Couldn't load codegen backend, specified module could not be found

[kafji]

Hi,

I'm trying to build the example described in the getting started guide on Windows but I got an error:

> cargo build
   Compiling rustc_codegen_nvvm v0.3.0 (https://github.com/rust-gpu/rust-cuda?rev=7fa76f3d717038a92c90bf4a482b0b8dd3259344#7fa76f3d)
   Compiling cuda_builder v0.3.0 (https://github.com/rust-gpu/rust-cuda?rev=7fa76f3d717038a92c90bf4a482b0b8dd3259344#7fa76f3d)
   Compiling rust-cuda-basic v0.1.0 (D:\workspace\rust-cuda-basic)
error: failed to run custom build command for `rust-cuda-basic v0.1.0 (D:\workspace\rust-cuda-basic)`
note: To improve backtraces for build dependencies, set the CARGO_PROFILE_DEV_BUILD_OVERRIDE_DEBUG=true environment variable to enable debug information generation.

Caused by:
  process didn't exit successfully: `D:\workspace\rust-cuda-basic\target\debug\build\rust-cuda-basic-11d5ae9c94c06876\build-script-build` (exit code: 101)
  --- stdout
  cargo::rerun-if-changed=build.rs
  cargo::rerun-if-changed=kernels
  cargo:rerun-if-changed=D:\workspace\rust-cuda-basic\kernels

  --- stderr
  error: failed to run `rustc` to learn about target-specific information

  Caused by:
    process didn't exit successfully: `C:\Users\Carrot\.rustup\toolchains\nightly-2025-08-04-x86_64-pc-windows-msvc\bin\rustc.exe - --crate-name ___ --print=file-names -Zcodegen-backend=D:\workspace\rust-cuda-basic\target\debug\rustc_codegen_nvvm.dll -Zcrate-attr=feature(register_tool) -Zcrate-attr=register_tool(nvvm_internal) -Zcrate-attr=no_std -Zsaturating_float_casts=false "-Cllvm-args=-arch=compute_75 --override-libm" --target nvptx64-nvidia-cuda --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=split-debuginfo --print=crate-name --print=cfg -Wwarnings` (exit code: 1)
    --- stderr
    error: couldn't load codegen backend \\?\D:\workspace\rust-cuda-basic\target\debug\rustc_codegen_nvvm.dll: LoadLibraryExW failed: The specified module could not be found. (os error 126) (The specified module could not be found. (os error 126)) (retried 5 times)


  thread 'main' panicked at build.rs:17:10:
  called `Result::unwrap()` on an `Err` value: BuildFailed
  stack backtrace:
     0: std::panicking::begin_panic_handler
               at /rustc/f34ba774c78ea32b7c40598b8ad23e75cdac42a6/library\std\src\panicking.rs:697
     1: core::panicking::panic_fmt
               at /rustc/f34ba774c78ea32b7c40598b8ad23e75cdac42a6/library\core\src\panicking.rs:75
     2: core::result::unwrap_failed
               at /rustc/f34ba774c78ea32b7c40598b8ad23e75cdac42a6/library\core\src\result.rs:1852
     3: build_script_build::main
     4: core::ops::function::FnOnce::call_once
  note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

But the codegen backend path does exist:

> ls D:\workspace\rust-cuda-basic\target\debug\rustc_codegen_nvvm.dll

    Directory: D:\workspace\rust-cuda-basic\target\debug

Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
-a---          30/11/2025  9:39 am       31755776 rustc_codegen_nvvm.dll

Any idea on what the actual problem is?

I'm using the prebuilt LLVM and CUDA 13.

Repro can be found here.

[nnethercote]

This looks very similar to #220. Can you try modifying PATH as was done in that issue?

[kafji]

Adding nvvm64_40_0.dll location to the path env var does fix the problem.

Mine is in C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v13.0\nvvm\bin\x64.

Thank you for the help!

[nnethercote]

Great, I will update the Getting Started Guide to include this information.

A semi-related question: is there a file called libdevice.10.bc, or similar, in your CUDA installation? (It might have number other than 10 in the name.) If so, can you tell me its path? I don't have easy access to a Windows machine and it would help me with something else I'm investigating. Thanks!

[kafji]

There is libdevice.10.bc. It's in C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v13.0\nvvm\libdevice. It's the only file in that directory and there are no other libdevice files in C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v13.0.

[nnethercote]

Thank you!

[kafji]

Happy to help!