[RFC] Explicit "_MLIR_CAPI_EXPORTED" macro

MLIR
#1

I would like to be able to build a shared library/DLL that only exports the C-API and hides everything else (either as part of LLVM/MLIR itself or as part of a larger project). I believe the relatively normal way to do this for this kind of project is something like:

  • On Windows:
    • Apply declspec(dllexport) to functions if building a library
    • Apply declspec(dllimport) to functions if depending on a library
  • On Others:
    • Apply __attribute__((__visibility__("default"))) to functions
    • Build the project with -fvisibility=hidden

There are many things and permutations that can be done if such annotations are applied correctly, and as a first step, we should decide on the macro and use it appropriately.

Libcxx has more complicated needs, but the basic structure and knobs from libcxx/include/__config provides the right recipe. I propose adding a _MLIR_CAPI_EXPORTED macro to a new mlir-c/Config.h and defining it roughly like the libcxx macro is defined (above). We can elaborate on the knobs and additional build setup to fully leverage it later.

#2

Is the request here 1) add such a macro, and 2) add only to functions that we want exported? (e.g., would it be a fairly local change to CAPI or extend beyond that?). Or question about preferred way to address this here?

#3

It is just #1 and then applying it to all functions in the CAPI. I see it as scoped just to the CAPI. Just raising as an RFC vs a patch since it is a design change and we have a somewhat higher bar on stability of the CAPI.