Souper is a superoptimizer for LLVM IR. It uses an SMT solver to help identify missing peephole optimizations in LLVM's midend optimizers.

Requirements

Souper should work on any reasonably modern Linux or OS X machine.

You will need a reasonably modern compiler toolchain. LLVM has instructions on how to get one for Linux: http://llvm.org/docs/GettingStarted.html#getting-a-modern-host-c-toolchain

You will also need CMake to build Souper and its dependencies.

To run Souper over a bitcode file, you will need an SMT solver. Souper can use Z3, Boolector, CVC4, or STP. We recommend Z3.

If you have Go installed, you will also need the Redigo Redis client:

$ go get github.com/gomodule/redigo/redis

Building Souper

1. Download and build dependencies:

$ ./build_deps.sh $buildtype $extra_cmake_flags

$buildtype is optional; it defaults to Release and may be set to any LLVM build type. $extra_cmake_flags is optional. It is passed to CMake.

2. Run CMake from a build directory:

$ mkdir /path/to/souper-build
$ cd /path/to/souper-build
$ cmake -DCMAKE_BUILD_TYPE=$buildtype /path/to/souper

Again, the build type is optional and defaults to Release. In any case it must match the build type used when compiling the dependencies. If you want to run Souper's full test suite, add this option to CMake (with the appropriate solver and path to the solver executable): -DTEST_SOLVER="-z3-path=/usr/bin/z3"

3. Run 'make' from the build directory.

4. Optionally run 'make check' to run Souper's test suite. To run the test suite under Valgrind, run 'make check LIT_ARGS="-v --vg --vg-leak"' instead. By default the solver is also run under Valgrind. This can be disabled by by adding --vg-arg=--trace-children-skip=/path/to/solver to LIT_ARGS.

Note that GCC 4.8 and earlier have a bug in handling multiline string literals. You should build Souper using GCC 4.9+ or Clang.

Using Souper

After following the above instructions, you will have a Souper executable in /path/to/souper-build/souper and a Clang executable in /path/to/souper/third_party/llvm/$buildtype/bin/clang. You can use the Clang executable to create an LLVM bitcode file like this:

$ /path/to/clang -emit-llvm -c -o /path/to/file.bc /path/to/file.c

Once you have a bitcode file you can invoke Souper with the path to the bitcode file as a command line argument. You will also need one of these flags to tell Souper where the SMT solver executable is:

-z3-path=<path>              - Path to Z3 executable
-boolector-path=<path>       - Path to Boolector executable
-cvc4-path=<path>            - Path to CVC4 executable
-stp-path=<path>             - Path to STP executable

For example:

$ /path/to/souper -z3-path=/usr/bin/z3 /path/to/file.bc

Souper will extract SMT queries from the bitcode file and pass them to a solver. Unsatisfiable queries (which represent missed optimization opportunities) will cause Souper to print its internal representation of the optimizable expression along with the shorter expression that refines the original one.

Alternatively, you may immediately let Souper modify the bitcode and let it apply the missed optimization opportunities by using the Souper llvm opt pass. When loaded the pass will automatically register itself to run after LLVM's regular peephole optimizations.

For example:

$ /path/to/clang -Xclang -load -Xclang /path/to/libsouperPass.so \
                 -mllvm -z3-path=/usr/bin/z3 /path/to/file.c

Or to run the pass on its own:

$ /path/to/opt -load /path/to/libsouperPass.so -souper \
               -z3-path=/usr/bin/z3 -o /path/to/file.opt.bc \
               /path/to/file.bc

Or use the drop-in compiler replacements sclang and sclang++:

$ export SOUPER_SOLVER=-z3-path=/usr/bin/z3
$ /path/to/configure CC=/path/to/sclang CXX=/path/to/sclang++
$ make

Compilation using Souper can be sped up by caching queries. By default, Souper uses a non-persistent RAM-based cache. The -souper-external-cache flag causes Souper to cache its queries in a Redis database. For this to work, Redis >= 1.2.0 must be installed on the machine where you are running Souper and a Redis server must be listening on the default port (6379).

sclang uses external caching by default since this often gives a substantial speedup for large compilations. This behavior may be disabled by setting the SOUPER_NO_EXTERNAL_CACHE environment variable. Souper's Redis cache does not yet have any support for versioning; you should stop Redis and delete its dump file any time Souper is upgraded.

Disclaimer

Please note that although some of the authors are employed by Google, this is not an official Google product.