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[codex] Allow DUT-only C++ builds without dummy testbenches #59

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1e8b70f
Allow DUT-only C++ builds without dummy testbenches
May 11, 2026
bceaf85
Keep DUT-only docs and entrypoint checks lint-clean
May 11, 2026
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67 changes: 45 additions & 22 deletions compiler/frontend/pycircuit/cli.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1440,6 +1440,10 @@ def _collect_testbench_payload(
)


def _has_testbench_entrypoint(mod: object) -> bool:
return hasattr(mod, "tb") and callable(getattr(mod, "tb"))


def _emit_testbench_pyc_file(
*,
out_dir: Path,
Expand Down Expand Up @@ -1751,6 +1755,15 @@ def _cmd_build(args: argparse.Namespace) -> int:
)
sys.stdout.write("jit-cache: miss\n")

target = str(args.target)
do_cpp = target in {"cpp", "both"}
do_v = target in {"verilator", "both"}
has_tb = _has_testbench_entrypoint(mod)
if not has_tb and do_v:
raise SystemExit(
"DUT-only build without @testbench currently supports --target cpp only; "
"Verilator/SV simulation requires `@testbench def tb(t: Tb): ...`"
)
pycc = _detect_pycc()
jobs = max(1, int(args.jobs))
if int(args.logic_depth) <= 0:
Expand All @@ -1761,10 +1774,6 @@ def _cmd_build(args: argparse.Namespace) -> int:
device_v_root = out_dir / "device" / "verilog"
device_cpp_root.mkdir(parents=True, exist_ok=True)
device_v_root.mkdir(parents=True, exist_ok=True)

target = str(args.target)
do_cpp = target in {"cpp", "both"}
do_v = target in {"verilator", "both"}
pycc_build_profile = "dev-fast" if str(args.profile) == "dev" else "release"
pycc_hard_hierarchy_flags = [
f"--build-profile={pycc_build_profile}",
Expand Down Expand Up @@ -1846,24 +1855,29 @@ def _cmd_build(args: argparse.Namespace) -> int:
except TraceConfigError as e:
raise SystemExit(f"trace config error: {e}") from e

tb_probes = TbProbes.from_probe_manifest(probe_manifest_obj)
tb_name, tb_payload_json = _collect_testbench_payload(
mod, iface, trace_plan=trace_plan, tb_probes=tb_probes
)
tb_pyc_path = _emit_testbench_pyc_file(
out_dir=out_dir, tb_name=tb_name, payload_json=tb_payload_json
)
manifest["testbench"] = {
"name": tb_name,
"pyc": str(tb_pyc_path.relative_to(out_dir)),
}
tb_name = ""
tb_pyc_path: Path | None = None
if has_tb:
tb_probes = TbProbes.from_probe_manifest(probe_manifest_obj)
tb_name, tb_payload_json = _collect_testbench_payload(
mod, iface, trace_plan=trace_plan, tb_probes=tb_probes
)
tb_pyc_path = _emit_testbench_pyc_file(
out_dir=out_dir, tb_name=tb_name, payload_json=tb_payload_json
)
manifest["testbench"] = {
"name": tb_name,
"pyc": str(tb_pyc_path.relative_to(out_dir)),
}
else:
manifest["testbench"] = None
if trace_plan is not None:
trace_path = out_dir / "trace_plan.json"
_save_json(trace_path, trace_plan.as_dict())
manifest["trace_plan"] = str(trace_path.relative_to(out_dir))

tb_cpp_out = out_dir / "tb" / f"{tb_name}.cpp"
tb_sv_out = out_dir / "tb" / f"{tb_name}.sv"
tb_cpp_out = out_dir / "tb" / f"{tb_name}.cpp" if has_tb else None
tb_sv_out = out_dir / "tb" / f"{tb_name}.sv" if has_tb else None
for sym in sorted(module_paths.keys()):
mp = module_paths[sym]
h = _module_hash(mp)
Expand Down Expand Up @@ -1915,7 +1929,7 @@ def _cmd_build(args: argparse.Namespace) -> int:
)
)

if do_cpp:
if do_cpp and has_tb and tb_pyc_path is not None and tb_cpp_out is not None:
tb_key = f"tb:{tb_name}"
tb_hash = _module_hash(tb_pyc_path)
module_hashes[tb_key] = tb_hash
Expand All @@ -1933,7 +1947,7 @@ def _cmd_build(args: argparse.Namespace) -> int:
],
)
)
if do_v:
if do_v and has_tb and tb_pyc_path is not None and tb_sv_out is not None:
tb_key = f"tb:{tb_name}"
tb_hash = module_hashes.get(tb_key) or _module_hash(tb_pyc_path)
module_hashes[tb_key] = tb_hash
Expand Down Expand Up @@ -1962,7 +1976,7 @@ def _cmd_build(args: argparse.Namespace) -> int:
cpp_sources = _gather_cpp_sources(device_cpp_root)
if not cpp_sources:
raise SystemExit("build(cpp): no generated C++ sources found")
if not tb_cpp_out.is_file():
if has_tb and (tb_cpp_out is None or not tb_cpp_out.is_file()):
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@gemini-code-assist gemini-code-assist Bot May 11, 2026

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medium

The check tb_cpp_out is None is redundant here. Since this is inside an if has_tb block, tb_cpp_out is guaranteed to be a Path object based on its initialization at line 1879.

Suggested change
if has_tb and (tb_cpp_out is None or not tb_cpp_out.is_file()):
if has_tb and not tb_cpp_out.is_file():

raise SystemExit(
f"build(cpp): missing generated TB C++ source: {tb_cpp_out}"
)
Expand All @@ -1979,7 +1993,7 @@ def _cmd_build(args: argparse.Namespace) -> int:
build_manifest = {
"version": 3,
"target_name": iface.sym,
"tb_cpp": str(tb_cpp_out),
"tb_cpp": str(tb_cpp_out) if tb_cpp_out is not None else None,
"sources": [str(p) for p in cpp_sources],
"headers": [str(p) for p in cpp_headers],
"include_dirs": include_dirs,
Expand All @@ -1989,6 +2003,15 @@ def _cmd_build(args: argparse.Namespace) -> int:
}
cpp_manifest = out_dir / "cpp_project_manifest.json"
_save_json(cpp_manifest, build_manifest)
manifest["cpp_project_manifest"] = str(cpp_manifest.relative_to(out_dir))

if not has_tb:
manifest["cpp_sources"] = [str(p.relative_to(out_dir)) for p in cpp_sources]
manifest["cpp_headers"] = [str(p.relative_to(out_dir)) for p in cpp_headers]
manifest["cpp_executable"] = None
_save_json(manifest_path, manifest)
sys.stdout.write(f"{manifest_path}\n")
return 0
Comment on lines +2010 to +2016
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@gemini-code-assist gemini-code-assist Bot May 11, 2026

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high

The early return for DUT-only builds bypasses the build cache update logic at the end of the function (lines 2149-2161). This means that subsequent builds will not benefit from the cache (e.g., module_hashes and jit_cache_key won't be persisted), leading to unnecessary re-compilations on every run.

Instead of returning early, consider wrapping the testbench-specific logic (the CMake generation/build and Verilator blocks) in an if has_tb: condition so the function can reach the common cache and manifest persistence logic at the end.


gen_script = _tool_script("gen_cmake_from_manifest.py")
cmake_src = out_dir / "cpp_build" / "src"
Expand Down Expand Up @@ -2042,7 +2065,7 @@ def _cmd_build(args: argparse.Namespace) -> int:
manifest["cpp_executable"] = str(cmake_build / "pyc_tb")

if do_v:
if not tb_sv_out.is_file():
if tb_sv_out is None or not tb_sv_out.is_file():
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medium

The check tb_sv_out is None is redundant. If do_v is true, the check at line 1762 ensures that has_tb is also true, which in turn guarantees that tb_sv_out is a Path object (initialized at line 1880).

Suggested change
if tb_sv_out is None or not tb_sv_out.is_file():
if not tb_sv_out.is_file():

raise SystemExit(
f"build(verilator): missing generated TB SV source: {tb_sv_out}"
)
Expand Down
11 changes: 10 additions & 1 deletion docs/PIPELINE.md
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Original file line number Diff line number Diff line change
Expand Up @@ -13,7 +13,7 @@ Frontend responsibilities:
- materialize `@module(value_params=...)` as runtime boundary input ports
- emit one `.pyc` per specialized module
- emit a deterministic `project_manifest.json`
- emit a testbench `.pyc` payload from `@testbench`
- emit a testbench `.pyc` payload from `@testbench` when present

All emitted modules are stamped with:
- `pyc.frontend.contract = "pycircuit"`
Expand Down Expand Up @@ -49,6 +49,15 @@ Build a project (multi-module + testbench):
python3 -m pycircuit.cli build <tb_or_top.py> --out-dir <dir> --target cpp|verilator|both --jobs <N>
```

Build a DUT only for an external C++ driver:

```bash
python3 -m pycircuit.cli build <top.py> --out-dir <dir> --target cpp --jobs <N>
```

In DUT-only mode the manifest has `"testbench": null`; C++ device sources and
headers are still generated, but no generated pyCircuit TB executable is built.

Simulation (Verilator):

```bash
Expand Down
8 changes: 7 additions & 1 deletion docs/TESTBENCH.md
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Original file line number Diff line number Diff line change
Expand Up @@ -30,7 +30,13 @@ def tb(t: Tb):
t.finish(at=10)
```

`pycircuit build` expects `tb` to be decorated with `@testbench`.
`pycircuit build` can also compile a DUT-only source that does not define
`tb(...)` when the target is `cpp`. In that mode pyCircuit emits the device
`.pyc` modules, generated C++ device sources, and `cpp_project_manifest.json`,
but it does not generate a testbench executable. `verilator` and `both` targets
still require a decorated `@testbench` because they produce simulation harnesses.

If a `tb(...)` function is present, it must be decorated with `@testbench`.

## Tb API (selected)

Expand Down
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