sc-cxxrtl

A CXXRTL cocotb simulator for SiliconCompiler — installed, not patched in.

pip install sc-cxxrtl adds the Yosys CXXRTL engine as a cocotb simulator to SiliconCompiler without modifying SiliconCompiler. SC executes whatever Task/Flowgraph subclasses a flow contains, and those live in this package — so no SC source change is needed.

Built on:

• cxxrtl-vpi — the engine adapter that makes cocotb drive a CXXRTL model (an IEEE-1364 VPI implementation over cxxrtl_capi),
• cocotb ≥ 2.0 — the Python testbench framework,
• Yosys — write_cxxrtl + a C++ compiler (external, like every SC simulator).

Install

pip install sc-cxxrtl

This pulls in cxxrtl-vpi, cocotb, and siliconcompiler. Requires Python ≤ 3.13 (cocotb's cap), plus Yosys (yosys / yosys-config) and a C++ compiler on PATH — the same external precondition as every SiliconCompiler simulator. (Latest from git: pip install git+https://github.com/lanserge/sc-cxxrtl.)

Use

from siliconcompiler import Design, Project
from sc_cxxrtl import CxxrtlDVFlow

design = Design("counter")
with design.active_fileset("rtl"):
    design.set_topmodule("counter")
    design.add_file("counter.v")
with design.active_fileset("tb"):
    design.add_file("test_counter.py")   # a cocotb test (filetype: python)

proj = Project(design)
proj.add_fileset("rtl")
proj.add_fileset("tb")
proj.set_flow(CxxrtlDVFlow())             # <- the only line that mentions cxxrtl
proj.run()

The flow is two nodes:

compile   (cxxrtl/cocotb_compile)  RTL -> CXXRTL model -> link cxxrtl-vpi + cocotb -> outputs/<top>.vexe
   │
simulate  (cxxrtl/exec_cocotb)     run the executable under cocotb -> results.xml

Tool-named fileset

Like SiliconCompiler's built-in verilator/icarus tasks, the compile task is tool-fileset-aware: if the design defines a fileset named after the tool (cxxrtl), RTL is sourced from that fileset's subtree. Give it add_depfileset(self, "rtl") to pull in the base RTL, and put any CXXRTL-specific includes/defines there:

with design.active_fileset("rtl"):
    design.set_topmodule("dut")
    design.add_file("dut.v")
with design.active_fileset("cxxrtl"):
    design.add_depfileset(design, "rtl")  # cxxrtl depends on rtl

This keeps a sibling verilator fileset (e.g. one carrying DPI/switchboard shims) out of the CXXRTL build — the same fileset convention used across the SC ecosystem. When the design has no cxxrtl fileset, all active filesets are used (unchanged behavior).

Init fuzzing (X-dependence detection)

CXXRTL is 2-state and inits flop state to 0, so a design that secretly relies on uninitialized state passes silently. The compile task exposes Verilator-style --x-initial unique fuzzing:

from sc_cxxrtl import CxxrtlCocotbCompileTask
CxxrtlCocotbCompileTask.find_task(proj).set_randomize_init(True, seed=42)

This seeds uninitialized flops with setundef -init -random; vary the seed across runs to expose X-dependence.

Waveform tracing

Enable a VCD dump (written to reports/<design>.vcd) on the exec task:

from sc_cxxrtl import CxxrtlCocotbExecTask
CxxrtlCocotbExecTask.find_task(proj).set_trace(True)

A complete, runnable example is in examples/counter/ (python make.py). It passes:

test_counter.test_count_up     PASS
TESTS=1 PASS=1 FAIL=0 SKIP=0
PASS: cocotb test ran on CXXRTL via SiliconCompiler

Why no SC patch is needed

SC's flow/task model is open by composition: a flow instantiates Task subclasses, which may live in any installed package. CxxrtlDVFlow wires CxxrtlCocotbCompileTask + CxxrtlCocotbExecTask (the latter reuses SC's own CocotbTask for environment setup, exactly like the built-in verilator-cocotb tasks). The only thing SC's built-in DVFlow(tool=…) can't do is grow a "cxxrtl" branch on its own — which is why this package ships its own flow. (Making the built-in DVFlow auto-discover plugins via a siliconcompiler.flows entry-point group would be a small, generic upstream contribution.)

License

ISC.