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C-Reduce for Julia

This repository contains a set of scripts to reduce Julia test cases with C-Reduce. It can be helpful if you want to minimize code that causes a crash or bug, without having to iteratively edit a script and manually verify the bug still occurs.

Note that testcase reduction works best if the testcase can be evaluated quickly (i.e., seconds rather than minutes). If your testcase takes a long time to run, you may want to do some manual editing first to get rid of dependencies that take a long time to load.


  • C-Reduce 2.8.0+, on PATH
  • Julia 1.0+, on PATH


Start by putting your test case in main.jl. If your script needs any packages, activate a Julia instance in the C-Reduce environment by executing the julia wrapper script from the repository's root directory:

$ cd /path/to/checkout
$ ./julia
] add ...
] dev ...

Packages that you add will be ignored by the reduction process; only packages you dev will take part in it.

Next, modify the run script to properly catch the error you are dealing with and return 0 if the reduced file is good. Often, you want to look for specific output in the standard error stream. If you need to use any Julia flags, or want to use a specific build, edit the julia wrapper script accordingly. You may also want to edit the reduce script to adjust the timeout, which defaults to 1 minute.

Optionally, preprocess the source to get rid of irrelevant source code by running the tools/preprocess.jl script. It is recommended to remove other irrelevant sources as well, such as tests, examples, or documentation. Always verify that the run script still returns 0 afterwards.

Finally, execute the reduce script. This should finalize the environment and start C-Reduce.


Let's start with a main.jl that prints Hello, world!:

println("Hello, World!")
println("1 + 1 = ", 1 + 1)

We can verify this executes correctly by running the julia wrapper script:

❯ ./julia main.jl
Hello, World!
1 + 1 = 2

Say we are only interested in the Hello, World!, so we adapt the run script as follows (keeping the surrounding boilerplate, which among other things ensures the process exited successfully):

$DIR/julia main.jl |& grep "example error message"

Let's verify this works as expected by executing the run script directly:

❯ ./run
Hello, World!
❯ echo $?

Notice how the exit code is 0, as expected. We're now ready to start the reduction:

❯ ./reduce
Reducing the following sources:
- main.jl
===================== done ====================
println("Hello, World!")

As expected, all lines except the one printing Hello, World! got removed.


Test case reduction happens in parallel, so make sure there's no global effects.

When you're reducing a large project, you often will need to do some manual editing to help the process. In that case, it can be useful to stage (git add) the depot/dev directory to keep track of changes by C-Reduce.

If you want to reduce a test case that involves a lot of packages, ensure you run the test case beforehand as to precompile all packages involved. The scripts in this repository use a layered depot, making it possible to load these precompilation images during the reduction.

Realistic example

After a refactor, CUDAnative's tests triggered an LLVM assertion when running with julia-debug. To reduce this, I started by installing the necessary packages:

creduce$ ./julia

] dev GPUCompiler
] dev CUDAnative
] add CUDAdrv
# the tests rely on CUDAdrv, but I'm certain the error isn't there

As the situation depends on running with julia-debug, I edited the julia script to use julia-debug and copied the failing test into main.jl:

creduce$ ./julia main.jl
PHI node entries do not match predecessors!
julia-debug: /home/tbesard/Julia/julia/src/codegen.cpp:1511: void jl_generate_fptr(jl_code_instance_t*): Assertion `specptr != NULL' failed.

To trap exactly this error condition, I edit the run script to look for this error:

$DIR/julia main.jl |& grep "PHI node entries do not match predecessors!"

To speed up the search, I ran the preprocess script to get rid of comments, whitespace, and documentation, and removed sources that are not used during execution:

creduce/tools$ julia --project
] instantiate

creduce/tools$ julia --project preprocess.jl

creduce$ rm -rf depot/dev/*/{test,examples,docs,res}

As a final check, I ran the run script (as it will be run by C-Reduce) and inspected the exit code (which initially should be 0):

creduce$ ./run

creduce$ echo $?

Kicking off the process, I staged all files with git (so that I can keep track of C-Reduce's progress) and launched the reduce script:

creduce$ git add main.jl depot

creduce$ ./reduce

After a day or two on a system with 32 cores / 64 threads (debug builds are slow, and this failure involved a fair number of packages and around 10.000 lines of code to reduce), all of GPUCompiler.jl and CUDA.jl got reduced to the following:

using CUDAnative
# /home/tbesard/Julia/tools/creduce/src/50.jl

function code_llvm(a) codegen(b, a) end
# /home/tbesard/Julia/tools/creduce/src/38.jl

module a

    using GPUCompiler

        include("reflection.jl") end

# /home/tbesard/Julia/tools/creduce/src/8.jl

for a in (:code_llvm, )     @eval GPUCompiler.$a(0 )         end
# /home/tbesard/Julia/tools/creduce/src/42.jl

Base.@kwdef struct a
    b::Union{Nothing,Int}  = c
similar(:, d) =

# /home/tbesard/Julia/tools/creduce/src/52.jl

module GPUCompiler


# /home/tbesard/Julia/tools/creduce/src/48.jl

function codegen(::Symbol, a)
        b = Dict()
        if for c in d
                e = similar(f, g)
                get(b, e) do

To reduce this even further, I then inlined the reduced code into main.jl, preserving modules and functions but removing the file structure. The error still reproduced, so this bug isn't sensitive to the way modules are compiled and loaded. I then removed the depot and kicked off another reduction that resulted with the following:

function codegen(::Symbol, a)
        b = Dict()
        if for c in d
                e = h(f, g)
                get(b, e) do
Base.@kwdef struct a
    b::Union{Nothing,Int}  = c
h(:, d) = a()
function i(a) codegen(b, a) end

After some manual clean-up, I ended up with the final reproducer:

struct a

function main()
    c = Dict()
    d = a(undefined)



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