Pono: A Flexible and Extensible SMT-Based Model Checker

Pono is a performant, adaptable, and extensible SMT-based model checker implemented in C++. It leverages Smt-Switch, a generic C++ API for SMT solving. Pono was developed as the next generation of CoSA and thus was originally named cosa2.

Pono is the Hawaiian word for proper, correct, or goodness. It is often used colloquially in the moral sense of "goodness" or "rightness," but also refers to "proper procedure" or "correctness." We use the word for multiple meanings. Our goal is that Pono can be a useful tool for people to verify the correctness of systems, which is surely the right thing to do.

Publications

• Makai Mann, Ahmed Irfan, Florian Lonsing, Yahan Yang, Hongce Zhang, Kristopher Brown, Aarti Gupta, Clark W. Barrett: Pono: A Flexible and Extensible SMT-Based Model Checker. CAV 2021.
  • Evaluated software artifact.
• Makai Mann, Amalee Wilson, Yoni Zohar, Lindsey Stuntz, Ahmed Irfan, Kristopher Brown, Caleb Donovick, Allison Guman, Cesare Tinelli, Clark W. Barrett: Smt-Switch: A Solver-Agnostic C++ API for SMT Solving. SAT 2021.
• Makai Mann: Augmenting transition systems for scalable symbolic model checking. PhD thesis, Stanford University, 2021.

Awards

Pono was awarded the Oski Award under its original name cosa2 at HWMCC'19 for solving the largest number of benchmarks overall.

Setup

• [optional] Install bison and flex
  • If you don't have bison and flex installed globally, run ./contrib/setup-bison.sh and ./contrib/setup-flex.sh
  • Even if you do have bison, you might get errors about not being able to load -ly. In such a case, run the bison setup script.
• Run ./contrib/setup-smt-switch.sh -- it will build smt-switch with boolector
  • [optional] to build with MathSAT (required for interpolant-based model checking) you need to obtain the libraries yourself
    • note that MathSAT is under a custom non-BSD compliant license and you must assume all responsibility for meeting the conditions
    • download the solver from https://mathsat.fbk.eu/download.html, unpack it and rename the directory to ./deps/mathsat
    • then add the --with-msat flag to the setup-smt-switch.sh command.
• Run ./contrib/setup-btor2tools.sh.
• Run ./configure.sh.
  • if building with mathsat, also include --with-msat as an option to configure.sh
• Run cd build.
• Run make.

Dependencies

• Please see the README of smt-switch for required dependencies.
• Note to Arch Linux users: building Pono will fail if the static library of GMP, which is required by cvc5, is not installed on your system. You can fix it by installing libgmp-static from AUR.

Profiling

We link against the gperftools library to generate profiling data. To enable profiling, run ./configure.sh with flag --with-profiling and recompile Pono by running make in the build directory. This assumes that you have installed the gperftools library before, e.g., on Ubuntu, run sudo apt-get install google-perftools libgoogle-perftools-dev.

To profile, run ./pono --profiling-log=<log-file> ... where <log-file> is the path to a file where profiling data is written. After normal or abnormal (e.g. via sending a signal) termination of Pono, the collected profile data can be analyzed by running, e.g., google-pprof --text ./pono <log-file> to produce a textual profile. See man google-pprof for further options.

In general, see https://github.com/gperftools/gperftools/tree/master/docs for further information about gperftools.

gperftools is licensed under a BSD 3-clause license, see https://github.com/gperftools/gperftools/blob/master/COPYING.

Existing code

Transition Systems

There are two Transition System interfaces:

• FunctionalTransitionSystem in fts.*
• TransitionSystem in rts.*

Smt-Switch

Smt-switch is a C++ solver-agnostic API for SMT solvers. The main thing to remember is that everything is a pointer. Objects might be "typedef-ed" with using statements, but they're still shared_ptrs. Thus, when using a solver or a term, you need to use -> accesses.

For more information, see the example usage in the smt-switch tests. Other useful files to visit include:

• smt-switch/include/solver.h: this is the main interface you will be using
• smt-switch/include/ops.h: this contains all the ops you might need
  • Note: create indexed ops like Op(Extract, 7, 4)

Python bindings

To build the pono python bindings, first make sure that you have Cython version >= 0.29 installed. Then ensure that smt-switch and its python bindings are installed. Finally, you can configure with ./configure.sh --python and then build normally. The sequence of commands would be as follows:

# Optional recommended step: start a python virtualenv
# If you install in the virtualenv, you will need to activate it each time before using pono
# and deactivate the virtualenv with: deactivate
python3 -m venv env
source ./env/bin/activate
pip install Cython==0.29 pytest
./contrib/setup-smt-switch.sh --python
./contrib/setup-btor2tools.sh
pip install -e ./deps/smt-switch/build/python
./configure.sh --python
cd build
make -j4
pip install -e ./python
cd ../
# Test the bindings
pytest ./tests

Generating BTOR2 from Verilog

The best tool for creating BTOR2 from Verilog is Yosys. Yosys has an excellent manual here. You can also run yosys interactively by running yosys with no arguments. Then you can view help messages for each command with: help <command>. Running help with no arguments lists all commands.

A particularly useful command if you're having trouble is show, which can show the current state of the circuit in Yosys.

Yosys Quick Start

Below is an example file with comments explaining each command that produces a BTOR2 file for ./samples/counter-false.v. This should be enough for most use cases.

Once you have yosys installed, copy the text below into gen-btor.ys in the top-level of this repository. Then, running yosys -s gen-btor.ys will produce the BTOR2 file.

# read in the file(s) -- there can be multiple
# whitespace separated files, and you can
# escape new lines if necessary
read -formal ./samples/counter-false.v;

# prep does a conservative elaboration
# of the top module provided
prep -top counter;

# this command just does a sanity check
# of the hierarchy
hierarchy -check;

# If an assumption is flopped, you might
# see strange behavior at the last state
# (because the clock hasn't toggled)
# this command ensures that assumptions
# hold at every state
chformal -assume -early;

# this processes memories
# nomap means it will keep them as arrays
memory -nomap;

# flatten the design hierarchy
flatten;

# (optional) uncomment and set values to simulate reset signal
# use -resetn for an active low pin
# -n configures the number of cycles to simulate
# -rstlen configures how long the reset is active (recommended to keep it active for the whole simulation)
# -w tells it to write back the final state of the simulation as the initial state in the btor2 file
# another useful option is -zinit which zero initializes any uninitialized state
# sim -clock <clockpin> -reset <resetpin> -n <number of cycles> -rstlen <number of cycles> -w <top_module>

# (optional) use an "explicit" clock
# e.g. every state is a half cycle of the
# fastest clock
# use this option if you see errors that
# refer to "adff" or asynchronous components
# IMPORTANT NOTE: the clocks are not
# automatically toggled if you use this option
# clk2fflogic;

# This turns all undriven signals into
# inputs
setundef -undriven -expose;

# This writes to a file in BTOR2 format
write_btor counter-false.btor2