasmElemSegment::Entry::kRefFuncEntry:
          return ValueType::Ref(module_->functions[entry.index].sig_index,
                                kNonNullable);
        case WasmElemSegment::Entry::kRefNullEntry:
          return ValueType::Ref(entry.index, kNullable);
      }
    }
  
    bool has_seen_unordered_section(SectionCode section_code) {
      return seen_unordered_sections_ & (1 << section_code);
    }
  
    void set_seen_unordered_section(SectionCode section_code) {
      seen_unordered_sections_ |= 1 << section_code;
    }
  
    uint32_t off(const byte* ptr) {
      return static_cast<uint32_t>(ptr - start_) + buffer_offset_;
    }
  
    bool AddTable(WasmModule* module) {
      if (enabled_features_.has_reftypes()) return true;
      if (module->tables.size() > 0) {
        error("At most one table is supported");
        return false;
      } else {
        return true;
      }
    }
  
    bool AddMemory(WasmModule* module) {
      if (module->has_memory) {
        error("At most one memory is supported");
        return false;
      } else {
        module->has_memory = true;
        return true;
      }
    }
  
    // Calculate individual global offsets and total size of globals table.
    // This function should be called after all globals have been defined, which
    // is after the import section and the global section, but before the global
    // offsets are accessed, e.g. by the function compilers. The moment when this
    // function should be called is not well-defined, as the global section may
    // not exist. Therefore this function is called multiple times.
    void CalculateGlobalOffsets(WasmModule* module) {
      if (module->globals.empty() || module->untagged_globals_buffer_size != 0 ||
          module->tagged_globals_buffer_size != 0) {
        // This function has already been executed before, so we don't have to
        // execute it again.
        return;
      }
      uint32_t untagged_offset = 0;
      uint32_t tagged_offset = 0;
      uint32_t num_imported_mutable_globals = 0;
      for (WasmGlobal& global : module->globals) {
        if (global.mutability && global.imported) {
          global.index = num_imported_mutable_globals++;
        } else if (global.type.is_reference()) {
          global.offset = tagged_offset;
          // All entries in the tagged_globals_buffer have size 1.
          tagged_offset++;
        } else {
          int size = global.type.element_size_bytes();
          untagged_offset = (untagged_offset + size - 1) & ~(size - 1);  // align
          global.offset = untagged_offset;
          untagged_offset += size;
        }
      }
      module->untagged_globals_buffer_size = untagged_offset;
      module->tagged_globals_buffer_size = tagged_offset;
    }
  
    // Verifies the body (code) of a given function.
    void VerifyFunctionBody(AccountingAllocator* allocator, uint32_t func_num,
                            const ModuleWireBytes& wire_bytes,
                            const WasmModule* module, WasmFunction* function) {
      WasmFunctionName func_name(function,
                                 wire_bytes.GetNameOrNull(function, module));
      if (FLAG_trace_wasm_decoder) {
        StdoutStream{} << "Verifying wasm function " << func_name << std::endl;
      }
      FunctionBody body = {
          function->sig, function->code.offset(),
          start_ + GetBufferRelativeOffset(function->code.offset()),
          start_ + GetBufferRelativeOffset(function->code.end_offset())};
  
      WasmFeatures unused_detected_features = WasmFeatures::None();
      DecodeResult result = VerifyWasmCode(allocator, enabled_features_, module,
                                           &unused_detected_features, body);
  
      // If the decode failed and this is the first error, set error code and
      // location.
      if (result.failed() && intermediate_error_.empty()) {
        // Wrap the error message from the function decoder.
        std::ostringstream error_msg;
        error_msg << "in function " << func_name << ": "
                  << result.error().message();
        intermediate_error_ = WasmError{result.error().offset(), error_msg.str()};
      }
    }
  
    uint32_t consume_sig_index(WasmModule* module, const FunctionSig** sig) {
      const byte* pos = pc_;
      uint32_t sig_index = consume_u32v("signature index");
      if (!module->has_signature(sig_index)) {
        errorf(pos, "signature index %u out of bounds (%d signatures)", sig_index,
               static_cast<int>(module->types.size()));
        *sig = nullptr;
        return 0;
      }
      *sig = module->signature(sig_index);
      return sig_index;
    }
  
    uint32_t consume_tag_sig_index(WasmModule* module, const FunctionSig** sig) {
      const byte* pos = pc_;
      uint32_t sig_index = consume_sig_index(module, sig);
      if (*sig && (*sig)->return_count() != 0) {
        errorf(pos, "tag signature %u has non-void return", sig_index);
        *sig = nullptr;
        return 0;
      }
      return sig_index;
    }
  
    uint32_t consume_count(const char* name, size_t maximum) {
      const byte* p = pc_;
      uint32_t count = consume_u32v(name);
      if (count > maximum) {
        errorf(p, "%s of %u exceeds internal limit of %zu", name, count, maximum);
        return static_cast<uint32_t>(maximum);
      }
      return count;
    }
  
    uint32_t consume_func_index(WasmModule* module, WasmFunction** func,
                                const char* name) {
      return consume_index(name, &module->functions, func);
    }
  
    uint32_t consume_global_index(WasmModule* module, WasmGlobal** global) {
      return consume_index("global index", &module->globals, global);
    }
  
    uint32_t consume_table_index(WasmModule* module, WasmTable** table) {
      return consume_index("table index", &module->tables, table);
    }
  
    uint32_t consume_tag_index(WasmModule* module, WasmTag** tag) {
      return consume_index("tag index", &module->tags, tag);
    }
  
    template <typename T>
    uint32_t consume_index(const char* name, std::vector<T>* vector, T** ptr) {
      const byte* pos = pc_;
      uint32_t index = consume_u32v(name);
      if (index >= vector->size()) {
        errorf(pos, "%s %u out of bounds (%d entr%s)", name, index,
               static_cast<int>(vector->size()),
               vector->size() == 1 ? "y" : "ies");
        *ptr = nullptr;
        return 0;
      }
      *ptr = &(*vector)[index];
      return index;
    }
  
    uint8_t validate_table_flags(const char* name) {
      uint8_t flags = consume_u8("table limits flags");
      STATIC_ASSERT(kNoMaximum < kWithMaximum);
      if (V8_UNLIKELY(flags > kWithMaximum)) {
        errorf(pc() - 1, "invalid %s limits flags", name);
      }
      return flags;
    }
  
    uint8_t validate_memory_flags(bool* has_shared_memory, bool* is_memory64) {
      uint8_t flags = consume_u8("memory limits flags");
      *has_shared_memory = false;
      switch (flags) {
        case kNoMaximum:
        case kWithMaximum:
          break;
        case kSharedNoMaximum:
        case kSharedWithMaximum:
          if (!enabled_features_.has_threads()) {
            errorf(pc() - 1,
                   "invalid memory limits flags 0x%x (enable via "
                   "--experimental-wasm-threads)",
                   flags);
          }
          *has_shared_memory = true;
          // V8 does not support shared memory without a maximum.
          if (flags == kSharedNoMaximum) {
            errorf(pc() - 1,
                   "memory limits flags must have maximum defined if shared is "
                   "true");
          }
          break;
        case kMemory64NoMaximum:
        case kMemory64WithMaximum:
          if (!enabled_features_.has_memory64()) {
            errorf(pc() - 1,
                   "invalid memory limits flags 0x%x (enable via "
                   "--experimental-wasm-memory64)",
                   flags);
          }
          *is_memory64 = true;
          break;
        default:
          errorf(pc() - 1, "invalid memory limits flags 0x%x", flags);
          break;
      }
      return flags;
    }
  
    void consume_resizable_limits(const char* name, const char* units,
                                  uint32_t max_initial, uint32_t* initial,
                                  bool* has_max, uint32_t max_maximum,
                                  uint32_t* maximum, uint8_t flags) {
      const byte* pos = pc();
      // For memory64 we need to read the numbers as LEB-encoded 64-bit unsigned
      // integer. All V8 limits are still within uint32_t range though.
      const bool is_memory64 =
          flags == kMemory64NoMaximum || flags == kMemory64WithMaximum;
      uint64_t initial_64 = is_memory64 ? consume_u64v("initial size")
                                        : consume_u32v("initial size");
      if (initial_64 > max_initial) {
        errorf(pos,
               "initial %s size (%" PRIu64
               " %s) is larger than implementation limit (%u)",
               name, initial_64, units, max_initial);
      }
      *initial = static_cast<uint32_t>(initial_64);
      if (flags & 1) {
        *has_max = true;
        pos = pc();
        uint64_t maximum_64 = is_memory64 ? consume_u64v("maximum size")
                                          : consume_u32v("maximum size");
        if (maximum_64 > max_maximum) {
          errorf(pos,
                 "maximum %s size (%" PRIu64
                 " %s) is larger than implementation limit (%u)",
                 name, maximum_64, units, max_maximum);
        }
        if (maximum_64 < *initial) {
          errorf(pos,
                 "maximum %s size (%" PRIu64 " %s) is less than initial (%u %s)",
                 name, maximum_64, units, *initial, units);
        }
        *maximum = static_cast<uint32_t>(maximum_64);
      } else {
        *has_max = false;
        *maximum = max_initial;
      }
    }
  
    bool expect_u8(const char* name, uint8_t expected) {
      const byte* pos = pc();
      uint8_t value = consume_u8(name);
      if (value != expected) {
        errorf(pos, "expected %s 0x%02x, got 0x%02x", name, expected, value);
        return false;
      }
      return true;
    }
  
    WireBytesRef consume_init_expr(WasmModule* module, ValueType expected) {
      FunctionBody body(FunctionSig::Build(&init_expr_zone_, {expected}, {}),
                        buffer_offset_, pc_, end_);
      WasmFeatures detected;
      WasmFullDecoder<Decoder::kFullValidation, InitExprInterface,
                      kInitExpression>
          decoder(&init_expr_zone_, module, enabled_features_, &detected, body,
                  module);
  
      uint32_t offset = this->pc_offset();
  
      decoder.DecodeFunctionBody();
  
      this->pc_ = decoder.end();
  
      if (decoder.failed()) {
        error(decoder.error().offset(), decoder.error().message().c_str());
        return {};
      }
  
      if (!decoder.interface().end_found()) {
        error("Initializer expression is missing 'end'");
        return {};
      }
  
      return {offset, static_cast<uint32_t>(decoder.end() - decoder.start())};
    }
  
    // Read a mutability flag
    bool consume_mutability() {
      byte val = consume_u8("mutability");
      if (val > 1) error(pc_ - 1, "invalid mutability");
      return val != 0;
    }
  
    ValueType consume_value_type() {
      uint32_t type_length;
      ValueType result = value_type_reader::read_value_type<kFullValidation>(
          this, this->pc(), &type_length, module_.get(),
          origin_ == kWasmOrigin ? enabled_features_ : WasmFeatures::None());
      consume_bytes(type_length, "value type");
      return result;
    }
  
    ValueType consume_storage_type() {
      uint8_t opcode = read_u8<kFullValidation>(this->pc());
      switch (opcode) {
        case kI8Code:
          consume_bytes(1, "i8");
          return kWasmI8;
        case kI16Code:
          consume_bytes(1, "i16");
          return kWasmI16;
        default:
          // It is not a packed type, so it has to be a value type.
          return consume_value_type();
      }
    }
  
    // Reads a reference type for tables and element segment headers.
    // Unless extensions are enabled, only funcref is allowed.
    // TODO(manoskouk): Replace this with consume_value_type (and checks against
    //                  the returned type at callsites as needed) once the
    //                  'reftypes' proposal is standardized.
    ValueType consume_reference_type() {
      if (!enabled_features_.has_reftypes()) {
        uint8_t ref_type = consume_u8("reference type");
        if (ref_type != kFuncRefCode) {
          error(pc_ - 1,
                "invalid table type. Consider using experimental flags.");
          return kWasmBottom;
        }
        return kWasmFuncRef;
      } else {
        const byte* position = pc();
        ValueType result = consume_value_type();
        if (!result.is_reference()) {
          error(position, "expected reference type");
        }
        return result;
      }
    }
  
    const FunctionSig* consume_sig(Zone* zone) {
      // Parse parameter types.
      uint32_t param_count =
          consume_count("param count", kV8MaxWasmFunctionParams);
      if (failed()) return nullptr;
      std::vector<ValueType> params;
      for (uint32_t i = 0; ok() && i < param_count; ++i) {
        params.push_back(consume_value_type());
      }
      std::vector<ValueType> returns;
  
      // Parse return types.
      uint32_t return_count =
          consume_count("return count", kV8MaxWasmFunctionReturns);
      if (failed()) return nullptr;
      for (uint32_t i = 0; ok() && i < return_count; ++i) {
        returns.push_back(consume_value_type());
      }
      if (failed()) return nullptr;
  
      // FunctionSig stores the return types first.
      ValueType* buffer = zone->NewArray<ValueType>(param_count + return_count);
      uint32_t b = 0;
      for (uint32_t i = 0; i < return_count; ++i) buffer[b++] = returns[i];
      for (uint32_t i = 0; i < param_count; ++i) buffer[b++] = params[i];
  
      return zone->New<FunctionSig>(return_count, param_count, buffer);
    }
  
    const StructType* consume_struct(Zone* zone) {
      uint32_t field_count = consume_count("field count", kV8MaxWasmStructFields);
      if (failed()) return nullptr;
      ValueType* fields = zone->NewArray<ValueType>(field_count);
      bool* mutabilities = zone->NewArray<bool>(field_count);
      for (uint32_t i = 0; ok() && i < field_count; ++i) {
        ValueType field = consume_storage_type();
        fields[i] = field;
        bool mutability = consume_mutability();
        mutabilities[i] = mutability;
      }
      if (failed()) return nullptr;
      uint32_t* offsets = zone->NewArray<uint32_t>(field_count);
      return zone->New<StructType>(field_count, offsets, fields, mutabilities);
    }
  
    const ArrayType* consume_array(Zone* zone) {
      ValueType field = consume_storage_type();
      if (failed()) return nullptr;
      bool mutability = consume_mutability();
      if (!V8_LIKELY(mutability)) {
        error(this->pc() - 1, "immutable arrays are not supported yet");
      }
      return zone->New<ArrayType>(field, mutability);
    }
  
    // Consume the attribute field of an exception.
    uint32_t consume_exception_attribute() {
      const byte* pos = pc_;
      uint32_t attribute = consume_u32v("exception attribute");
      if (attribute != kExceptionAttribute) {
        errorf(pos, "exception attribute %u not supported", attribute);
        return 0;
      }
      return attribute;
    }
  
    WasmElemSegment consume_element_segment_header(
        bool* expressions_as_elements) {
      const byte* pos = pc();
  
      // The mask for the bit in the flag which indicates if the segment is
      // active or not (0 is active).
      constexpr uint8_t kNonActiveMask = 1 << 0;
      // The mask for the bit in the flag which indicates:
      // - for active tables, if the segment has an explicit table index field.
      // - for non-active tables, whether the table is declarative (vs. passive).
      constexpr uint8_t kHasTableIndexOrIsDeclarativeMask = 1 << 1;
      // The mask for the bit in the flag which indicates if the functions of this
      // segment are defined as function indices (0) or init. expressions (1).
      constexpr uint8_t kExpressionsAsElementsMask = 1 << 2;
      constexpr uint8_t kFullMask = kNonActiveMask |
                                    kHasTableIndexOrIsDeclarativeMask |
                                    kExpressionsAsElementsMask;
  
      uint32_t flag = consume_u32v("flag");
      if ((flag & kFullMask) != flag) {
        errorf(pos, "illegal flag value %u. Must be between 0 and 7", flag);
        return {};
      }
  
      const WasmElemSegment::Status status =
          (flag & kNonActiveMask) ? (flag & kHasTableIndexOrIsDeclarativeMask)
                                        ? WasmElemSegment::kStatusDeclarative
                                        : WasmElemSegment::kStatusPassive
                                  : WasmElemSegment::kStatusActive;
      if (status == WasmElemSegment::kStatusDeclarative &&
          !enabled_features_.has_reftypes()) {
        error(
            "Declarative element segments require --experimental-wasm-reftypes");
        return {};
      }
      const bool is_active = status == WasmElemSegment::kStatusActive;
  
      *expressions_as_elements = flag & kExpressionsAsElementsMask;
  
      const bool has_table_index =
          is_active && (flag & kHasTableIndexOrIsDeclarativeMask);
      uint32_t table_index = has_table_index ? consume_u32v("table index") : 0;
      if (is_active && table_index >= module_->tables.size()) {
        errorf(pos, "out of bounds%s table index %u",
               has_table_index ? " implicit" : "", table_index);
        return {};
      }
      ValueType table_type =
          is_active ? module_->tables[table_index].type : kWasmBottom;
  
      WireBytesRef offset;
      if (is_active) {
        offset = consume_init_expr(module_.get(), kWasmI32);
        // Failed to parse offset initializer, return early.
        if (failed()) return {};
      }
  
      // Denotes an active segment without table index, type, or element kind.
      const bool backwards_compatible_mode =
          is_active && !(flag & kHasTableIndexOrIsDeclarativeMask);
      ValueType type;
      if (*expressions_as_elements) {
        type =
            backwards_compatible_mode ? kWasmFuncRef : consume_reference_type();
        if (is_active && !IsSubtypeOf(type, table_type, this->module_.get())) {
          errorf(pos,
                 "Element segment of type %s is not a subtype of referenced "
                 "table %u (of type %s)",
                 type.name().c_str(), table_index, table_type.name().c_str());
          return {};
        }
      } else {
        if (!backwards_compatible_mode) {
          // We have to check that there is an element kind of type Function. All
          // other element kinds are not valid yet.
          uint8_t val = consume_u8("element kind");
          if (static_cast<ImportExportKindCode>(val) != kExternalFunction) {
            errorf(pos, "illegal element kind 0x%x. Must be 0x%x", val,
                   kExternalFunction);
            return {};
          }
        }
        if (!is_active) {
          // Declarative and passive segments without explicit type are funcref.
          type = kWasmFuncRef;
        } else {
          type = table_type;
          // Active segments with function indices must reference a function
          // table. TODO(7748): Add support for anyref tables when we have them.
          if (!IsSubtypeOf(table_type, kWasmFuncRef, this->module_.get())) {
            errorf(pos,
                   "An active element segment with function indices as elements "
                   "must reference a table of %s. Instead, table %u of type %s "
                   "is referenced.",
                   enabled_features_.has_typed_funcref()
                       ? "a subtype of type funcref"
                       : "type funcref",
                   table_index, table_type.name().c_str());
            return {};
          }
        }
      }
  
      if (is_active) {
        return {type, table_index, std::move(offset)};
      } else {
        return {type, status == WasmElemSegment::kStatusDeclarative};
      }
    }
  
    void consume_data_segment_header(bool* is_active, uint32_t* index,
                                     WireBytesRef* offset) {
      const byte* pos = pc();
      uint32_t flag = consume_u32v("flag");
  
      // Some flag values are only valid for specific proposals.
      if (flag != SegmentFlags::kActiveNoIndex &&
          flag != SegmentFlags::kPassive &&
          flag != SegmentFlags::kActiveWithIndex) {
        errorf(pos, "illegal flag value %u. Must be 0, 1, or 2", flag);
        return;
      }
  
      // We know now that the flag is valid. Time to read the rest.
      ValueType expected_type = module_->is_memory64 ? kWasmI64 : kWasmI32;
      if (flag == SegmentFlags::kActiveNoIndex) {
        *is_active = true;
        *index = 0;
        *offset = consume_init_expr(module_.get(), expected_type);
        return;
      }
      if (flag == SegmentFlags::kPassive) {
        *is_active = false;
        return;
      }
      if (flag == SegmentFlags::kActiveWithIndex) {
        *is_active = true;
        *index = consume_u32v("memory index");
        *offset = consume_init_expr(module_.get(), expected_type);
      }
    }
  
    uint32_t consume_element_func_index() {
      WasmFunction* func = nullptr;
      uint32_t index =
          consume_func_index(module_.get(), &func, "element function index");
      if (failed()) return index;
      func->declared = true;
      DCHECK_NE(func, nullptr);
      DCHECK_EQ(index, func->func_index);
      return index;
    }
  
    // TODO(manoskouk): When reftypes lands, consider if we can implement this
    // with consume_init_expr(). It will require changes in module-instantiate.cc,
    // in {LoadElemSegmentImpl}.
    WasmElemSegment::Entry consume_element_expr() {
      uint8_t opcode = consume_u8("element opcode");
      if (failed()) return {};
      switch (opcode) {
        case kExprRefNull: {
          HeapTypeImmediate<kFullValidation> imm(WasmFeatures::All(), this,
                                                 this->pc(), module_.get());
          consume_bytes(imm.length, "ref.null immediate");
          expect_u8("end opcode", kExprEnd);
          return {WasmElemSegment::Entry::kRefNullEntry,
                  static_cast<uint32_t>(imm.type.representation())};
        }
        case kExprRefFunc: {
          uint32_t index = consume_element_func_index();
          if (failed()) return {};
          expect_u8("end opcode", kExprEnd);
          return {WasmElemSegment::Entry::kRefFuncEntry, index};
        }
        case kExprGlobalGet: {
          if (!enabled_features_.has_reftypes()) {
            errorf(
                "Unexpected opcode 0x%x in element. Enable with "
                "--experimental-wasm-reftypes",
                kExprGlobalGet);
            return {};
          }
          uint32_t index = this->consume_u32v("global index");
          if (failed()) return {};
          if (index >= module_->globals.size()) {
            errorf("Out-of-bounds global index %d", index);
            return {};
          }
          expect_u8("end opcode", kExprEnd);
          return {WasmElemSegment::Entry::kGlobalGetEntry, index};
        }
        default:
          error("invalid opcode in element");
          return {};
      }
    }
  };
  
  ModuleResult DecodeWasmModule(
      const WasmFeatures& enabled, const byte* module_start,
      const byte* module_end, bool verify_functions, ModuleOrigin origin,
      Counters* counters, std::shared_ptr<metrics::Recorder> metrics_recorder,
      v8::metrics::Recorder::ContextId context_id, DecodingMethod decoding_method,
      AccountingAllocator* allocator) {
    size_t size = module_end - module_start;
    CHECK_LE(module_start, module_end);
    size_t max_size = max_module_size();
    if (size > max_size) {
      return ModuleResult{
          WasmError{0, "size > maximum module size (%zu): %zu", max_size, size}};
    }
    // TODO(bradnelson): Improve histogram handling of size_t.
    auto size_counter =
        SELECT_WASM_COUNTER(counters, origin, wasm, module_size_bytes);
    size_counter->AddSample(static_cast<int>(size));
    // Signatures are stored in zone memory, which have the same lifetime
    // as the {module}.
    ModuleDecoderImpl decoder(enabled, module_start, module_end, origin);
    v8::metrics::WasmModuleDecoded metrics_event;
    base::ElapsedTimer timer;
    timer.Start();
    base::ThreadTicks thread_ticks = base::ThreadTicks::IsSupported()
                                         ? base::ThreadTicks::Now()
                                         : base::ThreadTicks();
    ModuleResult result =
        decoder.DecodeModule(counters, allocator, verify_functions);
  
    // Record event metrics.
    metrics_event.wall_clock_duration_in_us = timer.Elapsed().InMicroseconds();
    timer.Stop();
    if (!thread_ticks.IsNull()) {
      metrics_event.cpu_duration_in_us =
          (base::ThreadTicks::Now() - thread_ticks).InMicroseconds();
    }
    metrics_event.success = decoder.ok() && result.ok();
    metrics_event.async = decoding_method == DecodingMethod::kAsync ||
                          decoding_method == DecodingMethod::kAsyncStream;
    metrics_event.streamed = decoding_method == DecodingMethod::kSyncStream ||
                             decoding_method == DecodingMethod::kAsyncStream;
    if (result.ok()) {
      metrics_event.function_count = result.value()->num_declared_functions;
    } else if (auto&& module = decoder.shared_module()) {
      metrics_event.function_count = module->num_declared_functions;
    }
    metrics_event.module_size_in_bytes = size;
    metrics_recorder->DelayMainThreadEvent(metrics_event, context_id);
  
    return result;
  }
  
  ModuleDecoder::ModuleDecoder(const WasmFeatures& enabled)
      : enabled_features_(enabled) {}
  
  ModuleDecoder::~ModuleDecoder() = default;
  
  const std::shared_ptr<WasmModule>& ModuleDecoder::shared_module() const {
    return impl_->shared_module();
  }
  
  void ModuleDecoder::StartDecoding(
      Counters* counters, std::shared_ptr<metrics::Recorder> metrics_recorder,
      v8::metrics::Recorder::ContextId context_id, AccountingAllocator* allocator,
      ModuleOrigin origin) {
    DCHECK_NULL(impl_);
    impl_.reset(new ModuleDecoderImpl(enabled_features_, origin));
    impl_->StartDecoding(counters, allocator);
  }
  
  void ModuleDecoder::DecodeModuleHeader(base::Vector<const uint8_t> bytes,
                                         uint32_t offset) {
    impl_->DecodeModuleHeader(bytes, offset);
  }
  
  void ModuleDecoder::DecodeSection(SectionCode section_code,
                                    base::Vector<const uint8_t> bytes,
                                    uint32_t offset, bool verify_functions) {
    impl_->DecodeSection(section_code, bytes, offset, verify_functions);
  }
  
  void ModuleDecoder::DecodeFunctionBody(uint32_t index, uint32_t length,
                                         uint32_t offset, bool verify_functions) {
    impl_->DecodeFunctionBody(index, length, offset, verify_functions);
  }
  
  void ModuleDecoder::StartCodeSection() { impl_->StartCodeSection(); }
  
  bool ModuleDecoder::CheckFunctionsCount(uint32_t functions_count,
                                          uint32_t error_offset) {
    return impl_->CheckFunctionsCount(functions_count, error_offset);
  }
  
  ModuleResult ModuleDecoder::FinishDecoding(bool verify_functions) {
    return impl_->FinishDecoding(verify_functions);
  }
  
  void ModuleDecoder::set_code_section(uint32_t offset, uint32_t size) {
    return impl_->set_code_section(offset, size);
  }
  
  size_t ModuleDecoder::IdentifyUnknownSection(ModuleDecoder* decoder,
                                               base::Vector<const uint8_t> bytes,
                                               uint32_t offset,
                                               SectionCode* result) {
    if (!decoder->ok()) return 0;
    decoder->impl_->Reset(bytes, offset);
    *result = IdentifyUnknownSectionInternal(decoder->impl_.get());
    return decoder->impl_->pc() - bytes.begin();
  }
  
  bool ModuleDecoder::ok() { return impl_->ok(); }
  
  const FunctionSig* DecodeWasmSignatureForTesting(const WasmFeatures& enabled,
                                                   Zone* zone, const byte* start,
                                                   const byte* end) {
    ModuleDecoderImpl decoder(enabled, start, end, kWasmOrigin);
    return decoder.DecodeFunctionSignature(zone, start);
  }
  
  WireBytesRef DecodeWasmInitExprForTesting(const WasmFeatures& enabled,
                                            const byte* start, const byte* end,
                                            ValueType expected) {
    ModuleDecoderImpl decoder(enabled, start, end, kWasmOrigin);
    AccountingAllocator allocator;
    decoder.StartDecoding(nullptr, &allocator);
    return decoder.DecodeInitExprForTesting(expected);
  }
  
  FunctionResult DecodeWasmFunctionForTesting(
      const WasmFeatures& enabled, Zone* zone, const ModuleWireBytes& wire_bytes,
      const WasmModule* module, const byte* function_start,
      const byte* function_end, Counters* counters) {
    size_t size = function_end - function_start;
    CHECK_LE(function_start, function_end);
    if (size > kV8MaxWasmFunctionSize) {
      return FunctionResult{WasmError{0,
                                      "size > maximum function size (%zu): %zu",
                                      kV8MaxWasmFunctionSize, size}};
    }
    ModuleDecoderImpl decoder(enabled, function_start, function_end, kWasmOrigin);
    decoder.SetCounters(counters);
    return decoder.DecodeSingleFunction(zone, wire_bytes, module,
                                        std::make_unique<WasmFunction>());
  }
  
  AsmJsOffsetsResult DecodeAsmJsOffsets(
      base::Vector<const uint8_t> encoded_offsets) {
    std::vector<AsmJsOffsetFunctionEntries> functions;
  
    Decoder decoder(encoded_offsets);
    uint32_t functions_count = decoder.consume_u32v("functions count");
    // Consistency check.
    DCHECK_GE(encoded_offsets.size(), functions_count);
    functions.reserve(functions_count);
  
    for (uint32_t i = 0; i < functions_count; ++i) {
      uint32_t size = decoder.consume_u32v("table size");
      if (size == 0) {
        functions.emplace_back();
        continue;
      }
      DCHECK(decoder.checkAvailable(size));
      const byte* table_end = decoder.pc() + size;
      uint32_t locals_size = decoder.consume_u32v("locals size");
      int function_start_position = decoder.consume_u32v("function start pos");
      int function_end_position = function_start_position;
      int last_byte_offset = locals_size;
      int last_asm_position = function_start_position;
      std::vector<AsmJsOffsetEntry> func_asm_offsets;
      func_asm_offsets.reserve(size / 4);  // conservative estimation
      // Add an entry for the stack check, associated with position 0.
      func_asm_offsets.push_back(
          {0, function_start_position, function_start_position});
      while (decoder.pc() < table_end) {
        DCHECK(decoder.ok());
        last_byte_offset += decoder.consume_u32v("byte offset delta");
        int call_position =
            last_asm_position + decoder.consume_i32v("call position delta");
        int to_number_position =
            call_position + decoder.consume_i32v("to_number position delta");
        last_asm_position = to_number_position;
        if (decoder.pc() == table_end) {
          // The last entry is the function end marker.
          DCHECK_EQ(call_position, to_number_position);
          function_end_position = call_position;
        } else {
          func_asm_offsets.push_back(
              {last_byte_offset, call_position, to_number_position});
        }
      }
      DCHECK_EQ(decoder.pc(), table_end);
      functions.emplace_back(AsmJsOffsetFunctionEntries{
          function_start_position, function_end_position,
          std::move(func_asm_offsets)});
    }
    DCHECK(decoder.ok());
    DCHECK(!decoder.more());
  
    return decoder.toResult(AsmJsOffsets{std::move(functions)});
  }
  
  std::vector<CustomSectionOffset> DecodeCustomSections(const byte* start,
                                                        const byte* end) {
    Decoder decoder(start, end);
    decoder.consume_bytes(4, "wasm magic");
    decoder.consume_bytes(4, "wasm version");
  
    std::vector<CustomSectionOffset> result;
  
    while (decoder.more()) {
      byte section_code = decoder.consume_u8("section code");
      uint32_t section_length = decoder.consume_u32v("section length");
      uint32_t section_start = decoder.pc_offset();
      if (section_code != 0) {
        // Skip known sections.
        decoder.consume_bytes(section_length, "section bytes");
        continue;
      }
      uint32_t name_length = decoder.consume_u32v("name length");
      uint32_t name_offset = decoder.pc_offset();
      decoder.consume_bytes(name_length, "section name");
      uint32_t payload_offset = decoder.pc_offset();
      if (section_length < (payload_offset - section_start)) {
        decoder.error("invalid section length");
        break;
      }
      uint32_t payload_length = section_length - (payload_offset - section_start);
      decoder.consume_bytes(payload_length);
      if (decoder.failed()) break;
      result.push_back({{section_start, section_length},
                        {name_offset, name_length},
                        {payload_offset, payload_length}});
    }
  
    return result;
  }
  
  namespace {
  
  bool FindNameSection(Decoder* decoder) {
    static constexpr int kModuleHeaderSize = 8;
    decoder->consume_bytes(kModuleHeaderSize, "module header");
  
    WasmSectionIterator section_iter(decoder);
  
    while (decoder->ok() && section_iter.more() &&
           section_iter.section_code() != kNameSectionCode) {
      section_iter.advance(true);
    }
    if (!section_iter.more()) return false;
  
    // Reset the decoder to not read beyond the name section end.
    decoder->Reset(section_iter.payload(), decoder->pc_offset());
    return true;
  }
  
  }  // namespace
  
  void DecodeFunctionNames(const byte* module_start, const byte* module_end,
                           std::unordered_map<uint32_t, WireBytesRef>* names) {
    DCHECK_NOT_NULL(names);
    DCHECK(names->empty());
  
    Decoder decoder(module_start, module_end);
    if (FindNameSection(&decoder)) {
      while (decoder.ok() && decoder.more()) {
        uint8_t name_type = decoder.consume_u8("name type");
        if (name_type & 0x80) break;  // no varuint7
  
        uint32_t name_payload_len = decoder.consume_u32v("name payload length");
        if (!decoder.checkAvailable(name_payload_len)) break;
  
        if (name_type != NameSectionKindCode::kFunction) {
          decoder.consume_bytes(name_payload_len, "name subsection payload");
          continue;
        }
        uint32_t functions_count = decoder.consume_u32v("functions count");
  
        for (; decoder.ok() && functions_count > 0; --functions_count) {
          uint32_t function_index = decoder.consume_u32v("function index");
          WireBytesRef name = consume_string(&decoder, false, "function name");
  
          // Be lenient with errors in the name section: Ignore non-UTF8 names.
          // You can even assign to the same function multiple times (last valid
          // one wins).
          if (decoder.ok() && validate_utf8(&decoder, name)) {
            names->insert(std::make_pair(function_index, name));
          }
        }
      }
    }
  }
  
  NameMap DecodeNameMap(base::Vector<const uint8_t> module_bytes,
                        uint8_t name_section_kind) {
    Decoder decoder(module_bytes);
    if (!FindNameSection(&decoder)) return NameMap{{}};
  
    std::vector<NameAssoc> names;
    while (decoder.ok() && decoder.more()) {
      uint8_t name_type = decoder.consume_u8("name type");
      if (name_type & 0x80) break;  // no varuint7
  
      uint32_t name_payload_len = decoder.consume_u32v("name payload length");
      if (!decoder.checkAvailable(name_payload_len)) break;
  
      if (name_type != name_section_kind) {
        decoder.consume_bytes(name_payload_len, "name subsection payload");
        continue;
      }
  
      uint32_t count = decoder.consume_u32v("names count");
      for (uint32_t i = 0; i < count; i++) {
        uint32_t index = decoder.consume_u32v("index");
        WireBytesRef name = consume_string(&decoder, false, "name");
        if (!decoder.ok()) break;
        if (index > kMaxInt) continue;
        if (!validate_utf8(&decoder, name)) continue;
        names.emplace_back(static_cast<int>(index), name);
      }
    }
    std::stable_sort(names.begin(), names.end(), NameAssoc::IndexLess{});
    return NameMap{std::move(names)};
  }
  
  IndirectNameMap DecodeIndirectNameMap(base::Vector<const uint8_t> module_bytes,
                                        uint8_t name_section_kind) {
    Decoder decoder(module_bytes);
    if (!FindNameSection(&decoder)) return IndirectNameMap{{}};
  
    std::vector<IndirectNameMapEntry> entries;
    while (decoder.ok() && decoder.more()) {
      uint8_t name_type = decoder.consume_u8("name type");
      if (name_type & 0x80) break;  // no varuint7
  
      uint32_t name_payload_len = decoder.consume_u32v("name payload length");
      if (!decoder.checkAvailable(name_payload_len)) break;
  
      if (name_type != name_section_kind) {
        decoder.consume_bytes(name_payload_len, "name subsection payload");
        continue;
      }
  
      uint32_t outer_count = decoder.consume_u32v("outer count");
      for (uint32_t i = 0; i < outer_count; ++i) {
        uint32_t outer_index = decoder.consume_u32v("outer index");
        if (outer_index > kMaxInt) continue;
        std::vector<NameAssoc> names;
        uint32_t inner_count = decoder.consume_u32v("inner count");
        for (uint32_t k = 0; k < inner_count; ++k) {
          uint32_t inner_index = decoder.consume_u32v("inner index");
          WireBytesRef name = consume_string(&decoder, false, "name");
          if (!decoder.ok()) break;
          if (inner_index > kMaxInt) continue;
          // Ignore non-utf8 names.
          if (!validate_utf8(&decoder, name)) continue;
          names.emplace_back(static_cast<int>(inner_index), name);
        }
        // Use stable sort to get deterministic names (the first one declared)
        // even in the presence of duplicates.
        std::stable_sort(names.begin(), names.end(), NameAssoc::IndexLess{});
        entries.emplace_back(static_cast<int>(outer_index), std::move(names));
      }
    }
    std::stable_sort(entries.begin(), entries.end(),
                     IndirectNameMapEntry::IndexLess{});
    return IndirectNameMap{std::move(entries)};
  }
  
  #undef TRACE
  
  }  // namespace wasm
  }  // namespace internal
  }  // namespace v8
  