# Reporting backend crashes

If after a compilation failure you are greeted by a large amount of llvm-ir code, then our enzyme backend likely failed to compile your code. These cases are harder to debug, so your help is highly appreciated. Please also keep in mind that release builds are usually much more likely to work at the moment.

The final goal here is to reproduce your bug in the enzyme [compiler explorer](https://enzyme.mit.edu/explorer/), in order to create a bug report in the [Enzyme](https://github.com/enzymead/enzyme/issues) repository.

We have an `autodiff` flag which you can pass to `rustflags` to help with this. it will print the whole llvm-ir module, along with some `__enzyme_fwddiff` or `__enzyme_autodiff` calls. A potential workflow on linux could look like:  

## Controlling llvm-ir generation

Before generating the llvm-ir, keep in mind two techniques that can help ensure the relevant rust code is visible for debugging:

- **`std::hint::black_box`**: wrap rust variables or expressions in `std::hint::black_box()` to prevent rust and llvm from optimizing them away. This is useful when you need to inspect or manually manipulate specific values in the llvm-ir.
- **`extern "rust"` or `extern "c"`**: if you want to see how a specific function declaration is lowered to llvm-ir, you can declare it as `extern "rust"` or `extern "c"`. You can also look for existing `__enzyme_autodiff` or similar declarations within the generated module for examples.

## 1) Generate an llvm-ir reproducer

```sh
rustflags="-z autodiff=enable,printmodbefore" cargo +enzyme build --release &> out.ll 
```

This also captures a few warnings and info messages above and below your module. open out.ll and remove every line above `; moduleid = <somehash>`. Now look at the end of the file and remove everything that's not part of llvm-ir, i.e. remove errors and warnings. The last line of your llvm-ir should now start with `!<somenumber> = `, i.e. `!40831 = !{i32 0, i32 1037508, i32 1037538, i32 1037559}` or `!43760 = !dilocation(line: 297, column: 5, scope: !43746)`.

The actual numbers will depend on your code.  

## 2) Check your llvm-ir reproducer

To confirm that your previous step worked, we will use llvm's `opt` tool. find your path to the opt binary, with a path similar to `<some_dir>/rust/build/<x86/arm/...-target-tripple>/build/bin/opt`. also find `llvmenzyme-19.<so/dll/dylib>` path, similar to `/rust/build/target-tripple/enzyme/build/enzyme/llvmenzyme-19`. Please keep in mind that llvm frequently updates it's llvm backend, so the version number might be higher (20, 21, ...). Once you have both, run the following command:

```sh
<path/to/opt> out.ll -load-pass-plugin=/path/to/llvmenzyme-19.so -passes="enzyme" -s
```

If the previous step succeeded, you are going to see the same error that you saw when compiling your rust code with cargo. 

If you fail to get the same error, please open an issue in the rust repository. If you succeed, congrats! the file is still huge, so let's automatically minimize it.

## 3) Minimize your llvm-ir reproducer

First find your `llvm-extract` binary, it's in the same folder as your opt binary. then run:

```sh
<path/to/llvm-extract> -s --func=<name> --recursive --rfunc="enzyme_autodiff*" --rfunc="enzyme_fwddiff*" --rfunc=<fnc_called_by_enzyme> out.ll -o mwe.ll 
```

This command creates `mwe.ll`, a minimal working example.

Please adjust the name passed with the last `--func` flag. You can either apply the `#[no_mangle]` attribute to the function you differentiate, then you can replace it with the rust name. otherwise you will need to look up the mangled function name. To do that, open `out.ll` and search for `__enzyme_fwddiff` or `__enzyme_autodiff`. the first string in that function call is the name of your function. example:

```llvm-ir 
define double @enzyme_opt_helper_0(ptr %0, i64 %1, double %2) {
  %4 = call double (...) @__enzyme_fwddiff(ptr @_zn2ad3_f217h3b3b1800bd39fde3e, metadata !"enzyme_const", ptr %0, metadata !"enzyme_const", i64 %1, metadata !"enzyme_dup", double %2, double %2)