cxxrtl-vpi

An IEEE-1364 VPI interface for CXXRTL, the Yosys C++ simulation backend.

CXXRTL compiles a (System)Verilog design into a fast, embeddable C++ model, but exposes only its own cxxrtl_capi C API — it does not speak VPI. This project implements a VPI provider on top of cxxrtl_capi, turning any CXXRTL-generated model into a VPI-capable simulator.

The immediate motivation is cocotb (Python testbenches) — cocotb drives any VPI/VHPI/FLI simulator, but ships no CXXRTL backend. With cxxrtl-vpi, cocotb's existing generic VPI consumer can drive a CXXRTL model unmodified. But the VPI layer is deliberately client-agnostic: any VPI tool (debuggers, custom harnesses) can use it.

Status: working. Unmodified cocotb testbenches run against CXXRTL models and pass. Measured against cocotb 2.0's own core regression suite, the scheduler/timing/callback core is at Verilator parity (~91% of the runnable core suite; the rest is the inherent CXXRTL-is-a-synthesizer boundary — real/string, folded parameters), and ahead on force/release (which cocotb does not support on Verilator). Implemented over cxxrtl_capi: object access, callbacks/time/control, hierarchy + parent-nav, generate-scope arrays (dut.lane[i].sig), signal arrays (dut.stage[i]), memories, wide signals, net/reg classification, ranges, picosecond time, init-fuzzing, VCD tracing, and an optional yosys-slang frontend for full SystemVerilog. See docs/vpi-coverage.md for the coverage matrix and limits.

Why this is an engine adapter, not a cocotb plugin

This mirrors how Verilator integrates with cocotb:

The heavy, reusable piece — making CXXRTL speak VPI — is a simulator capability, so its natural home is the engine (Yosys), exactly as Verilator owns its own VPI. This repo is the incubator: it depends only on two stable, public contracts and is unblocked regardless of either project's review cycle:

• cxxrtl/capi/cxxrtl_capi.h (Yosys — stable, external-facing)
• vpi_user.h (IEEE 1364 — standard)

See docs/design.md for the full rationale and upstreaming plan.

Layout

include/cxxrtl_vpi/   public headers for the model wrapper + VPI provider
src/
  model.cc            thin wrapper over cxxrtl_capi (create/eval/step/signals)
  vpi_provider.cc     IEEE-1364 VPI entry points implemented over cxxrtl_capi
  harness.cc          main(): build model, run VPI startup, drive the sim loop
examples/cocotb_counter/   end-to-end cocotb example (the first consumer)
cxxrtl_vpi/           small Python helper: locate includes, build, run
docs/                 design rationale + VPI coverage matrix

Install

pip install cxxrtl-vpi

The cocotb path requires Python ≤ 3.13 (cocotb's cap), plus Yosys (yosys / yosys-config) and a C++14 compiler on PATH — the same external toolchain every simulator needs. (Latest from git: pip install git+https://github.com/lanserge/cxxrtl-vpi.) The quickest way to see it work is the cocotb example:

PYTHON=/path/to/venv/bin/python bash examples/cocotb_counter/run_cocotb.sh
# -> test_count_up passed ... TESTS=1 PASS=1 FAIL=0

The provider library alone also builds standalone with CMake:

cmake -S . -B build && cmake --build build

License

ISC — the same license as Yosys/CXXRTL. This keeps the VPI core trivially upstreamable into the Yosys tree (matching file headers, no relicensing), and is permissively compatible with cocotb (BSD-3-Clause) for the consumer glue.