Matching on enums with optimizations enabled causes rustc-LLVM errors on the nvptx64-nvidia-cuda target

[FractalFir]

This code:

#![no_std]
pub enum Bug {
    A,
    B,
    C,
}
pub fn debug(t: &Bug) {
    core::hint::black_box(match t {
        Bug::A => "A",
        Bug::B => "B",
        Bug::C => "C",
    });
}

Compiles just fine for the nvptx64 target... if optimizations are disabled.

--target=nvptx64-nvidia-cuda -Copt-level=0

However, as soon as optimizations are turned on, this code raises rustc-LLVM errors.

--target=nvptx64-nvidia-cuda -Copt-level=1

rustc-LLVM ERROR: Circular dependency found in global variable set
Compiler returned: 101

Meta

This bug can be triggered on the newest nightly, and stable versions back to 1.87.

My reproducer does not trigger the issue in 1.86, but I will have to check if the larger codebase I extracted this from works there.

[FractalFir]

I have confoirmed this has been an issue since at least nightly-2025-06-23(reproducer does not trigger the issue, but the original codebase does) - will have to check further back. There is a chance this never worked.

[FractalFir]

Issue seems to be caused by LLVM using relative reallocations here:
(IR simplifed for readiblity, names changed).

@table = private unnamed_addr constant [3 x i32] [
i32 trunc (i64 sub (i64 ptrtoint (ptr @alloc0 to i64), i64 ptrtoint (ptr @table to i64)) to i32), 
i32 trunc (i64 sub (i64 ptrtoint (ptr @alloc1 to i64), i64 ptrtoint (ptr @table to i64)) to i32), 
i32 trunc (i64 sub (i64 ptrtoint (ptr @alloc2 to i64), i64 ptrtoint (ptr @table to i64)) to i32)], align 4

Since @table uses relative pointers here, the difference between the symbol table & alloc0 has to be computed.

PTX does not allow for self-referential globals, I belive - which is the cause of the issue.

[nikic]

llvm/llvm-project#159748