Reify is a random program generator based on semantic reification. It generates C functions and programs free of undefined behaviors (UBs), making it suitable for testing C compilers and, potentially, other language virtual machines. Using Reify, 59 bugs have been discovered and reported in GCC and LLVM.

Currently, Reify supports only i32, i32 arrays, and i32 structs. We are extending it to support additional primitive and aggregate types. We are also experimenting with generating Java bytecode and eBPF bytecode. Even in their early stages, these experimental attempts have already revealed one JIT compiler bug in OpenJ9 and two bugs in Linux's eBPF runtime.

Additionally, we are experiment with a full-fledged SymIR called symlang, based on which Reify has a re-implementation with richer feature support such as pointers, vectors, and intrinsics.

Prerequisites:

Start fuzzing C compilers:

make -j8 all
python scripts/fuzz.py -j 8 'gcc -O3 -fno-tree-slsr -fno-tree-ch'

Reify is capable of generating leaf functions that does not call other functions and whole programs composed of multiple functions.

Use the following script to generate 512 leaf functions:

python scripts/rysmith.py --output generated --limit 512

The script rotates through the recommended function-generation configurations (number of basic blocks per function, etc.) so that the 512 functions expose various shapes.

The output directory (given by the --output option) includes multiple sub-directories, each for an individual function:

• func_<uuid>_<sno>: Contains all artifacts for an individual leaf function, including:
  • func.c: Generated function in C (not compilable alone).
  • inout.jsonl: Input-output mappings ensuring UB-free execution.
  • func.sexp: S-expression of the generated function.
  • main.c: A driver program to test the function (compilable with func.c).
  • func.log: Generation logs.

Or use the following command to generate an individual leaf function:

timeout 3s ./build/bin/rysmith -A -U -m -S --output generated --Xbitwuzla-threads 4 --sno 0 $(uuidgen)

Note that generating leaf functions may (1) take a long time due to complicated constraints or (2) fail due to unsatisfiable constraints. This is why the above command is prefixed with timeout 3s.

Experimental: Leaf functions can be generated from existing control flow graphs (CFGs). This feature is experimental.

First install Clang:

Then extract 512 CFGs from other programs such as those generated by Csmith or YARPGen:

python scripts/ggen.py -l /path/to/clang -L 512 -g csmith --csmith /path/to/csmith /path/to/csmith_db.jsonl

Finally integrate the generated CFG database into leaf function generation:

python scripts/rysmith.py ... --extra '--unstable-graphdb /path/to/csmith_db.jsonl' ...

Use the following script to generate 512 whole programs from a set of previously generated leaf functions (in particular, their S expressions, given by the --input option):

python scripts/rylink.py --input generated --limit 512

The script rotates through the recommended program-generation configurations (number of functions per program) so that the 512 programs span a range of sizes.

The generated programs are placed alongside the input functions in --input. Each program has its own directory:

• prog_<uuid>_<sno>: Contains all artifacts for a whole program, including:
  • main.c: The entry point.
  • chksum.c: Checksum utilities.
  • proto.h: Prototypes of used leaf functions.
  • func_*.c: Individual function files.

Or use the following command to generate programs in a single invocation:

./build/bin/rylink --input generated --limit 512 --Xfunction-depth 10 $(uuidgen)

GCC and Clang/LLVM can be tested with one command with scripts/fuzz.py. Start fuzzing with:

python scripts/fuzz.py -o fuzzdir -j 10 -s 0 'gcc -O3 -fno-tree-slsr -fno-tree-ch'

• -o fuzzdir: Output directory
• -j 10: Run 10 jobs in parallel
• -s 0: Seed for random number generator

Use the following command to fuzz a Java virtual machine:

./scripts/fuzz_jvm.sh --nproc 8 --java-home /path/to/java/home

This process adapted leaf function generation to generate Java bytecode. An individual Java class can also be generated via --unstable-javaclass into build/bin/rysmith:

timeout 3s ./build/bin/rysmith ... --unstable-javaclass ...

The generated Java class is placed inside the javaclasses subdirectory.

Navigate to bugs.

If you found this work helpful, please consider citing our work:

@inproceedings{reify,
  title={Semantic Reification: A New Paradigm for Random Program Generation},
  author={Kavya Chopra and Cong Li and Thodoris Sotiropoulos and Zhendong Su},
  year={2026},
  booktitle={Proceedings of the 2026 ACM SIGPLAN Conference on Programming Language Design and Implementation},
  series={PLDI '26},
  doi={10.1145/3808268},
}

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Kavya Chopra ([email protected])
Cong Li ([email protected])

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