[llvm-dev] Exposing the New Pass Manager in the LLVM C API

[Leonard Chan via llvm-dev]

Hi all,

At some time in the near future we would like to allow for Rust to use the
new PM. As I understand it, Rust uses LLVM through the stable LLVM C API,
which currently only provides an interface to the legacy PM. We wanted to
ask what people think about how we should go about exposing the new PM
through this API. We can think of 3 possibilities for now:

1) Just replace the API to expose the new PM instead of the legacy one.

I imagine we wouldn't want to do this since it would be a pretty sudden
change and there wouldn't be new PM equivalents for the
Initialize/FinalizeFunctionPassManager functions.

2) Add a new interface that exposes the new PM while still keeping the
current one.

The interface would probably be similar to the current one but just not
have the corresponding Initialize/FinalizeFunctionPassManager functions. I
think with this method we could also slowly roll out usage of the new PM
through the C API and eventually deprecate the legacy PM interface in favor
of the new one.

3) Add a build flag that switches between PMs used in the C API.

This could just be some flag in CMake that users can change at build time.

Any other suggestions/thoughts?

- Leo
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