mir_llvm/get_declared_value: fix null pointer dereference

[tskaar]

This fixes a panic on Arch Linux when I ran the integration tests (seen when just running cargo test).

Running RUST_BACKTRACE=1 cargo test -p openvaf --test integration gave me the BT:

0: __rustc::rust_begin_unwind
1: core::panicking::panic_nounwind_fmt
2: core::panicking::panic_null_pointer_dereference
3: mir_llvm::declarations::<impl mir_llvm::context::CodegenCx>::get_declared_value
         at /home/torleif/git/AUR/openvaf-reloaded/src/openvaf-reloaded/openvaf/mir_llvm/src/declarations.rs:142:18
4: mir_llvm::declarations::<impl mir_llvm::context::CodegenCx>::get_defined_value
         at /home/torleif/git/AUR/openvaf-reloaded/src/openvaf-reloaded/openvaf/mir_llvm/src/declarations.rs:152:9
5: mir_llvm::declarations::<impl mir_llvm::context::CodegenCx>::define_global
         at /home/torleif/git/AUR/openvaf-reloaded/src/openvaf-reloaded/openvaf/mir_llvm/src/declarations.rs:108:12
....

With the PR it works as expected.

[gemini-code-assist]

Summary of Changes

Hello @tskaar, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical runtime panic that occurred during integration tests, specifically on Arch Linux. The issue stemmed from an unsafe dereference of a potentially null pointer returned by an LLVM API function. The change introduces a robust null-safety check when retrieving declared values, significantly improving the stability and reliability of the LLVM codegen component.

Highlights

• Null Pointer Dereference Fix: The get_declared_value function in mir_llvm/src/declarations.rs was updated to correctly handle the return value of LLVMGetNamedGlobal. Previously, it directly dereferenced the raw pointer returned by the LLVM API call, which could be null if the named global was not found, leading to a panic. The change now uses NonNull::new to safely convert the raw pointer into an Option<&'ll Value>, ensuring that dereferencing only occurs when a valid pointer is present.

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[gemini-code-assist]

Code Review

This pull request correctly fixes a critical null pointer dereference that occurred when looking up a global LLVM value that might not exist. The change is sound and effectively resolves the panic. I've added one suggestion to make the code slightly more concise and idiomatic by using a standard library helper for handling nullable raw pointers.

[Kreijstal]

Thank you a lot for the PR, I am a bit short on time but I promise I'll work on this

[tskaar]

No problems!
I can see that when I re-run the test-suite locally I sometimes get failing tests as well, so it seems like there is some non-determinism going on there 🤔

[Kreijstal]

@tskaar I believe the latest mob branch does not have this problem anymore.

[tskaar]

@Kreijstal: Tested the branch, seems to be working as expected (i.e. no panic on test), thanks!

In terms of the determinism (failing tests), I can still get it to randomly fail by re-running multiple times, but I think that's a completely separate issue