Minimizing LLVM IR Reproducers and Manual Investigation

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) ret double %4 } ``` Here, `_zn2ad3_f217h3b3b1800bd39fde3e` is the correct name. make sure to not copy the leading `@`. redo step 2) by running the `opt` command again, but this time passing `mwe.ll` as the input file instead of `out.ll`. Check if this minimized example still reproduces the crash. ## 4) (Optional) Minimize your llvm-ir reproducer further. After the previous step you should have an `mwe.ll` file with ~5k loc. let's try to get it down to 50. find your `llvm-reduce` binary next to `opt` and `llvm-extract`. Copy the first line of your error message, an example could be: ```sh opt: /home/manuel/prog/rust/src/llvm-project/llvm/lib/ir/instructions.cpp:686: void llvm::callinst::init(llvm::functiontype*, llvm::value*, llvm::arrayref<llvm::value*>, llvm::arrayref<llvm::operandbundledeft<llvm::value*> >, const llvm::twine&): assertion `(args.size() == fty->getnumparams() || (fty->isvararg() && args.size() > fty->getnumparams())) && "calling a function with bad signature!"' failed. ``` If you just get a `segfault` there is no sensible error message and not much to do automatically, so continue to 5). otherwise, create a `script.sh` file containing ```sh #!/bin/bash <path/to/your/opt> $1 -load-pass-plugin=/path/to/llvmenzyme-19.so -passes="enzyme" \ |& grep "/some/path.cpp:686: void llvm::callinst::init" ``` Experiment a bit with which error message you pass to grep. it should be long enough to make sure that the error is unique. However, for longer errors including `(` or `)` you will need to escape them correctly which can become annoying. Run ```sh <path/to/llvm-reduce> --test=script.sh mwe.ll ``` If you see `input isn't interesting! verify interesting-ness test`, you got the error message in script.sh wrong, you need to make sure that grep matches your actual error. If all works out, you will see a lot of iterations, ending with a new `reduced.ll` file. Verify with `opt` that you still get the same error. ### Advanced debugging: manual llvm-ir investigation Once you have a minimized reproducer (`mwe.ll` or `reduced.ll`), you can delve deeper:

This section describes how to further minimize LLVM IR code after generating a minimal working example (mwe.ll) using `llvm-extract`. It explains how to use `llvm-reduce` with a script to automatically reduce the code while ensuring it still reproduces the original error. It also provides guidance on manually investigating the minimized LLVM IR code for advanced debugging, focusing on identifying the mangled function name and verifying the minimized example with `opt`.