Julia Segfault In Juliac: Const Statement Evaluation

Aug 5, 2025 by Mei Lin 53 views

Resolving Julia Segfaults During `const` Statement Evaluation

Hey guys! Let's dive into a tricky issue encountered while using juliac to trim Julia code. We're talking about segfaults happening during the evaluation of const statements, specifically when these statements are outside a package definition. It's a bit of a rabbit hole, but let's break it down and see what's going on.

Understanding the Issue

So, the core problem is this: juliac is crashing when it hits a somewhat complex const = ... statement that isn't part of a package. Imagine you're building a cool app, and you've got some constants defined to make things run smoothly. But when you try to trim your code using juliac, boom! Segfault. This only happens when the code is in a module or simply included, not when it's part of a package definition. Sounds weird, right? Let's dig deeper.

The Scenario

The issue was first spotted while working on a --trim CI test for NonlinearSolve.jl. The code in question looks something like this:

const autodiff = AutoForwardDiff(; chunksize = 1)
const alg = TrustRegion(; autodiff, linsolve = LS.CholeskyFactorization())
const prob = NonlinearLeastSquaresProblem{false}(...)
const cache = init(prob, alg)

function minimize(x)
    reinit!(cache, x)
    solve!(cache)
    return cache.u
end

Here, we're defining a few constants: autodiff, alg, prob, and cache. The cache constant is initialized using the init function, which takes prob and alg as arguments. The minimize function then reinitializes and solves the cache. All straightforward, you might think. And you'd be right, except when juliac gets involved.

The Segfault Trigger

Now, here's the kicker. This code trims perfectly fine when it's inside a package definition. But if you wrap it in a module or simply include it, the cache = init(prob, alg) line causes juliac to segfault. It's like juliac is saying, "Hey, I can handle this in a package, but outside? Nope, I'm out!" This inconsistency is what makes it a particularly nasty bug.

For example, this code:

include("optimization_trimmable.jl")
function (@main)(argv::Vector{String})::Cint
    minimize(1.0)
    return 0
end

Or even this:

mod MyMod
    include("optimization_trimmable.jl")
end
function (@main)(argv::Vector{String})::Cint
    MyMod.minimize(1.0)
    return 0
end

...will cause juliac to crash. It's a bit like a temperamental artist; it works under specific conditions but throws a tantrum otherwise.

Expected Behavior

Ideally, juliac should trim this code without any issues, just like it does when the code is part of a package. The expected behavior is that it processes the code, optimizes it, and spits out a trimmed executable. No segfaults, no drama. But alas, that's not what's happening here.

Reproducing the Issue: A Step-by-Step Guide

Okay, let's get our hands dirty and reproduce this bug. Here's how you can see it in action:

Clone the Repository:

First, you'll need to clone the NonlinearSolve.jl repository. This is where the bug was initially discovered.
```
git clone [email protected]:RomeoV/NonlinearSolve.jl.git
```
If you are using https, you can use the following command
```
git clone https://github.com/RomeoV/NonlinearSolve.jl.git
```
This command downloads the repository to your local machine.
Navigate to the Directory:

Next, move into the NonlinearSolve.jl directory and check out the specific commit where the issue was identified.
```
cd NonlinearSolve.jl
git checkout ca0d0f2a8cf752a68c48ab88cf4c850bb1acbc49
cd test/trim
```
This ensures you're working with the exact code version that triggers the bug. This step is crucial because software bugs often depend on specific versions of the code and its dependencies. By checking out the exact commit, you create the correct environment to reproduce the issue consistently. This consistency is invaluable for debugging and confirming that the fix works correctly.
Set the JULIAC Environment Variable:

Now, you need to set the JULIAC environment variable. This tells the system where to find the juliac executable. The command below figures out the path to juliac using Julia itself and then sets the environment variable.
```
export JULIAC=$(julia +1.12 -E 'joinpath(Sys.BINDIR, Base.DATAROOTDIR, 
```