vj

vj is an event-driven, 4-state SystemVerilog simulator written in Rust. It targets IEEE 1800-2023, parses and elaborates with slang (through the sv-lang bindings) and compiles designs to native code with LLVM, either just in time (vj run) or ahead of time (vj build-aot).

Status: 0.1.0 is a preview release. It is not yet fully IEEE 1800-2023 compliant; the numbers below say exactly where it stands.

Conformance (measured, October 2026)

vj tracks every clause of IEEE 1800-2023 (1300 rows) in a compliance matrix:

Open clauses: 6.9.1 (array methods and class-property arrays on values wider than 65535 bits), 15.5 (a class event term's iff is judged when the waiting process resumes rather than at the trigger), 16.9.3 (sampled value functions inside class methods and continuous assigns), 18.5 (a fixed middle dimension below a dynamic outer one in a nested random array), 23.3.3.2 (a ref port actual with a variable index) and 37.15 (some VPI reference objects). Every unsupported construct stops with an error naming it; vj does not knowingly produce a silent wrong answer.

External suites:

• sv-tests, non-UVM: 914 pass, 8 fail (all 8 are test or oracle defects).
• sv-tests UVM tests: 101 of 104 pass (the 3 failures are test defects).
• ivtest: 3218 pass, 330 fail, 66 skipped; the failures are Icarus-specific behaviour, not IEEE 1800 requirements.

External designs, run against iverilog 12 and Verilator 5.032 with matching traces or signatures (every difference traced to an oracle or testbench bug): SERV, darkriscv, Hazard3, cv32e40p, picorv32, Ibex and the OpenTitan primitives, uart and rv_timer; alexforencich verilog-axi, verilog-axis, verilog-ethernet, verilog-pcie, verilog-uart and verilog-i2c; ZipCPU wb2axip bridges and FIFOs. CVA6 elaborates and runs to completion (neither oracle builds it).

Install

vj needs LLVM 22 (development libraries) and a C++ compiler for slang.

LLVM_SYS_221_PREFIX=/path/to/llvm-22 cargo install vj

This installs the vj binary.

Use

vj run top.sv                      # compile and simulate
vj run -I inc -D WIDTH=8 a.sv b.sv --top tb
vj run tb.sv --vpi ./libcocotb.so  # VPI / cocotb
vj run tb.sv dpi.c                 # DPI-C
vj --help

vj build-aot produces a shared library or executable from a design. It links a runner archive, libvj_aot.a, that cargo install does not build: build it from the vj-aot crate (cargo build --release -p vj-aot) and pass --runtime-lib or set VJ_AOT_RT.

Crates

vj is the command-line simulator. The other vj-* crates are its parts (vj-sim netlist and scheduler, vj-codegen LLVM code generation, vj-runtime, vj-exec, vj-vpi, vj-sdf, vj-abi, vj-aot, vj-harness), imported in Rust as vj_sim, vj_codegen and so on.

License

Licensed under either of Apache License 2.0 or MIT license at your option.