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README.md

C-Reduce for Julia

Helper scripts to reduce Julia test cases with C-Reduce

Requirements

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

Usage

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. If you don't need any packages, make sure you don't have a depot folder before starting the reduction.

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.

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.

Notes

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.

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

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 tp grep on 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 $?
0

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 CUDAnative.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
end
similar(:, d) =
    a()

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

module GPUCompiler

include("ptx.jl")
include("driver.jl")
include("reflection.jl")
end

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

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

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)
    begin
        b = Dict()
        if for c in d
                e = h(f, g)
                get(b, e) do
                 end
            end
        end
    end
end
Base.@kwdef struct a
    b::Union{Nothing,Int}  = c
end
h(:, d) = a()
function i(a) codegen(b, a) end
i(0)

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

struct a
    b::Union{Nothing,Int}
end

function main()
    c = Dict()
    d = a(undefined)
    c[d]
end

main()

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