Enzyme support example

[nkoukpaizan]

This builds on earlier work from Asher and Slaven to add examples using the Enzyme automatic differentiation library.
There are two new tests compared to develop that should pass.

Building Enzyme from source is discussed in #23. I tested that approach as well as building LLVM and Enzyme through Spack, and both approaches work for this use-case.

[pelesh]

CC @ashermancinelli

[ashermancinelli]

Very cool to see!

[ashermancinelli]

If you don't want such a manual pipeline, you can use the plugin registration hooks that the opt pass plugin api provides (like registerOptimizerLastEPCallback, for example). See the docs here. Sorry I left you with such cursed cmake! 😅 You could also load the plugin from the command line like this:

clang++ -fpass-plugin=LLVMEnzyme.so -mllvm '-passes=default<O1>,enzyme,default<O3>' -o ./a.out

[pelesh]

So, in CMake configuration you would set something like -DCMAKE_CXX_FLAGS="-fpass-plugin=LLVMEnzyme.so" -mllvm '-passes=default<O1>,enzyme,default<O3>'? That instead of creating a custom command in CMake?

[ashermancinelli]

Exactly. Instead of dumping the IR between phases, you can first register the enzyme pass as a plugin and then use the -mllvm -passes= argument to build a custom phase ordering so enzyme is thrown in there.