generator.py

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#  Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
#  See https://llvm.org/LICENSE.txt for license information.
#  SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
"""Code generation from ESI manifests to source code.
 
Uses a two-pass approach for C++: first collect and name all reachable types,
then emit structs/aliases in a dependency-ordered sequence so headers are
standalone and deterministic.
"""
 
# C++ header support included with the runtime, though it is intended to be
# extensible for other languages.
 
from typing import Any, Callable, Dict, List, Set, TextIO, Tuple, Type, Optional, cast
from ..accelerator import AcceleratorConnection, Context, Instance
from ..esiCppAccel import AppID, ModuleInfo
from .. import types
 
import sys
import os
import textwrap
import argparse
from pathlib import Path
 
from .indented_writer import IndentedWriter
from .ports import CppPortGroup, CppPortKind, cpp_port_kind
 
 
class Generator:
  """Base class for all generators."""
 
  language: Optional[str] = None
 
  def __init__(self, conn: AcceleratorConnection) -> None:
    self.manifest = conn.manifest()
 
  def generate(self, output_dir: Path, system_name: str) -> None:
    raise NotImplementedError("Generator.generate() must be overridden")
 
 
class CppGenerator(Generator):
  """Generate C++ headers from an ESI manifest."""
 
  language = "C++"
 
  def __init__(self, conn: AcceleratorConnection) -> None:
    super().__init__(conn)
    self._conn = conn
    self.type_planner = CppTypePlanner(self.manifest.type_table)
    self.type_emitter = CppTypeEmitter(self.type_planner)
 
  def get_consts_str(self, module_info: ModuleInfo) -> str:
    """Get the C++ code for a constant in a module."""
    const_strs: List[str] = [
        f"static constexpr {self.type_emitter.type_identifier(const.type)} "
        f"{name} = 0x{const.value:x};"
        for name, const in module_info.constants.items()
    ]
    return "\n".join(const_strs)
 
  # ---------------------------------------------------------------------------
  # Port-emission helpers
  # ---------------------------------------------------------------------------
 
  @staticmethod
  def _sanitize_id(name: str) -> str:
    """Return a C++-safe identifier from an AppID name."""
    result = []
    for ch in name:
      result.append(ch if (ch.isalnum() or ch == "_") else "_")
    if not result:
      return "_port"
    if result[0].isdigit():
      result.insert(0, "_")
    return "".join(result)
 
  def _build_module_instance_map(self) -> Dict[str, Instance]:
    """Walk the live hierarchy and return {module_name: first Instance}."""
    accel = self._conn.build_accelerator()
    result: Dict[str, Instance] = {}
    queue = list(accel.children.values())
    while queue:
      inst = queue.pop(0)
      info = inst.cpp_hwmodule.info
      if info is not None:
        name = info.name
        if name is not None and name not in result:
          result[name] = inst
      queue.extend(inst.children.values())
    return result
 
  def _port_using_aliases(self, kind: CppPortKind, alias_prefix: str,
                          port: types.BundlePort) -> List[Tuple[str, str]]:
    """Return (alias_name, type_id) pairs to emit as `using` declarations at
    module-class scope for the typed-port template parameters."""
    if kind.alias_kind == "func":
      # func kinds are always FunctionPort / CallbackPort, which share the
      # arg/result attributes accessed here.
      func_port = cast(types.FunctionPort, port)
      arg = self.type_emitter.type_identifier(func_port.arg_window_type or
                                              func_port.arg_type)
      res = self.type_emitter.type_identifier(func_port.result_window_type or
                                              func_port.result_type)
      return [
          (f"{alias_prefix}Args", arg),
          (f"{alias_prefix}Result", res),
      ]
    if kind.alias_kind == "chan":
      # chan kinds are always ToHostPort / FromHostPort, which share the
      # data attributes accessed here.
      chan_port = cast(types.ToHostPort, port)
      data = self.type_emitter.type_identifier(chan_port.data_window_type or
                                               chan_port.data_type)
      return [(f"{alias_prefix}Data", data)]
    return []
 
  @staticmethod
  def _appid_expr(appid: AppID) -> str:
    """Return `esi::AppID(...)` expression for an AppID."""
    name = appid.name
    idx = appid.idx
    if idx is None:
      return f'esi::AppID("{name}")'
    return f'esi::AppID("{name}", {idx})'
 
  def _scalar_port_group(self, member_name: str, port: types.BundlePort,
                         appid: AppID) -> CppPortGroup:
    """Build a CppPortGroup for a single scalar (non-indexed) port."""
    kind = cpp_port_kind(port)
    aliases = self._port_using_aliases(kind, member_name, port)
    alias_prefix = member_name if aliases else None
    member_type = kind.member_type(alias_prefix=alias_prefix)
    is_ref = member_type.endswith(" &")
    param_name = f"{member_name}{kind.param_suffix}"
 
    if is_ref:
      member_decl = f"{member_type}{member_name};"
    else:
      member_decl = f"{member_type} {member_name};"
 
    post = ""
    if kind.connectable:
      post = f"connected->{member_name}.connect();"
 
    return CppPortGroup(
        member_decl=member_decl,
        ctor_params=[f"{kind.param_type} {param_name}"],
        init_entry=f"{member_name}({param_name})",
        find_code=kind.scalar_find_code(param_name, self._appid_expr(appid)),
        make_unique_args=[param_name],
        post_connect=post,
        using_aliases=aliases,
    )
 
  def _indexed_ports_group(
      self, member_name: str, appid_name: str,
      port_list: List[Tuple[AppID, types.BundlePort]]) -> CppPortGroup:
    """Build a CppPortGroup for a same-name, same-type indexed port array."""
    # Derive the element type from the first port.
    first_port = port_list[0][1]
    kind = cpp_port_kind(first_port)
    aliases = self._port_using_aliases(kind, member_name, first_port)
    alias_prefix = member_name if aliases else None
    elem_type = kind.indexed_elem_type(alias_prefix=alias_prefix)
    indexed_type = f"esi::IndexedPorts<{elem_type}>"
    map_var = f"{member_name}_backing"
    map_type = f"std::map<int, {elem_type}>"
    indexed_var = f"{member_name}_map"
 
    # Build the find code: per-index resolve and try_emplace, then freeze
    # into the IndexedPorts wrapper. The body of the loop differs by port
    # kind: channel ports need an extra `getRawRead("data")` /
    # `getRawWrite("data")` step, MMIO regions and metrics store raw
    # pointers.
    find_parts = [
        f"{map_type} {map_var};",
        f"for (uint32_t idx : esi::findPortIndices(rawModule, "
        f"\"{appid_name}\")) {{",
    ]
    appid_expr = f'esi::AppID("{appid_name}", idx)'
    find_parts.append(kind.indexed_find_code(map_var, appid_expr))
    find_parts.append("}")
    find_parts.append(f"{indexed_type} {indexed_var}(std::move({map_var}));")
 
    post = ""
    if kind.connectable:
      # IndexedPorts now exposes mutable iteration so `port.connect()` is fine.
      post = (f"for (auto &[idx, port] : connected->{member_name})\n"
              f"  port.connect();")
 
    return CppPortGroup(
        member_decl=f"{indexed_type} {member_name};",
        ctor_params=[f"{indexed_type} {indexed_var}"],
        init_entry=f"{member_name}(std::move({indexed_var}))",
        find_code="\n".join(find_parts),
        make_unique_args=[f"std::move({indexed_var})"],
        post_connect=post,
        using_aliases=aliases,
    )
 
  def _mixed_struct_group(
      self, member_name: str, appid_name: str,
      port_list: List[Tuple[AppID, types.BundlePort]]) -> CppPortGroup:
    """Build a CppPortGroup for a same-name, mixed-type indexed port group."""
    struct_name = f"{member_name}_ports"
    sub_member_decls: List[str] = []
    ctor_params: List[str] = []
    init_args: List[str] = []
    find_parts: List[str] = []
    make_args: List[str] = []
    post_parts: List[str] = []
    using_aliases: List[Tuple[str, str]] = []
 
    for appid, port in port_list:
      kind = cpp_port_kind(port)
      idx = appid.idx if appid.idx is not None else 0
      sub_name = f"_{idx}"
      sub_alias_prefix = f"{member_name}_{idx}"
      sub_aliases = self._port_using_aliases(kind, sub_alias_prefix, port)
      using_aliases.extend(sub_aliases)
      alias_prefix = sub_alias_prefix if sub_aliases else None
      member_type = kind.member_type(alias_prefix=alias_prefix)
      is_ref = member_type.endswith(" &")
      if is_ref:
        sub_member_decls.append(f"{member_type}{sub_name};")
      else:
        sub_member_decls.append(f"{member_type} {sub_name};")
 
      param_name = f"{member_name}_{idx}{kind.param_suffix}"
      ctor_params.append(f"{kind.param_type} {param_name}")
      init_args.append(param_name)
      # The connect() local mirrors the ctor param name plus the kind's
      # suffix; the find snippet declares it and make_unique passes it.
      find_local = f"{param_name}{kind.param_suffix}"
      find_parts.append(
          kind.scalar_find_code(find_local, self._appid_expr(appid)))
      make_args.append(find_local)
      if kind.connectable:
        post_parts.append(f"connected->{member_name}.{sub_name}.connect();")
 
    struct_decl = (f"struct {struct_name} {{\n" +
                   "".join(f"      {d}\n" for d in sub_member_decls) + "    };")
    init_entry = f"{member_name}{{{', '.join(init_args)}}}"
 
    return CppPortGroup(
        struct_decls=[struct_decl],
        member_decl=f"{struct_name} {member_name};",
        ctor_params=ctor_params,
        init_entry=init_entry,
        find_code="\n".join(find_parts),
        make_unique_args=make_args,
        post_connect="\n".join(post_parts),
        using_aliases=using_aliases,
    )
 
  def _collect_port_groups(
      self, ports: Dict[AppID, types.BundlePort]) -> List[CppPortGroup]:
    """Group `ports` (AppID → BundlePort) into CppPortGroup list."""
    # Group by AppID name, preserving sorted order.
    groups_by_name: Dict[str, list] = {}
    for appid, port in ports.items():
      n = appid.name
      if n not in groups_by_name:
        groups_by_name[n] = []
      groups_by_name[n].append((appid, port))
 
    result: List[CppPortGroup] = []
    for appid_name, port_list in groups_by_name.items():
      member_name = self._sanitize_id(appid_name)
      # Sort by idx (None → -1 so scalar ports sort first).
      port_list.sort(key=lambda x: x[0].idx if x[0].idx is not None else -1)
 
      if len(port_list) == 1:
        appid, port = port_list[0]
        if appid.idx is None:
          result.append(self._scalar_port_group(member_name, port, appid))
        else:
          result.append(
              self._indexed_ports_group(member_name, appid_name, port_list))
        continue
 
      # Multiple ports with the same name.
      all_indexed = all(a.idx is not None for a, _ in port_list)
      if not all_indexed:
        # Degenerate: mix of indexed and non-indexed with the same name.
        # Emit as a mixed struct for safety.
        result.append(
            self._mixed_struct_group(member_name, appid_name, port_list))
        continue
 
      all_same_type = len({type(p) for _, p in port_list}) == 1
      if all_same_type:
        result.append(
            self._indexed_ports_group(member_name, appid_name, port_list))
      else:
        result.append(
            self._mixed_struct_group(member_name, appid_name, port_list))
 
    return result
 
  def _emit_module_class(self, name: str, system_name: str,
                         module_info: ModuleInfo,
                         port_groups: List[CppPortGroup], out: TextIO) -> None:
    """Emit the full module class to `out`."""
    out.write(f"/// Generated header for {system_name} module {name}.\n"
              "#pragma once\n"
              '#include "types.h"\n'
              '#include "esi/TypedPorts.h"\n'
              "\n"
              "#include <any>\n"
              "#include <map>\n"
              "#include <optional>\n"
              "#include <string>\n"
              "\n"
              f"namespace {system_name} {{\n"
              "\n")
 
    # Module metadata as a Doxygen comment block above the class.
    metadata_lines: List[str] = []
    summary = getattr(module_info, "summary", None)
    if summary:
      for line in summary.splitlines():
        metadata_lines.append(line)
    for label, attr in (("Version", "version"), ("Repository", "repo"),
                        ("Commit", "commit_hash")):
      val = getattr(module_info, attr, None)
      if val:
        metadata_lines.append(f"{label}: {val}")
    if metadata_lines:
      out.write("///\n")
      for line in metadata_lines:
        out.write(f"/// {line}\n" if line else "///\n")
      out.write("///\n")
 
    out.write(f"class {name} {{\n"
              "public:\n")
 
    consts = self.get_consts_str(module_info)
    if consts:
      out.write("  // Module constants.\n")
      out.write(f"  {consts}\n\n")
 
    # Type aliases for typed-port template parameters, hoisted to module scope
    # so the long mangled names don't appear inline as template arguments.
    aliases = [a for grp in port_groups for a in grp.using_aliases]
    if aliases:
      for alias_name, alias_type in aliases:
        out.write(f"  using {alias_name} = {alias_type};\n")
      out.write("\n")
 
    if port_groups:
      out.write(
          "  /// Holds the resolved, typed ports for this module instance.\n"
          "  /// Returned by `connect()`.\n"
          "  class Connected {\n  public:\n")
 
      # Struct declarations for mixed groups.
      for grp in port_groups:
        for decl in grp.struct_decls:
          out.write(f"    {decl}\n")
      if any(grp.struct_decls for grp in port_groups):
        out.write("\n")
 
      # Member declarations.
      for grp in port_groups:
        out.write(f"    {grp.member_decl}\n")
      out.write("\n")
 
      # Constructor.
      all_params = [p for grp in port_groups for p in grp.ctor_params]
      out.write("    Connected(\n")
      for i, param in enumerate(all_params):
        comma = "," if i < len(all_params) - 1 else ""
        out.write(f"        {param}{comma}\n")
      out.write("        )\n        : ")
      inits = [grp.init_entry for grp in port_groups]
      out.write(",\n          ".join(inits))
      out.write(" {}\n  };\n\n")
 
    # Outer constructor.
    out.write(
        f"  {name}(esi::HWModule *rawModule) : rawModule(rawModule) {{}}\n\n")
 
    # Module-metadata accessors. These read from the live HWModule's ModuleInfo
    # so callers can verify that the connected accelerator is compatible with
    # the build the software was generated against.
    out.write(
        "  /// The connected module's name as reported by the manifest, or\n"
        "  /// std::nullopt if the module has no metadata.\n"
        "  std::optional<std::string> name() const {\n"
        "    auto info = rawModule->getInfo();\n"
        "    return info ? info->name : std::nullopt;\n"
        "  }\n"
        "  /// The connected module's summary string, if any.\n"
        "  std::optional<std::string> summary() const {\n"
        "    auto info = rawModule->getInfo();\n"
        "    return info ? info->summary : std::nullopt;\n"
        "  }\n"
        "  /// The connected module's version string, if any.\n"
        "  std::optional<std::string> version() const {\n"
        "    auto info = rawModule->getInfo();\n"
        "    return info ? info->version : std::nullopt;\n"
        "  }\n"
        "  /// The connected module's source repository, if any.\n"
        "  std::optional<std::string> repo() const {\n"
        "    auto info = rawModule->getInfo();\n"
        "    return info ? info->repo : std::nullopt;\n"
        "  }\n"
        "  /// The connected module's source commit hash, if any.\n"
        "  std::optional<std::string> commitHash() const {\n"
        "    auto info = rawModule->getInfo();\n"
        "    return info ? info->commitHash : std::nullopt;\n"
        "  }\n"
        "  /// Designer-specified constants for the connected module.\n"
        "  /// Returns an empty map if the module has no metadata.\n"
        "  std::map<std::string, esi::Constant> constants() const {\n"
        "    auto info = rawModule->getInfo();\n"
        "    return info ? info->constants\n"
        "                : std::map<std::string, esi::Constant>{};\n"
        "  }\n"
        "  /// Free-form designer-supplied metadata for the connected module.\n"
        "  /// Returns an empty map if the module has no metadata.\n"
        "  std::map<std::string, std::any> extra() const {\n"
        "    auto info = rawModule->getInfo();\n"
        "    return info ? info->extra : std::map<std::string, std::any>{};\n"
        "  }\n\n")
 
    if port_groups:
      out.write("  std::unique_ptr<Connected> connect() {\n")
 
      # Find / resolve phase.
      for grp in port_groups:
        if grp.find_code:
          for line in grp.find_code.splitlines():
            out.write(f"    {line}\n")
          out.write("\n")
 
      # Construct Connected.
      all_args = [a for grp in port_groups for a in grp.make_unique_args]
      out.write("    auto connected = std::make_unique<Connected>(\n")
      for i, arg in enumerate(all_args):
        comma = "," if i < len(all_args) - 1 else ""
        out.write(f"        {arg}{comma}\n")
      out.write("    );\n\n")
 
      # Post-construction connects.
      for grp in port_groups:
        if grp.post_connect:
          for line in grp.post_connect.splitlines():
            out.write(f"    {line}\n")
 
      out.write("    return connected;\n  }\n\n")
 
    out.write("private:\n  esi::HWModule *rawModule;\n};\n\n")
    out.write(f"}} // namespace {system_name}\n")
 
  def write_modules(self, output_dir: Path, system_name: str) -> None:
    """Write the C++ header. One for each module in the manifest."""
    module_instances = self._build_module_instance_map()
 
    for module_info in self.manifest.module_infos:
      if module_info.name is None:
        continue
      name = module_info.name
      instance = module_instances.get(name)
      try:
        if instance is not None:
          port_groups = self._collect_port_groups(instance.ports)
        else:
          port_groups = []
      except (NotImplementedError, ValueError) as e:
        hdr_file = output_dir / f"{name}.h"
        with open(hdr_file, "w", encoding="utf-8") as hdr:
          hdr.write(f"// Skipped: {e}\n")
        continue
 
      hdr_file = output_dir / f"{name}.h"
      with open(hdr_file, "w", encoding="utf-8") as hdr:
        self._emit_module_class(name, system_name, module_info, port_groups,
                                hdr)
 
  def generate(self, output_dir: Path, system_name: str) -> None:
    self.type_emitter.write_header(output_dir, system_name)
    self.write_modules(output_dir, system_name)
 
 
class CppTypePlanner:
  """Plan C++ type naming and ordering from an ESI manifest."""
 
  def __init__(self, type_table: List[types.ESIType]) -> None:
    """Initialize the generator with the manifest and target namespace."""
    # Map manifest type ids to their preferred C++ names.
    self.type_id_map: Dict[types.ESIType, str] = {}
    # Track all names already taken to avoid collisions. True => alias-based.
    self.used_names: Dict[str, bool] = {}
    # Track alias base names to warn on collisions.
    self.alias_base_names: Set[str] = set()
    self.ordered_types: List[types.ESIType] = []
    # Types the planner chose not to emit, paired with the reason. The
    # emitter writes a `// Unsupported type ...` comment for each so the
    # generated header explains the omission.
    self.skipped_types: List[Tuple[types.ESIType, str]] = []
    self.has_cycle = False
    self._prepare_types(type_table)
 
  def _prepare_types(self, type_table: List[types.ESIType]) -> None:
    """Name the types and prepare for emission by registering all reachable
    types and assigning."""
    visited: Set[str] = set()
    for t in type_table:
      self._collect_aliases(t, visited)
 
    visited = set()
    for t in type_table:
      self._collect_structs(t, visited)
 
    visited = set()
    for t in type_table:
      self._collect_windows(t, visited)
 
    self.ordered_types, self.has_cycle = self._ordered_emit_types()
 
  def _sanitize_name(self, name: str) -> str:
    """Create a C++-safe identifier from the manifest-provided name."""
    name = name.replace("::", "_")
    if name.startswith("@"):
      name = name[1:]
    sanitized = []
    for ch in name:
      if ch.isalnum() or ch == "_":
        sanitized.append(ch)
      else:
        sanitized.append("_")
    if not sanitized:
      return "Type"
    if sanitized[0].isdigit():
      sanitized.insert(0, "_")
    return "".join(sanitized)
 
  def _reserve_name(self, base: str, is_alias: bool) -> str:
    """Reserve a globally unique identifier using the sanitized base name."""
    base = self._sanitize_name(base)
    if is_alias and base in self.alias_base_names:
      sys.stderr.write(
          f"Warning: duplicate alias name '{base}' detected; disambiguating.\n")
    if is_alias:
      self.alias_base_names.add(base)
    name = base
    idx = 1
    while name in self.used_names:
      name = f"{base}_{idx}"
      idx += 1
    self.used_names[name] = is_alias
    return name
 
  def _auto_struct_name(self, struct_type: types.StructType) -> str:
    """Derive a deterministic name for anonymous structs from their fields."""
    parts = ["_struct"]
    for field_name, field_type in struct_type.fields:
      parts.append(field_name)
      parts.append(self._sanitize_name(field_type.id))
    return self._reserve_name("_".join(parts), is_alias=False)
 
  def _auto_union_name(self, union_type: types.UnionType) -> str:
    """Derive a deterministic name for anonymous unions from their fields."""
    parts = ["_union"]
    for field_name, field_type in union_type.fields:
      parts.append(field_name)
      parts.append(self._sanitize_name(field_type.id))
    return self._reserve_name("_".join(parts), is_alias=False)
 
  def _auto_window_name(self, window_type: types.WindowType) -> str:
    """Derive a deterministic name for generated window helpers.
 
    Two distinct windows can wrap the same `into` struct (e.g. serial and
    parallel encodings of the same payload), so the helper name must be
    derived from BOTH the inner type's name and the window's own name/id.
    """
    into_type = self._unwrap_aliases(window_type.into_type)
    into_name = self.type_id_map.get(into_type)
    window_part = (window_type.name
                   if window_type.name else self._sanitize_name(window_type.id))
    if into_name:
      base = f"{into_name}_{window_part}"
    elif window_type.name:
      base = window_part
    else:
      base = f"_window_{window_part}"
    return self._reserve_name(base, is_alias=False)
 
  def _unwrap_aliases(self, wrapped: types.ESIType) -> types.ESIType:
    while isinstance(wrapped, types.TypeAlias):
      wrapped = wrapped.inner_type
    return wrapped
 
  def _is_supported_window(self, current_type: types.ESIType) -> bool:
    if not isinstance(current_type, types.WindowType):
      return False
    into_type = self._unwrap_aliases(current_type.into_type)
    if not isinstance(into_type, types.StructType):
      return False
 
    # The generated window helper only supports struct-shaped payloads with a
    # single logical list field to stream across multiple frames.
    list_fields = []
    for field_name, field_type in into_type.fields:
      if isinstance(self._unwrap_aliases(field_type), types.ListType):
        list_fields.append(field_name)
    if len(list_fields) != 1:
      return False
 
    list_field_name = list_fields[0]
    header_field = None
    data_field = None
    # That list must appear exactly once as a bulk-count field and exactly once
    # as a single-item data field so the helper can synthesize header/data/footer.
    for frame in current_type.frames:
      for field in frame.fields:
        if field.name != list_field_name:
          continue
        if field.bulk_count_width > 0:
          if header_field is not None:
            return False
          header_field = field
        elif field.num_items > 0:
          if data_field is not None:
            return False
          data_field = field
    return (header_field is not None and data_field is not None and
            data_field.num_items == 1)
 
  def _iter_type_children(self, t: types.ESIType) -> List[types.ESIType]:
    """Return child types in a stable order for traversal."""
    if isinstance(t, types.TypeAlias):
      return [t.inner_type] if t.inner_type is not None else []
    if isinstance(t, types.BundleType):
      return [channel.type for channel in t.channels]
    if isinstance(t, types.ChannelType):
      return [t.inner]
    if isinstance(t, types.StructType):
      return [field_type for _, field_type in t.fields]
    if isinstance(t, types.UnionType):
      return [field_type for _, field_type in t.fields]
    if isinstance(t, types.ListType):
      return [t.element_type]
    if isinstance(t, types.WindowType):
      return [t.into_type]
    if isinstance(t, types.ArrayType):
      return [t.element_type]
    return []
 
  def _visit_types(self, t: types.ESIType, visited: Set[str],
                   visit_fn: Callable[[types.ESIType], None]) -> None:
    """Traverse types with alphabetical child ordering in post-order."""
    if not isinstance(t, types.ESIType):
      raise TypeError(f"Expected ESIType, got {type(t)}")
    tid = t.id
    if tid in visited:
      return
    visited.add(tid)
    children = sorted(self._iter_type_children(t), key=lambda child: child.id)
    for child in children:
      self._visit_types(child, visited, visit_fn)
    visit_fn(t)
 
  def _collect_aliases(self, t: types.ESIType, visited: Set[str]) -> None:
    """Scan for aliases and reserve their names (recursive)."""
 
    # Visit callback: reserve alias names and map aliases to identifiers.
    def visit(alias_type: types.ESIType) -> None:
      if not isinstance(alias_type, types.TypeAlias):
        return
      if alias_type not in self.type_id_map:
        alias_name = self._reserve_name(alias_type.name, is_alias=True)
        self.type_id_map[alias_type] = alias_name
 
    self._visit_types(t, visited, visit)
 
  def _collect_structs(self, t: types.ESIType, visited: Set[str]) -> None:
    """Scan for structs/unions needing auto-names and reserve them."""
 
    # Visit callback: assign auto-names to unnamed structs and unions.
    def visit(current_type: types.ESIType) -> None:
      if current_type in self.type_id_map:
        return
      if isinstance(current_type, types.StructType):
        self.type_id_map[current_type] = self._auto_struct_name(current_type)
      elif isinstance(current_type, types.UnionType):
        self.type_id_map[current_type] = self._auto_union_name(current_type)
 
    self._visit_types(t, visited, visit)
 
  def _collect_windows(self, t: types.ESIType, visited: Set[str]) -> None:
    """Scan for supported window types and reserve helper names."""
 
    def visit(current_type: types.ESIType) -> None:
      if not self._is_supported_window(current_type):
        return
      assert isinstance(current_type, types.WindowType)
      if current_type in self.type_id_map:
        return
      self.type_id_map[current_type] = self._auto_window_name(current_type)
 
    self._visit_types(t, visited, visit)
 
  def _collect_decls_from_type(self,
                               wrapped: types.ESIType) -> Set[types.ESIType]:
    """Collect types that require top-level declarations for a given type."""
    deps: Set[types.ESIType] = set()
 
    # Visit callback: collect structs, unions, and aliases used by a type.
    def visit(current: types.ESIType) -> None:
      if isinstance(current, types.TypeAlias):
        # A type that references an alias is emitted using the alias *name*
        # (see `_cpp_type`), so it must be ordered after the alias's own
        # `using` declaration. Depend on the alias itself, not the type it
        # unwraps to; the alias in turn depends on that underlying struct /
        # union / window, so the full chain (underlying -> alias -> user) is
        # ordered correctly. (Depending on the unwrapped inner type instead
        # left the alias free to sort *after* a user that referenced it --
        # e.g. a nested `array<array<Alias>>` field, where it only happened
        # to work for the un-nested case by alphabetical luck.)
        deps.add(current)
      elif isinstance(current, (types.StructType, types.UnionType)):
        deps.add(current)
      elif self._is_supported_window(current):
        deps.add(current)
 
    self._visit_types(wrapped, set(), visit)
    return deps
 
  def _collect_decls_from_window(
      self, window_type: types.WindowType) -> Set[types.ESIType]:
    """Collect only the declarations referenced by a generated window helper."""
    deps: Set[types.ESIType] = set()
    into_type = self._unwrap_aliases(window_type.into_type)
    if not isinstance(into_type, types.StructType):
      return deps
 
    for _, field_type in into_type.fields:
      unwrapped = self._unwrap_aliases(field_type)
      if isinstance(unwrapped, types.ListType):
        deps.update(self._collect_decls_from_type(unwrapped.element_type))
      else:
        deps.update(self._collect_decls_from_type(field_type))
    return deps
 
  def _contains_window(self, esi_type: types.ESIType) -> bool:
    """Return True if `esi_type` is or transitively contains a WindowType.
 
    Structs (and aliases/unions that reference them) which embed a window
    cannot be emitted as C++ packed structs because the C++ window helper
    is a variable-size multi-frame container. This helper is used to
    exclude such types from the emission list entirely.
    """
    unwrapped = self._unwrap_aliases(esi_type)
    if isinstance(unwrapped, types.WindowType):
      return True
    if isinstance(unwrapped, types.StructType):
      return any(self._contains_window(ft) for _, ft in unwrapped.fields)
    if isinstance(unwrapped, types.UnionType):
      return any(self._contains_window(ft) for _, ft in unwrapped.fields)
    if isinstance(unwrapped, types.ArrayType):
      return self._contains_window(unwrapped.element_type)
    return False
 
  def _contains_unbounded(self, esi_type: types.ESIType) -> bool:
    """Return True if `esi_type` is or transitively contains a type with
    no bounded bit width (e.g. `!esi.any`, or a list that the window
    helper can't wrap).
 
    A struct can't be emitted as a fixed-size raw-bytes buffer if any
    field's width is unbounded — there's no `std::array<uint8_t, N>` size
    that would match the wire layout — so the planner excludes such
    structs (and aliases/unions/arrays that reach one) from the emission
    list, the same way `_contains_window` does for nested windows.
    """
    unwrapped = self._unwrap_aliases(esi_type)
    try:
      if unwrapped.bit_width < 0:
        return True
    except Exception:
      return True
    if isinstance(unwrapped, types.StructType):
      return any(self._contains_unbounded(ft) for _, ft in unwrapped.fields)
    if isinstance(unwrapped, types.UnionType):
      return any(self._contains_unbounded(ft) for _, ft in unwrapped.fields)
    if isinstance(unwrapped, types.ArrayType):
      return self._contains_unbounded(unwrapped.element_type)
    return False
 
  def _ordered_emit_types(self) -> Tuple[List[types.ESIType], bool]:
    """Collect and order types for deterministic emission."""
    window_into_types: Set[types.ESIType] = set()
    for esi_type in self.type_id_map.keys():
      if not self._is_supported_window(esi_type):
        continue
      assert isinstance(esi_type, types.WindowType)
      window_into_types.add(self._unwrap_aliases(esi_type.into_type))
 
    # Build the skip set up front so the DFS dep-walk below can filter
    # against it too — otherwise a top-level type can pull a skipped
    # inner struct back into the emission list via `_collect_decls_*`.
    skip_set: Set[types.ESIType] = set()
    for esi_type in self.type_id_map.keys():
      if isinstance(esi_type,
                    types.StructType) and esi_type in window_into_types:
        skip_set.add(esi_type)
        self.skipped_types.append(
            (esi_type,
             "into-type of a windowed helper; subsumed by the helper class"))
        continue
      if isinstance(esi_type, types.TypeAlias):
        inner = esi_type.inner_type
        if inner is not None and self._unwrap_aliases(
            inner) in window_into_types:
          skip_set.add(esi_type)
          self.skipped_types.append(
              (esi_type, "alias of a windowed helper's into-type"))
          continue
      # Skip structs/unions/aliases that transitively embed a WindowType
      # field. WindowType itself is fine — it emits as its own helper
      # class.
      if (not isinstance(self._unwrap_aliases(esi_type), types.WindowType) and
          self._contains_window(esi_type)):
        skip_set.add(esi_type)
        self.skipped_types.append((esi_type, "contains a windowed sub-type"))
        continue
      # Skip structs/unions/aliases that transitively contain an
      # unbounded type (e.g. `!esi.any`). No fixed-size raw-bytes buffer
      # can hold them, so the per-field emitter has no width to embed.
      # WindowType itself reports unbounded width but emits as its own
      # helper class, so leave that branch alone.
      if (not isinstance(self._unwrap_aliases(esi_type), types.WindowType) and
          self._contains_unbounded(esi_type)):
        skip_set.add(esi_type)
        self.skipped_types.append(
            (esi_type, "contains an unbounded sub-type (e.g. !esi.any)"))
        continue
 
    emit_types: List[types.ESIType] = []
    for esi_type in self.type_id_map.keys():
      if esi_type in skip_set:
        continue
      if (isinstance(esi_type,
                     (types.StructType, types.UnionType, types.TypeAlias)) or
          self._is_supported_window(esi_type)):
        emit_types.append(esi_type)
 
    # Prefer alias-reserved names first, then lexicographic for determinism.
    name_to_type = {self.type_id_map[t]: t for t in emit_types}
    sorted_names = sorted(name_to_type.keys(),
                          key=lambda name:
                          (0 if self.used_names.get(name, False) else 1, name))
 
    ordered: List[types.ESIType] = []
    visited: Set[types.ESIType] = set()
    visiting: Set[types.ESIType] = set()
    has_cycle = False
 
    # Visit callback: DFS to emit dependencies before their users. Skipped
    # types are filtered out of the dep set so they can't sneak back in
    # via a non-skipped type that happens to reference them (e.g. an
    # alias whose dep walk reaches the into-struct of a window helper).
    def visit(current: types.ESIType) -> None:
      nonlocal has_cycle
      if current in visited:
        return
      if current in visiting:
        has_cycle = True
        return
      visiting.add(current)
 
      deps: Set[types.ESIType] = set()
      if isinstance(current, types.TypeAlias):
        inner = current.inner_type
        if inner is not None:
          deps.update(self._collect_decls_from_type(inner))
      elif isinstance(current, (types.StructType, types.UnionType)):
        for _, field_type in current.fields:
          deps.update(self._collect_decls_from_type(field_type))
      elif self._is_supported_window(current):
        assert isinstance(current, types.WindowType)
        deps.update(self._collect_decls_from_window(current))
      for dep in sorted(deps, key=lambda dep: self.type_id_map[dep]):
        if dep in skip_set:
          continue
        visit(dep)
 
      visiting.remove(current)
      visited.add(current)
      ordered.append(current)
 
    for name in sorted_names:
      visit(name_to_type[name])
 
    return ordered, has_cycle
 
 
class CppTypeEmitter:
  """Emit C++ headers from precomputed type ordering."""
 
  def __init__(self, planner: CppTypePlanner) -> None:
    self.type_id_map = planner.type_id_map
    self.ordered_types = planner.ordered_types
    self.skipped_types = planner.skipped_types
    self.has_cycle = planner.has_cycle
 
  def type_identifier(self, type: types.ESIType) -> str:
    """Get the C++ type string for an ESI type."""
    return self._cpp_type(type)
 
  def _cpp_string_literal(self, value: str) -> str:
    """Escape a Python string for use as a C++ string literal."""
    escaped = value.replace("\\", "\\\\").replace('"', '\\"')
    return f'"{escaped}"'
 
  def _get_bitvector_str(self, type: types.ESIType) -> str:
    """Get the textual code for the C++ type used to represent an integer
    field's value at the API boundary.
 
    Integers up to 64 bits map to the native `int{N}_t` / `uint{N}_t`
    (or `bool` for a single bit) storage types so common scalars stay
    zero-overhead, behave identically to plain C ints, and stay valid as
    template parameters for the existing `TypedReadPort<T>` /
    `TypedWritePort<T>` / `TypedFunction<...>` machinery.
 
    Wider integers (signed > 64 bits, unsigned > 64 bits, or any
    `BitsType` > 64 bits) fall back to the non-owning view classes from
    `esi/Values.h`: `esi::BitVector` for `BitsType`, `esi::IntView` for
    signed integers, `esi::UIntView` for unsigned. All three are
    non-owning views over the parent struct's bytes, so generated
    getters are zero-allocation; see the lifetime note on `BitVector`
    and at the top of the generated header.
    """
    assert isinstance(type, (types.BitsType, types.IntType))
 
    if type.bit_width > 64:
      if isinstance(type, types.BitsType):
        return "esi::BitVector"
      if isinstance(type, types.UIntType):
        return "esi::UIntView"
      return "esi::IntView"
 
    return self._storage_type(
        type.bit_width, not isinstance(type, (types.BitsType, types.UIntType)))
 
  def _storage_type(self, bit_width: int, signed: bool) -> str:
    """Get the textual code for a native byte-addressable integer storage
    type. Only valid for widths 1..64; wider integers are handled by
    `_get_bitvector_str` via the `esi::IntView` / `esi::UIntView` view
    classes.
    """
 
    if bit_width == 1:
      return "bool"
    elif bit_width <= 8:
      storage_width = 8
    elif bit_width <= 16:
      storage_width = 16
    elif bit_width <= 32:
      storage_width = 32
    elif bit_width <= 64:
      storage_width = 64
    else:
      raise ValueError(f"Unsupported native integer width: {bit_width}")
 
    if not signed:
      return f"uint{storage_width}_t"
    return f"int{storage_width}_t"
 
  def _is_value_class_type(self, field_type: types.ESIType) -> bool:
    """True if the C++ representation of this integer field is one of
    the `esi::{BitVector,IntView,UIntView}` view classes from
    `esi/Values.h` rather than a native integer.
    """
    wrapped = self._unwrap_aliases(field_type)
    if not isinstance(wrapped, (types.BitsType, types.IntType)):
      return False
    return wrapped.bit_width > 64
 
  def _array_base_and_dims(
      self, array_type: types.ArrayType) -> Tuple[str, List[int]]:
    """Return the base C++ type and outer-to-inner dimensions of a nested array."""
    dims: List[int] = []
    inner: types.ESIType = array_type
    while isinstance(inner, types.ArrayType):
      dims.append(inner.size)
      inner = inner.element_type
    base_cpp = self._cpp_type(inner)
    return base_cpp, dims
 
  def _std_array_type(self, array_type: types.ArrayType) -> str:
    """Return the equivalent nested `std::array<...>` type for an array.
 
    `std::array<T, N>` is layout-compatible in practice with `T[N]` on every
    major implementation (and identical under `#pragma pack(1)`), so the
    generator uses it everywhere a fixed-size array would appear.  This keeps
    field/value/ctor types storable in `std::vector` and assignable with `=`.
    """
    base_cpp, dims = self._array_base_and_dims(array_type)
    result = base_cpp
    for size in reversed(dims):
      result = f"std::array<{result}, {size}>"
    return result
 
  def _cpp_type(self, wrapped: types.ESIType) -> str:
    """Resolve an ESI type to its C++ identifier."""
    if isinstance(wrapped, types.WindowType) and wrapped in self.type_id_map:
      return self.type_id_map[wrapped]
    if isinstance(wrapped,
                  (types.TypeAlias, types.StructType, types.UnionType)):
      # Zero-width composite types (e.g. a struct of only void fields, or an
      # alias to such a struct) collapse to `void`. C++ structs are defined to
      # have `sizeof >= 1`, so there is no meaningful storage to emit; treat them
      # as void everywhere they appear so callers comment them out exactly
      # like a direct `VoidType` field.
      if wrapped.bit_width == 0:
        return "void"
      return self.type_id_map[wrapped]
    if isinstance(wrapped, types.BundleType):
      return "void"
    if isinstance(wrapped, types.ChannelType):
      return self._cpp_type(wrapped.inner)
    if isinstance(wrapped, types.ListType):
      raise ValueError("List types require a generated window wrapper")
    if isinstance(wrapped, types.VoidType):
      return "void"
    if isinstance(wrapped, types.AnyType):
      return "std::any"
    if isinstance(wrapped, (types.BitsType, types.IntType)):
      # A zero-width integer carries no data; emit it as `void` so it gets
      # commented out in field positions like any other void.
      if wrapped.bit_width == 0:
        return "void"
      return self._get_bitvector_str(wrapped)
    if isinstance(wrapped, types.ArrayType):
      # `std::array<void, N>` is ill-formed; arrays of zero-width elements
      # also collapse to `void`.
      if wrapped.bit_width == 0:
        return "void"
      return self._std_array_type(wrapped)
    if type(wrapped) is types.ESIType:
      return "std::any"
    raise NotImplementedError(
        f"Type '{wrapped}' not supported for C++ generation")
 
  def _unwrap_aliases(self, wrapped: types.ESIType) -> types.ESIType:
    """Strip alias wrappers to reach the underlying type."""
    while isinstance(wrapped, types.TypeAlias):
      wrapped = wrapped.inner_type
    return wrapped
 
  def _format_window_ctor_param(self, field_name: str,
                                field_type: types.ESIType) -> str:
    """Emit a constructor parameter for generated window helpers.
 
    Small scalar header fields are cheaper to pass by value than by reference.
    Larger aggregates stay as const references.
    """
    field_cpp = self._cpp_type(field_type)
    wrapped = self._unwrap_aliases(field_type)
    if isinstance(wrapped, (types.BitsType, types.IntType)):
      if self._is_value_class_type(field_type):
        return f"const {field_cpp} &{field_name}"
      return f"{field_cpp} {field_name}"
    return f"const {field_cpp} &{field_name}"
 
  def _field_byte_width(self, field_type: types.ESIType) -> int:
    """Compute the byte width of a field type, rounding up to full bytes."""
    return (field_type.bit_width + 7) // 8
 
  def _safe_byte_width(self, esi_type: types.ESIType) -> Optional[int]:
    """Return the bounded byte width of `esi_type`, or `None` if it has no
    well-defined static size (e.g. unbounded `!esi.any` or recursive types).
    """
    try:
      bit_width = esi_type.bit_width
    except Exception:
      return None
    if bit_width is None or bit_width < 0:
      return None
    return (bit_width + 7) // 8
 
  def _emit_size_assert(self, w: IndentedWriter, type_name: str,
                        expected_bytes: Optional[int]) -> None:
    """Emit a `static_assert` that pins the C++ `sizeof` of a packed type to
    the byte width derived from the manifest.
 
    `std::array` and bit-field layout are technically implementation-defined,
    so this assertion is the safety net that catches a toolchain that lays
    them out differently from the wire format.  Skipped silently for types
    without a bounded static size.
    """
    if expected_bytes is None:
      return
    w.line(f"static_assert(sizeof({type_name}) == {expected_bytes},")
    w.line(f'              "{type_name}: packed layout does not match '
           f'manifest size");')
 
  def _analyze_window(self, window_type: types.WindowType) -> Dict[str, Any]:
    """Extract the metadata needed to emit a bulk list window wrapper."""
    into_type = self._unwrap_aliases(window_type.into_type)
    if not isinstance(into_type, types.StructType):
      raise ValueError("window codegen currently requires a struct into-type")
 
    field_map = {name: field_type for name, field_type in into_type.fields}
    list_fields = [
        (name, self._unwrap_aliases(field_type))
        for name, field_type in into_type.fields
        if isinstance(self._unwrap_aliases(field_type), types.ListType)
    ]
    if len(list_fields) != 1:
      raise ValueError("window codegen currently supports exactly one list")
 
    list_field_name, list_type = list_fields[0]
    assert isinstance(list_type, types.ListType)
 
    header_frame = None
    header_field = None
    data_frame = None
    data_field = None
    for frame in window_type.frames:
      for field in frame.fields:
        if field.name != list_field_name:
          continue
        if field.bulk_count_width > 0:
          header_frame = frame
          header_field = field
        elif field.num_items > 0:
          data_frame = frame
          data_field = field
 
    if header_frame is None or header_field is None:
      raise ValueError("window codegen requires a bulk-count header frame")
    if data_frame is None or data_field is None:
      raise ValueError("window codegen requires a data frame for the list")
    if data_field.num_items != 1:
      raise ValueError("window codegen currently supports numItems == 1")
 
    ctor_params = [(name, field_type)
                   for name, field_type in into_type.fields
                   if name != list_field_name]
 
    # Frame fields are kept in declared (MSB-first) order and laid out
    # MSB-aligned within the frame/union bit width, matching how CIRCT
    # lowers the frame union (content packed from the most-significant bit
    # down, with any slack as low-end padding). Widths are exact bit widths
    # -- no per-field byte rounding -- so sub-byte count/static fields land
    # at the same offsets the hardware uses.
    header_fields = []
    header_bits = 0
    count_field_name = f"{list_field_name}_count"
    count_width = header_field.bulk_count_width
    count_cpp = self._storage_type(count_width, signed=False)
    for field in header_frame.fields:
      if field.name == list_field_name:
        header_fields.append((count_field_name, None))
        header_bits += count_width
      else:
        field_type = field_map[field.name]
        header_fields.append((field.name, field_type))
        header_bits += field_type.bit_width
 
    data_fields = []
    data_bits = 0
    for field in data_frame.fields:
      if field.name == list_field_name:
        data_fields.append((list_field_name, list_type.element_type))
        data_bits += list_type.element_type.bit_width
      else:
        field_type = field_map[field.name]
        data_fields.append((field.name, field_type))
        data_bits += field_type.bit_width
 
    # The frame/union width is the wider of the two variants' content. The
    # byte-array storage rounds that up to whole bytes (any extra high bits
    # stay zero); both frames share it so `sizeof(header) == sizeof(data)`.
    frame_bits = max(header_bits, data_bits)
    frame_bytes = (frame_bits + 7) // 8
 
    return {
        "ctor_params": ctor_params,
        "count_cpp": count_cpp,
        "count_field_name": count_field_name,
        "count_width": count_width,
        "data_fields": data_fields,
        "element_cpp": self._cpp_type(list_type.element_type),
        "frame_bits": frame_bits,
        "frame_bytes": frame_bytes,
        "header_fields": header_fields,
        "list_field_name": list_field_name,
        "window_name": self.type_id_map[window_type],
    }
 
  def _compute_field_bit_offsets(
      self, struct_type: types.StructType) -> Tuple[Dict[str, int], int]:
    """Return `(bit_offset_by_name, total_bits)` for non-void fields.
 
    Fields are laid out LSB-first in wire order, which is reversed manifest
    order when `cpp_type.reverse` is set. Caller already knows each field's
    type/width from `struct_type.fields`; we only need the per-name offset
    and the resulting total bit width to size the storage array.
    """
    declared = struct_type.fields
    if struct_type.cpp_type.reverse:
      declared = list(reversed(declared))
    bit_offsets: Dict[str, int] = {}
    bit_offset = 0
    for field_name, field_type in declared:
      if self._cpp_type(field_type) == "void":
        continue
      bit_offsets[field_name] = bit_offset
      bit_offset += field_type.bit_width
    return bit_offsets, bit_offset
 
  def _is_signed_int_field(self, field_type: types.ESIType) -> bool:
    """True if the field is a signed integer (only `IntType`, not `UIntType`
    or `BitsType`)."""
    wrapped = self._unwrap_aliases(field_type)
    return (isinstance(wrapped, types.IntType) and
            not isinstance(wrapped, types.UIntType))
 
  def _emit_view_store(self, w: IndentedWriter, raw_member: str, src: str,
                       off_expr: str, width: int) -> None:
    """Emit a loop writing the low `width` bits of the view `src` into
    `raw_member`, with bit `b` landing at wire bit `off_expr + b`. Shared by
    the scalar view-class field setter and the per-element view-array setter.
    """
    w.line(f"const std::size_t n = "
           f"std::min<std::size_t>({src}.width(), {width});")
    with w.block(f"for (std::size_t b = 0; b < {width}; ++b)"):
      w.line(f"const std::size_t g = {off_expr} + b;")
      w.line(f"const bool val = (b < n) && {src}.getBit(b);")
      w.line("if (val)")
      with w.indented():
        w.line(f"{raw_member}[g / 8] |= "
               f"static_cast<uint8_t>(uint8_t{{1}} << (g % 8));")
      w.line("else")
      with w.indented():
        w.line(f"{raw_member}[g / 8] &= "
               f"static_cast<uint8_t>(~(uint8_t{{1}} << (g % 8)));")
 
  def _emit_field_accessor(self, w: IndentedWriter, raw_member: str,
                           self_type: str, field_name: str,
                           field_type: types.ESIType, bit_offset: int,
                           bit_width: int) -> None:
    """Emit a getter/setter pair that reads/writes `field_name` out of the
    raw bytes member `raw_member` at `bit_offset` / `bit_width`.
 
    Setters share the field's name (no `set_` prefix) and return
    `self_type &` so the caller can chain calls (e.g.
    `Foo{}.a(1).b(2).inner(x)`).
 
    For integer fields the emitter picks between three inline strategies:
 
      * Native ints (<= 64 bits). Delegate to the compile-time
        `esi::detail::{read,write}{Un,}signedBits` helpers from
        `esi/BitAccess.h`, which loop over the constituent bits at the
        field's constant offset/width. The non-type template parameters
        let the optimiser fully unroll the loop, so a byte-aligned
        standard-width field still collapses to a single load/store on
        -O1+ -- no special-cased `memcpy` / per-byte fast path required.
      * `bool` (a single bit). Same helpers, specialised to read/write one
        bit and hand back a `bool`.
      * View-class fields. Triggered when `_is_value_class_type` is true --
        currently widths above 64 bits, for any of `BitsType`, signed, or
        unsigned. Returns a non-owning
        `esi::BitVector` / `esi::IntView` / `esi::UIntView` view *into*
        the parent struct's `_bytes` -- zero allocation, no copy. The
        setter accepts any `esi::BitVector` (so views and owning
        subclasses both work) and writes back bit-by-bit. The returned
        view dangles when the parent buffer dies; see the lifetime warning
        at the top of the generated header.
    """
    field_cpp = self._cpp_type(field_type)
    if field_cpp == "void":
      return
 
    wrapped = self._unwrap_aliases(field_type)
 
    if isinstance(wrapped, (types.BitsType, types.IntType)):
      # View-class field. Today this is exactly the wider-than-64-bit
      # integer / Bits cases; gated by `_is_value_class_type` so the rule
      # lives in one place.
      if self._is_value_class_type(field_type):
        byte_offset = bit_offset // 8
        sub_bit_index = bit_offset % 8
        # Number of bytes the view needs to span: the high bit is at
        # `sub_bit_index + bit_width - 1`, so we cover
        # ceil((sub_bit_index + bit_width) / 8) bytes starting at
        # `_bytes.data() + byte_offset`.
        span_bytes = (sub_bit_index + bit_width + 7) // 8
        with w.block(f"{field_cpp} {field_name}() const"):
          w.line(f"return {field_cpp}(")
          w.line(f"    std::span<const uint8_t>("
                 f"{raw_member}.data() + {byte_offset}, {span_bytes}),")
          w.line(f"    static_cast<std::size_t>({bit_width}),")
          w.line(f"    static_cast<uint8_t>({sub_bit_index}));")
        with w.block(f"{self_type} &{field_name}(const {field_cpp} &v)"):
          # Walk the input view bit-by-bit and write each bit straight
          # into `_bytes` at its target position.
          self._emit_view_store(w, raw_member, "v", str(bit_offset), bit_width)
          w.line("return *this;")
        return
 
      # `bool` is the storage choice for a single bit; bit-precise helpers
      # are the simplest fit and the optimiser collapses them on -O2.
      if bit_width == 1 and field_cpp == "bool":
        with w.block(f"bool {field_name}() const"):
          w.line(f"return esi::detail::readUnsignedBits<uint8_t, "
                 f"{bit_offset}, 1>({raw_member}.data()) != 0;")
        with w.block(f"{self_type} &{field_name}(bool v)"):
          w.line(f"esi::detail::writeUnsignedBits<uint8_t, "
                 f"{bit_offset}, 1>({raw_member}.data(), "
                 f"static_cast<uint8_t>(v) & uint8_t{{1}});")
          w.line("return *this;")
        return
 
      signed = self._is_signed_int_field(field_type)
      kind = "Signed" if signed else "Unsigned"
 
      # Delegate to the generic compile-time bit helpers from
      # `esi/BitAccess.h`. They read/write `Width` bits starting at the
      # field's constant `BitOffset` (LSB-first) for any width up to 64 --
      # byte-aligned or not -- and sign-extend signed fields. Because
      # `BitOffset` / `Width` are non-type template parameters the optimiser
      # fully unrolls the per-bit loop, so byte-aligned standard-width fields
      # (`ui8` / `ui32` / ...) still collapse to a single load/store on
      # -O1+, so a single helper covers every width without any
      # hand-written byte-copy fast paths.
      with w.block(f"{field_cpp} {field_name}() const"):
        w.line(f"return esi::detail::read{kind}Bits<{field_cpp}, "
               f"{bit_offset}, {bit_width}>({raw_member}.data());")
      with w.block(f"{self_type} &{field_name}({field_cpp} v)"):
        w.line(f"esi::detail::write{kind}Bits<{field_cpp}, "
               f"{bit_offset}, {bit_width}>({raw_member}.data(), v);")
        w.line("return *this;")
      return
 
    # Aggregate field (struct / union / array). The inner aggregate stores
    # its bits LSB-first in its own `_bytes` buffer, so reading/writing a
    # struct or union reduces to copying `bit_width` bits between the parent
    # buffer at `bit_offset` and the inner buffer at bit 0. Arrays are
    # delegated to `_emit_array_accessor`, a recursive per-element (un)packer
    # that places every scalar leaf at its true wire offset -- so sub-byte
    # ints, odd widths, sub-byte aggregates, and *arbitrarily nested* arrays
    # of those all round-trip, instead of being flat-copied (which only works
    # when the C++ element layout already matches the wire, e.g. `4 x ui8`).
    assert bit_width >= 0, (
        f"field '{field_name}': unbounded aggregate field reached the "
        f"emitter; the planner should have excluded the parent struct "
        f"via `_contains_unbounded`")
 
    # Arrays of plain (non view-class) elements: one uniform recursion.
    if (isinstance(wrapped, types.ArrayType) and
        not self._is_value_class_type(wrapped.element_type)):
      self._emit_array_accessor(w, raw_member, self_type, field_name, wrapped,
                                bit_offset)
      return
 
    # An array whose element is a >64-bit view class is handled by the
    # dedicated span / lazy-range accessors at the end of this method.
    is_view_array = (isinstance(wrapped, types.ArrayType) and
                     self._is_value_class_type(wrapped.element_type))
    byte_aligned = (bit_offset % 8 == 0 and bit_width % 8 == 0)
    byte_offset = bit_offset // 8
    byte_width = (bit_width + 7) // 8
 
    if not is_view_array:
      if byte_aligned:
        # Byte-aligned: direct byte copy between the parent's buffer slice
        # and the inner aggregate's `_bytes`. An explicit per-byte loop
        # avoids `memcpy` while still collapsing to one on -O2.
        with w.block(f"{field_cpp} {field_name}() const"):
          w.line(f"{field_cpp} out{{}};")
          w.line(f"for (std::size_t i = 0; i < {byte_width}; ++i)")
          with w.indented():
            w.line(f"reinterpret_cast<uint8_t *>(&out)[i] = "
                   f"{raw_member}[{byte_offset} + i];")
          w.line("return out;")
        with w.block(f"{self_type} &{field_name}(const {field_cpp} &v)"):
          w.line(f"for (std::size_t i = 0; i < {byte_width}; ++i)")
          with w.indented():
            w.line(f"{raw_member}[{byte_offset} + i] = "
                   f"reinterpret_cast<const uint8_t *>(&v)[i];")
          w.line("return *this;")
      else:
        # Non-byte-aligned: delegate to the per-bit copy helpers. The
        # inner's `out{}` zero-initialiser is required by `copyBitsIn`,
        # which only OR-sets the `1` bits.
        with w.block(f"{field_cpp} {field_name}() const"):
          w.line(f"{field_cpp} out{{}};")
          w.line(f"esi::detail::copyBitsIn<{bit_offset}, "
                 f"{bit_width}>({raw_member}.data(), "
                 f"reinterpret_cast<uint8_t *>(&out));")
          w.line("return out;")
        with w.block(f"{self_type} &{field_name}(const {field_cpp} &v)"):
          w.line(f"esi::detail::copyBitsOut<{bit_offset}, "
                 f"{bit_width}>({raw_member}.data(), "
                 f"reinterpret_cast<const uint8_t *>(&v));")
          w.line("return *this;")
 
    # Indexed accessors for arrays whose element is one of the view
    # classes (`BitVector` / `IntView` / `UIntView`). The whole-array
    # byte-copy path is intentionally skipped (the view's storage layout
    # doesn't match the wire layout). Instead we emit three accessors:
    #
    #   * `field(i)` / `field(i, v)` -- per-element getter (zero-copy
    #     view into the relevant slice of `_bytes`) and setter
    #     (runtime per-bit copy, since `copyBitsOut` needs a
    #     compile-time offset and the array index is runtime).
    #   * `field()` -- a lazy `std::views::iota | std::views::transform`
    #     range that yields the per-element views on demand. Random
    #     access, no allocation, no element materialised until accessed.
    #   * `field(const std::array<view, N> &)` -- whole-array setter
    #     that delegates to the per-element setter N times. Lets the
    #     emplace-style struct ctor accept view-array fields as a
    #     single argument.
    #
    # All three share the parent struct's lifetime; see the warning at
    # the top of the generated header.
    if is_view_array:
      assert isinstance(wrapped, types.ArrayType)
      elem_type = wrapped.element_type
      elem_cpp = self._cpp_type(elem_type)
      elem_width = elem_type.bit_width
      elem_count = wrapped.size
      with w.block(f"{elem_cpp} {field_name}(std::size_t i) const"):
        w.line(f"const std::size_t bit_off = {bit_offset} + i * {elem_width};")
        w.line(f"return {elem_cpp}(")
        w.line(f"    std::span<const uint8_t>("
               f"{raw_member}.data() + bit_off / 8,")
        w.line(f"                             "
               f"(bit_off % 8 + {elem_width} + 7) / 8),")
        w.line(f"    static_cast<std::size_t>({elem_width}),")
        w.line(f"    static_cast<uint8_t>(bit_off % 8));")
      with w.block(f"{self_type} &{field_name}(std::size_t i, "
                   f"const {elem_cpp} &v)"):
        w.line(f"const std::size_t bit_off = {bit_offset} + i * {elem_width};")
        self._emit_view_store(w, raw_member, "v", "bit_off", elem_width)
        w.line("return *this;")
      # Lazy whole-array view: `iota(0, N) | transform([this](i){ ... })`
      # gives a random-access range of `elem_cpp` views computed on
      # access, with zero up-front allocation.
      with w.block(f"auto {field_name}() const"):
        w.line(f"return std::views::iota("
               f"std::size_t{{0}}, std::size_t{{{elem_count}}}) |")
        w.line("       std::views::transform([this](std::size_t i) {")
        w.line(f"         return this->{field_name}(i);")
        w.line("       });")
      # Whole-array setter: forwards to the per-element setter for
      # each index.
      with w.block(f"{self_type} &{field_name}("
                   f"const std::array<{elem_cpp}, {elem_count}> &v)"):
        w.line(f"for (std::size_t i = 0; i < {elem_count}; ++i)")
        with w.indented():
          w.line(f"{field_name}(i, v[i]);")
        w.line("return *this;")
 
  def _emit_array_accessor(self, w: IndentedWriter, raw_member: str,
                           self_type: str, field_name: str,
                           array_type: types.ArrayType,
                           bit_offset: int) -> None:
    """Emit the accessor set for a fixed-size array field.
 
    Four accessors are emitted, matching the existing array API:
 
      * `Elem field(std::size_t i)` / `Self &field(std::size_t i, const Elem &)`
        -- per-element get/set, where `Elem` is the (possibly itself an
        array) element type.
      * `Whole field()` / `Self &field(const Whole &)` -- whole-array
        get/set, expressed in terms of the per-element accessors.
 
    The per-element get/set bodies are produced by `_emit_unpack` /
    `_emit_pack`, which recurse to the scalar leaves and place each leaf at
    its true wire offset `bit_offset + i * elemWidth (+ ...)`. This is what
    lets sub-byte, odd-width, sub-byte-aggregate, and *arbitrarily nested*
    array elements round-trip even when the C++ element layout does not
    match the wire layout.
    """
    elem_type = array_type.element_type
    elem_cpp = self._cpp_type(elem_type)
    elem_width = elem_type.bit_width
    elem_count = array_type.size
    whole_cpp = self._cpp_type(array_type)
    elem_off = f"{bit_offset} + i * {elem_width}"
 
    with w.block(f"{elem_cpp} {field_name}(std::size_t i) const"):
      w.line(f"{elem_cpp} out{{}};")
      self._emit_unpack(w, raw_member, "out", elem_off, elem_type, 1)
      w.line("return out;")
 
    with w.block(f"{self_type} &{field_name}(std::size_t i, "
                 f"const {elem_cpp} &v)"):
      self._emit_pack(w, raw_member, "v", elem_off, elem_type, 1)
      w.line("return *this;")
 
    with w.block(f"{whole_cpp} {field_name}() const"):
      w.line(f"{whole_cpp} out{{}};")
      w.line(f"for (std::size_t i = 0; i < {elem_count}; ++i)")
      with w.indented():
        w.line(f"out[i] = {field_name}(i);")
      w.line("return out;")
 
    with w.block(f"{self_type} &{field_name}(const {whole_cpp} &v)"):
      w.line(f"for (std::size_t i = 0; i < {elem_count}; ++i)")
      with w.indented():
        w.line(f"{field_name}(i, v[i]);")
      w.line("return *this;")
 
  def _emit_unpack(self, w: IndentedWriter, raw_member: str, dst: str, off: str,
                   esi_type: types.ESIType, depth: int) -> None:
    """Emit statements that read a value of `esi_type` out of `raw_member`
    at wire bit offset `off` (a C++ `size_t` expression) into the
    already-declared, zero-initialised C++ lvalue `dst`.
 
    Recurses through arrays (one loop per dimension) down to scalar / struct
    / union leaves, which are read with the runtime-offset bit helpers.
    """
    wrapped = self._unwrap_aliases(esi_type)
    if isinstance(wrapped, types.ArrayType):
      idx = f"i{depth}"
      ew = wrapped.element_type.bit_width
      with w.block(f"for (std::size_t {idx} = 0; {idx} < "
                   f"{wrapped.size}; ++{idx})"):
        self._emit_unpack(w, raw_member, f"{dst}[{idx}]",
                          f"{off} + {idx} * {ew}", wrapped.element_type,
                          depth + 1)
      return
    if isinstance(wrapped, (types.StructType, types.UnionType)):
      # The inner aggregate's own `_bytes` already mirror the wire layout, so
      # copy its bits straight in. `dst` is zero-initialised by the caller,
      # as `copyBitsInDyn` only OR-sets the `1` bits.
      w.line(f"esi::detail::copyBitsInDyn({raw_member}.data(), {off},")
      w.line(f"    reinterpret_cast<uint8_t *>(&{dst}), {wrapped.bit_width});")
      return
    if isinstance(wrapped, (types.BitsType, types.IntType)):
      if self._is_value_class_type(esi_type):
        raise NotImplementedError(
            "arrays of integers wider than 64 bits are not supported")
      cpp = self._cpp_type(esi_type)
      if cpp == "bool":
        w.line(f"{dst} = esi::detail::readUnsignedBitsDyn<uint8_t>("
               f"{raw_member}.data(), {off}, 1) != 0;")
      elif self._is_signed_int_field(esi_type):
        w.line(f"{dst} = esi::detail::readSignedBitsDyn<{cpp}>("
               f"{raw_member}.data(), {off}, {wrapped.bit_width});")
      else:
        w.line(f"{dst} = esi::detail::readUnsignedBitsDyn<{cpp}>("
               f"{raw_member}.data(), {off}, {wrapped.bit_width});")
      return
    raise NotImplementedError(
        f"unsupported array element type '{wrapped}' for C++ generation")
 
  def _emit_pack(self, w: IndentedWriter, raw_member: str, src: str, off: str,
                 esi_type: types.ESIType, depth: int) -> None:
    """Inverse of `_emit_unpack`: write the C++ value `src` of `esi_type`
    into `raw_member` at wire bit offset `off`."""
    wrapped = self._unwrap_aliases(esi_type)
    if isinstance(wrapped, types.ArrayType):
      idx = f"i{depth}"
      ew = wrapped.element_type.bit_width
      with w.block(f"for (std::size_t {idx} = 0; {idx} < "
                   f"{wrapped.size}; ++{idx})"):
        self._emit_pack(w, raw_member, f"{src}[{idx}]", f"{off} + {idx} * {ew}",
                        wrapped.element_type, depth + 1)
      return
    if isinstance(wrapped, (types.StructType, types.UnionType)):
      w.line(f"esi::detail::copyBitsOutDyn({raw_member}.data(), {off},")
      w.line(f"    reinterpret_cast<const uint8_t *>(&{src}), "
             f"{wrapped.bit_width});")
      return
    if isinstance(wrapped, (types.BitsType, types.IntType)):
      if self._is_value_class_type(esi_type):
        raise NotImplementedError(
            "arrays of integers wider than 64 bits are not supported")
      cpp = self._cpp_type(esi_type)
      if cpp == "bool":
        w.line(f"esi::detail::writeUnsignedBitsDyn<uint8_t>("
               f"{raw_member}.data(), "
               f"static_cast<uint8_t>({src}) & uint8_t{{1}}, {off}, 1);")
      elif self._is_signed_int_field(esi_type):
        w.line(f"esi::detail::writeSignedBitsDyn<{cpp}>("
               f"{raw_member}.data(), {src}, {off}, {wrapped.bit_width});")
      else:
        w.line(f"esi::detail::writeUnsignedBitsDyn<{cpp}>("
               f"{raw_member}.data(), {src}, {off}, {wrapped.bit_width});")
      return
    raise NotImplementedError(
        f"unsupported array element type '{wrapped}' for C++ generation")
 
  def _ctor_param_type(self, field_type: types.ESIType) -> str:
    """Return the C++ constructor-parameter type for a setter call."""
    field_cpp = self._cpp_type(field_type)
    wrapped = self._unwrap_aliases(field_type)
    if isinstance(wrapped, (types.BitsType, types.IntType)):
      return field_cpp
    return f"const {field_cpp} &"
 
  def _emit_struct(self, w: IndentedWriter,
                   struct_type: types.StructType) -> None:
    """Emit a packed struct as a raw byte buffer with bit-precise accessors.
 
    The struct's storage is a single `std::array<uint8_t, N>` (where `N`
    is the byte width derived from the manifest) whose layout matches the
    on-wire bit-packing exactly. Per-field accessors (generated below)
    read/write the bits so the wire format does not depend on C++ bit-field
    allocation rules, which differ between the Itanium ABI (GCC/Clang) and MSVC.
    """
    # Zero-width composite types collapse to `void` everywhere they appear
    # (see _cpp_type), so there is nothing meaningful to emit here.
    if struct_type.bit_width == 0:
      return
    struct_name = self.type_id_map[struct_type]
    bit_offsets, total_bits = self._compute_field_bit_offsets(struct_type)
    total_bytes = (total_bits + 7) // 8
 
    # Logical-order field list (manifest order) for the constructor and the
    # public accessor list.
    logical_fields = [(name, ftype)
                      for name, ftype in struct_type.fields
                      if self._cpp_type(ftype) != "void"]
 
    # A struct whose every field collapses to void carries no data the
    # generated header can expose. Emit nothing rather than a 1-byte
    # placeholder, since (a) the placeholder doesn't reflect any real wire
    # layout and (b) a size_assert built from the manifest's `bit_width`
    # (which includes void padding) would fail to compile.
    if not logical_fields:
      return
 
    with w.block(f"struct {struct_name}", tail=";"):
      # `_bytes` holds the wire layout and is private; the only legitimate
      # external view of those bytes is via `MessageData::from()` /
      # `MessageData::as()` which reinterpret-cast through the whole struct
      # and don't need member-level access.
      w.access("private:")
      w.line(f"std::array<uint8_t, {max(total_bytes, 1)}> _bytes{{}};")
      w.line()
      w.access("public:")
      w.line(f"{struct_name}() = default;")
      ctor_params = ", ".join(f"{self._ctor_param_type(ftype)} {name}"
                              for name, ftype in logical_fields)
      with w.block(f"{struct_name}({ctor_params})"):
        # `this->` disambiguates the chained setter calls from the like-named
        # parameters that shadow the member functions inside the ctor body.
        chain = ".".join(f"{name}({name})" for name, _ in logical_fields)
        w.line(f"this->{chain};")
      w.line()
 
      # Per-field accessors in logical (manifest) order so the user-facing
      # API mirrors the manifest field order rather than the wire reversal.
      for name, ftype in logical_fields:
        self._emit_field_accessor(w, "_bytes", struct_name, name, ftype,
                                  bit_offsets[name], ftype.bit_width)
        w.line()
 
      w.line(f"static constexpr std::string_view _ESI_ID = "
             f"{self._cpp_string_literal(struct_type.id)};")
    expected_bytes = self._safe_byte_width(struct_type)
    if expected_bytes is not None and expected_bytes > 0:
      self._emit_size_assert(w, struct_name, expected_bytes)
    w.line()
 
  def _emit_union(self, w: IndentedWriter, union_type: types.UnionType) -> None:
    """Emit a union as a raw byte buffer with per-variant accessors.
 
    The union's storage is a single `std::array<uint8_t, N>` sized to the
    widest variant. Each variant lives at the MSB end of the buffer
    (matching the existing SV-style packed union layout where padding
    occupies the lower bytes), so the byte offset for a variant of size
    V is `union_bytes - V`. Sub-byte integer variants are byte-padded to
    full bytes within that region, matching the Python runtime's union
    serialization.
    """
    # Zero-width unions collapse to `void` (see _cpp_type) so there is
    # nothing meaningful to emit here.
    if union_type.bit_width == 0:
      return
    union_name = self.type_id_map[union_type]
    union_bytes = self._field_byte_width(union_type)
 
    with w.block(f"struct {union_name}", tail=";"):
      # See `_emit_struct` for the access-control rationale.
      w.access("private:")
      w.line(f"std::array<uint8_t, {union_bytes}> _bytes{{}};")
      w.line()
      w.access("public:")
      w.line(f"{union_name}() = default;")
      w.line()
 
      for field_name, field_type in union_type.fields:
        field_cpp = self._cpp_type(field_type)
        if field_cpp == "void":
          continue
        field_bytes = self._field_byte_width(field_type)
        byte_offset = union_bytes - field_bytes
        bit_offset = byte_offset * 8
        bit_width = field_type.bit_width
        # Each variant is reached at the same MSB-aligned position regardless
        # of width. Reuse `_emit_field_accessor` so we share the integer /
        # aggregate code paths and don't duplicate the bit-access boilerplate.
        self._emit_field_accessor(w, "_bytes", union_name, field_name,
                                  field_type, bit_offset, bit_width)
        w.line()
 
      w.line(f"static constexpr std::string_view _ESI_ID = "
             f"{self._cpp_string_literal(union_type.id)};")
    if union_bytes > 0:
      self._emit_size_assert(w, union_name, union_bytes)
    w.line()
 
  def _compute_window_frame_layout(
      self, fields: List[Tuple[str, Optional[types.ESIType]]], frame_bits: int,
      count_type_synth: types.ESIType
  ) -> List[Tuple[str, types.ESIType, int, int]]:
    """Compute (name, type, bit_offset, bit_width) for each window frame field.
 
    Fields are listed in declared (MSB-first) order and laid out MSB-aligned
    within `frame_bits`, matching how CIRCT lowers the serial-window frame
    union: the first field occupies the highest bits, each subsequent field
    sits immediately below it with no inter-field padding, and any slack
    (`frame_bits - content_bits`) is left as zero padding at the LSB end.
    The returned `bit_offset` is the field's LSB position within the frame's
    little-endian `_bytes` array, ready to hand straight to
    `_emit_field_accessor`.
 
    The sentinel `(name, None)` count field is materialised as
    `count_type_synth` so it can flow through the standard
    `_emit_field_accessor` path.
 
    Raises `ValueError` if a field is unbounded (`bit_width < 0`) or if the
    accumulated content overflows `frame_bits` -- either would produce bogus
    offsets, so fail fast rather than emit silently-wrong accessors.
    """
    layout = []
    bit_top = frame_bits
    for name, ftype in fields:
      actual_type = count_type_synth if ftype is None else ftype
      bit_width = actual_type.bit_width
      if bit_width < 0:
        raise ValueError(
            f"window frame field '{name}' has an unbounded width; window "
            f"codegen requires every frame field to be fixed-width")
      bit_top -= bit_width
      if bit_top < 0:
        raise ValueError(
            f"window frame field '{name}' overflows the {frame_bits}-bit frame "
            f"(content is wider than the frame width)")
      layout.append((name, actual_type, bit_top, bit_width))
    return layout
 
  def _emit_window_frame(
      self, w: IndentedWriter, frame_name: str, frame_bytes: int,
      layout: List[Tuple[str, types.ESIType, int, int]]) -> None:
    """Emit a window header/data frame as a raw-bytes struct with accessors.
 
    Nested inside the window helper class, it uses the same accessor pattern
    as top-level structs/unions: a private `_bytes` array plus per-field
    getter/setter pairs returning `frame_name &` to allow chaining.
    """
    with w.block(f"struct {frame_name}", tail=";"):
      w.access("private:")
      w.line(f"std::array<uint8_t, {frame_bytes}> _bytes{{}};")
      w.line()
      w.access("public:")
      for name, ftype, bit_offset, bit_width in layout:
        self._emit_field_accessor(w, "_bytes", frame_name, name, ftype,
                                  bit_offset, bit_width)
        w.line()
    self._emit_size_assert(w, frame_name, frame_bytes)
 
  def _emit_window(self, hdr: IndentedWriter,
                   window_type: types.WindowType) -> None:
    """Emit a SegmentedMessageData helper for a serial list window.
 
    This emitter builds its own pre-indented text, so it uses the writer
    purely as a verbatim `hdr.write(...)` sink. The nested frame structs,
    however, are produced by the line-based `_emit_window_frame`, so their
    calls are wrapped in `hdr.indented()` to place them at the window
    class's member indent.
    """
    info = self._analyze_window(window_type)
    ctor_params = [
        self._format_window_ctor_param(name, field_type)
        for name, field_type in info["ctor_params"]
    ]
    value_ctor_params = list(ctor_params)
    value_ctor_params.append(
        f"const std::vector<value_type> &{info['list_field_name']}")
    value_ctor_signature = ", ".join(value_ctor_params)
    frame_ctor_params = list(ctor_params)
    frame_ctor_params.append("std::vector<data_frame> frames")
    frame_ctor_signature = ", ".join(frame_ctor_params)
    helper_args = ", ".join(name for name, _ in info["ctor_params"])
    helper_call = f"{helper_args}, std::move(frames)" if helper_args else "std::move(frames)"
 
    # Synthesise an unsigned integer type for the sentinel "count" header
    # field so it flows through `_emit_field_accessor` like any other
    # integer.
    count_type_synth = types.UIntType(f"_count_ui{info['count_width']}",
                                      info["count_width"])
    data_layout = self._compute_window_frame_layout(info["data_fields"],
                                                    info["frame_bits"],
                                                    count_type_synth)
    header_layout = self._compute_window_frame_layout(info["header_fields"],
                                                      info["frame_bits"],
                                                      count_type_synth)
 
    # Largest list length encodable in a single burst's count field. Lists
    # longer than this are split across multiple header/data bursts (the
    # read-side `SerialListTypeDeserializer` reassembles them), each burst
    # carrying at most this many data frames. `count_width` is <= 64 (the
    # count field's storage type tops out at 64 bits), so this literal always
    # fits in a `size_t`.
    max_batch_val = (1 << info["count_width"]) - 1
 
    hdr.write(
        f"struct {info['window_name']} : public esi::SegmentedMessageData {{\n")
    hdr.write("public:\n")
    hdr.write(f"  using value_type = {info['element_cpp']};\n")
    hdr.write(f"  using count_type = {info['count_cpp']};\n\n")
    with hdr.indented():
      self._emit_window_frame(hdr, "data_frame", info["frame_bytes"],
                              data_layout)
    hdr.write("\n")
    hdr.write("private:\n")
    with hdr.indented():
      self._emit_window_frame(hdr, "header_frame", info["frame_bytes"],
                              header_layout)
    hdr.write("\n")
    hdr.write("  // One header per burst (bursts 2..N are continuations whose\n"
              "  // only meaningful field is the count); the data frames of\n"
              "  // every burst are stored contiguously in `data_frames` and\n"
              "  // sliced per burst by `segment()`.\n")
    hdr.write("  std::vector<header_frame> headers;\n")
    hdr.write("  std::vector<data_frame> data_frames;\n")
    hdr.write("  header_frame footer{};\n\n")
    hdr.write("  // Largest list length encodable in one burst's count field.\n"
              "  // Longer lists are chunked across multiple bursts.\n")
    hdr.write(f"  static constexpr size_t _kMaxBatch = {max_batch_val}ULL;\n\n")
    hdr.write(f"  void construct({frame_ctor_signature}) {{\n")
    hdr.write("    if (frames.empty())\n")
    hdr.write(
        f"      throw std::invalid_argument(\"{info['window_name']}: bulk windowed lists cannot be empty\");\n"
    )
    hdr.write("    data_frames = std::move(frames);\n")
    hdr.write("    // Split the list into bursts no larger than the count\n"
              "    // field can encode. The read side reassembles the bursts\n"
              "    // back into a single list.\n")
    hdr.write("    const size_t total = data_frames.size();\n")
    hdr.write(
        "    const size_t numBatches = (total + _kMaxBatch - 1) / _kMaxBatch;\n"
    )
    hdr.write("    headers.assign(numBatches, header_frame{});\n")
    hdr.write("    for (size_t b = 0; b < numBatches; ++b) {\n")
    hdr.write(
        "      const size_t batchSize =\n"
        "          std::min<size_t>(_kMaxBatch, total - b * _kMaxBatch);\n")
    hdr.write(
        f"      headers[b].{info['count_field_name']}(static_cast<count_type>(batchSize));\n"
    )
    hdr.write("    }\n")
    hdr.write(
        "    // Only the first burst's header carries the static fields.\n")
    for name, _ in info["ctor_params"]:
      hdr.write(f"    headers.front().{name}({name});\n")
    hdr.write(f"    footer.{info['count_field_name']}(0);\n")
    hdr.write("  }\n\n")
    hdr.write("public:\n")
    hdr.write(f"  {info['window_name']}({frame_ctor_signature}) {{\n")
    hdr.write(f"    construct({helper_call});\n")
    hdr.write("  }\n\n")
    hdr.write(f"  {info['window_name']}({value_ctor_signature}) {{\n")
    hdr.write("    std::vector<data_frame> frames;\n")
    hdr.write(f"    frames.reserve({info['list_field_name']}.size());\n")
    hdr.write(f"    for (const auto &element : {info['list_field_name']}) {{\n")
    hdr.write("      auto &frame = frames.emplace_back();\n")
    hdr.write(f"      frame.{info['list_field_name']}(element);\n")
    hdr.write("    }\n")
    hdr.write(f"    construct({helper_call});\n")
    hdr.write("  }\n\n")
    hdr.write("  size_t numSegments() const override {\n"
              "    return 2 * headers.size() + 1;\n"
              "  }\n")
    hdr.write("  esi::Segment segment(size_t idx) const override {\n")
    hdr.write("    const size_t footerIdx = 2 * headers.size();\n")
    hdr.write("    if (idx == footerIdx)\n")
    hdr.write(
        "      return {reinterpret_cast<const uint8_t *>(&footer), sizeof(footer)};\n"
    )
    hdr.write("    if (idx > footerIdx)\n")
    hdr.write(
        f"      throw std::out_of_range(\"{info['window_name']}: invalid segment index\");\n"
    )
    hdr.write("    const size_t b = idx / 2;\n")
    hdr.write("    if (idx % 2 == 0)\n")
    hdr.write("      return {reinterpret_cast<const uint8_t *>(&headers[b]),\n"
              "              sizeof(header_frame)};\n")
    hdr.write("    const size_t start = b * _kMaxBatch;\n")
    hdr.write(
        "    const size_t batchSize =\n"
        "        std::min<size_t>(_kMaxBatch, data_frames.size() - start);\n")
    hdr.write(
        "    return {reinterpret_cast<const uint8_t *>(data_frames.data() + start),\n"
        "            batchSize * sizeof(data_frame)};\n")
    hdr.write("  }\n\n")
    hdr.write(
        f"  static constexpr std::string_view _ESI_ID = {self._cpp_string_literal(self._unwrap_aliases(window_type.into_type).id)};\n"
    )
    # The into-type id alone cannot distinguish two different windows over
    # the same underlying struct (e.g. serial vs. parallel list encoding).
    # Emit the window id so the runtime can verify the wire format too.
    hdr.write(
        f"  static constexpr std::string_view _ESI_WINDOW_ID = {self._cpp_string_literal(window_type.id)};\n"
    )
    self._emit_window_data_accessors(hdr, info)
    self._emit_window_deserializer(hdr, info)
    hdr.write("};\n\n")
 
  def _emit_window_data_accessors(self, hdr: IndentedWriter,
                                  info: Dict[str, Any]) -> None:
    """Emit accessors for the header and data fields of a window helper.
 
    Exposes each static header field as a scalar accessor, the count of data
    frames, and one vector-valued accessor per data field so decoded values
    are easy to inspect on the read side.
    """
    list_field_name = info["list_field_name"]
    hdr.write("\n")
    for field_name, field_type in info["header_fields"]:
      # Skip the synthetic bulk-count field; it is exposed via
      # `<list>_count()` below.
      if field_type is None:
        continue
      cpp = self._cpp_type(field_type)
      # The header-frame accessor returns by value (it pulls bytes out of
      # the raw buffer), so this is also returned by value regardless of
      # whether the underlying type is a scalar or an aggregate. The static
      # fields live on the first burst's header.
      hdr.write(
          f"  {cpp} {field_name}() const {{ return headers.front().{field_name}(); }}\n"
      )
    hdr.write(
        f"  size_t {list_field_name}_count() const {{ return data_frames.size(); }}\n"
    )
    for field_name, field_type in info["data_fields"]:
      if field_name == list_field_name:
        elem_cpp = "value_type"
      else:
        elem_cpp = self._cpp_type(field_type)
      # The data-frame accessor is now a member function returning by value
      # (the underlying `_bytes` is private; getters reconstruct the field
      # value from its wire bytes), so the projection has to call it
      # rather than form a pointer-to-data-member.
      #
      # TODO: For byte-aligned aggregate data fields this means iterating
      # `field()` copies each element; the pre-B3 pointer-to-member form
      # could yield references for free. If profiling shows it matters,
      # we can add a separate `field_ref()` accessor on `data_frame` for
      # byte-aligned aggregate fields (they're stored contiguously inside
      # `_bytes` and the inner type's only member is itself a byte array,
      # so a `reinterpret_cast`-backed reference is well-defined) and
      # project on that instead.
      projection = (f"[](const data_frame &f) -> {elem_cpp} "
                    f"{{ return f.{field_name}(); }}")
      hdr.write(
          f"  auto {field_name}() const {{\n"
          f"    return std::views::transform(data_frames, {projection});\n"
          f"  }}\n")
      hdr.write(f"  std::vector<{elem_cpp}> {field_name}_vector() const {{\n"
                f"    std::vector<{elem_cpp}> out;\n"
                f"    out.reserve(data_frames.size());\n"
                f"    for (const auto &frame : data_frames)\n"
                f"      out.push_back(frame.{field_name}());\n"
                f"    return out;\n"
                f"  }}\n")
 
  def _emit_window_deserializer(self, hdr: IndentedWriter,
                                info: Dict[str, Any]) -> None:
    """Emit a few bridge helpers + a `TypeDeserializer` alias.
 
    The actual decoder lives in `esi::SerialListTypeDeserializer<T>`, which
    walks the header/data/footer burst protocol generically. Each window
    helper only has to expose:
 
      - `_headerCount(const header_frame &)` -> `count_type`
      - `_fromFrames(const header_frame &, std::vector<data_frame> &&)`
        -> `std::unique_ptr<T>`
 
    plus a `friend class esi::SerialListTypeDeserializer<T>;` so the template
    can reach the (private) `header_frame` definition.
    """
    window_name = info["window_name"]
    count_field_name = info["count_field_name"]
    ctor_args = ", ".join(f"h.{name}()" for name, _ in info["ctor_params"])
    if ctor_args:
      ctor_args = f"{ctor_args}, std::move(frames)"
    else:
      ctor_args = "std::move(frames)"
 
    hdr.write("\n")
    hdr.write("private:\n")
    hdr.write(
        "  // Bridge helpers used by esi::SerialListTypeDeserializer<T>; the\n")
    hdr.write(
        "  // template walks the serial-list burst protocol generically and\n")
    hdr.write(
        "  // reaches into `header_frame` via the friend declaration below.\n")
    hdr.write("  static count_type _headerCount(const header_frame &h) {\n")
    hdr.write(f"    return h.{count_field_name}();\n")
    hdr.write("  }\n")
    hdr.write(f"  static std::unique_ptr<{window_name}> _fromFrames(\n")
    hdr.write(
        "      const header_frame &h, std::vector<data_frame> &&frames) {\n")
    hdr.write(f"    return std::make_unique<{window_name}>({ctor_args});\n")
    hdr.write("  }\n")
    hdr.write(
        f"  friend class esi::SerialListTypeDeserializer<{window_name}>;\n\n")
    hdr.write("public:\n")
    hdr.write(
        f"  using TypeDeserializer = esi::SerialListTypeDeserializer<{window_name}>;\n"
    )
 
  def _emit_alias(self, w: IndentedWriter, alias_type: types.TypeAlias) -> None:
    """Emit a using alias when the alias targets a different C++ type."""
    inner_wrapped = alias_type.inner_type
    alias_name = self.type_id_map[alias_type]
    inner_cpp = None
    if inner_wrapped is not None:
      inner_cpp = self._cpp_type(inner_wrapped)
    if inner_cpp is None:
      inner_cpp = self.type_id_map[alias_type]
    if inner_cpp != alias_name:
      w.line(f"using {alias_name} = {inner_cpp};")
      w.line()
 
  def write_header(self, output_dir: Path, system_name: str) -> None:
    """Emit the fully ordered types.h header into the output directory."""
    if self.has_cycle:
      sys.stderr.write("Warning: cyclic type dependencies detected.\n")
      sys.stderr.write("  Logically this should not be possible.\n")
      sys.stderr.write(
          "  Emitted code may fail to compile due to ordering issues.\n")
 
    hdr_file = output_dir / "types.h"
    with open(hdr_file, "w", encoding="utf-8") as hdr:
      w = IndentedWriter(hdr)
      w.write(
          textwrap.dedent(f"""
        // Generated header for {system_name} types.
        //
        // Lifetime note: accessors that return `esi::BitVector` (for
        // `Bits<N>` fields), `esi::IntView` (for `Int<N>`, N > 64), or
        // `esi::UIntView` (for `UInt<N>`, N > 64) hand back non-owning
        // views into the parent struct's bytes. The view is only valid
        // while the parent is alive. Bind the parent to a named local
        // first, or construct an owning `esi::Int` / `esi::UInt` /
        // `esi::MutableBitVector` from the view if you need the value
        // to outlive its source. See `esi/Values.h` for details.
        #pragma once
 
        #include <cstdint>
        #include <cstddef>
        #include <cstring>
        #include <algorithm>
        #include <any>
        #include <array>
        #include <limits>
        #include <ranges>
        #include <stdexcept>
        #include <string_view>
        #include <utility>
        #include <vector>
 
        #include "esi/BitAccess.h"
        #include "esi/Common.h"
        #include "esi/TypedPorts.h"
        #include "esi/Values.h"
 
        namespace {system_name} {{
 
      """))
 
      for skipped_type, reason in self.skipped_types:
        w.write(f"// Unsupported type '{skipped_type}': {reason}\n\n")
 
      for emit_type in self.ordered_types:
        # Anything that wasn't expressible should have been caught by
        # CppTypePlanner and recorded in `skipped_types`; if a problem
        # leaks past that, treat it as a planner bug rather than emitting
        # a half-written header.
        if isinstance(emit_type, types.StructType):
          self._emit_struct(w, emit_type)
        elif isinstance(emit_type, types.UnionType):
          self._emit_union(w, emit_type)
        elif isinstance(emit_type, types.WindowType):
          self._emit_window(w, emit_type)
        elif isinstance(emit_type, types.TypeAlias):
          self._emit_alias(w, emit_type)
 
      w.write(textwrap.dedent(f"""
    }} // namespace {system_name}
    """))
 
 
def run(generator: Type[Generator] = CppGenerator,
        cmdline_args: List[str] = sys.argv) -> int:
  """Create and run a generator reading options from the command line."""
 
  argparser = argparse.ArgumentParser(
      description=f"Generate {generator.language} headers from an ESI manifest",
      formatter_class=argparse.RawDescriptionHelpFormatter,
      epilog=textwrap.dedent("""
        Can read the manifest from either a file OR a running accelerator.
 
        Usage examples:
          # To read the manifest from a file:
          esi-cppgen --file /path/to/manifest.json
 
          # To read the manifest from a running accelerator:
          esi-cppgen --platform cosim --connection localhost:1234
      """))
 
  argparser.add_argument("--file",
                         type=str,
                         default=None,
                         help="Path to the manifest file.")
  argparser.add_argument(
      "--platform",
      type=str,
      help="Name of platform for live accelerator connection.")
  argparser.add_argument(
      "--connection",
      type=str,
      help="Connection string for live accelerator connection.")
  argparser.add_argument(
      "--output-dir",
      type=str,
      default="esi",
      help="Output directory for generated files. Recommend adding either `esi`"
      " or the system name to the end of the path so as to avoid header name"
      "conflicts. Defaults to `esi`")
  argparser.add_argument(
      "--system-name",
      type=str,
      default="esi_system",
      help="Name of the ESI system. For C++, this will be the namespace.")
 
  if (len(cmdline_args) <= 1):
    argparser.print_help()
    return 1
  args = argparser.parse_args(cmdline_args[1:])
 
  if args.file is not None and args.platform is not None:
    print("Cannot specify both --file and --platform")
    return 1
 
  conn: AcceleratorConnection
  if args.file is not None:
    # Use os.pathsep (';' on Windows, ':' on Unix) to avoid conflicts with
    # drive letters.
    conn = Context.default().connect("trace", f"-{os.pathsep}{args.file}")
  elif args.platform is not None:
    if args.connection is None:
      print("Must specify --connection with --platform")
      return 1
    conn = Context.default().connect(args.platform, args.connection)
  else:
    print("Must specify either --file or --platform")
    return 1
 
  output_dir = Path(args.output_dir)
  if output_dir.exists() and not output_dir.is_dir():
    print(f"Output directory {output_dir} is not a directory")
    return 1
  if not output_dir.exists():
    output_dir.mkdir(parents=True)
 
  gen = generator(conn)
  gen.generate(output_dir, args.system_name)
  return 0