Per discussion on discord and various feature requests across bindings (Haskell and Rust bindings authors have asked me directly), we should be building a link-ready MLIR-C dylib which exports the C API and can be used without linking to anything else.
This patch:
* Adds a new MLIR-C aggregate shared library (libMLIR-C.so), which is similar in name and function to libLLVM-C.so.
* It is guarded by the new CMake option MLIR_BUILD_MLIR_C_DYLIB, which has a similar purpose/name to the LLVM_BUILD_LLVM_C_DYLIB option.
* On all platforms, this will work with both static, BUILD_SHARED_LIBS, and libMLIR builds, if supported:
* In static builds: libMLIR-C.so will export the CAPI symbols and statically link all dependencies into itself.
* In BUILD_SHARED_LIBS: libMLIR-C.so will export the CAPI symbols and have dynamic dependencies on implementation shared libraries.
* In libMLIR.so mode: same as static. libMLIR.so was not finished for actual linking use within the project. An eventual relayering so that libMLIR-C.so depends on libMLIR.so is possible but requires first re-engineering the latter to use the aggregate facility.
* On Linux, exported symbols are filtered to only the CAPI. On others (MacOS, Windows), all symbols are exported. A CMake status is printed unless if global visibility is hidden indicating that this has not yet been implemented. The library should still work, but it will be larger and more likely to conflict until fixed. Someone should look at lifting the corresponding support from libLLVM-C.so and adapting. Or, for special uses, just build with `-DCMAKE_CXX_VISIBILITY_PRESET=hidden -DCMAKE_C_VISIBILITY_PRESET=hidden`.
* Includes fixes to execution engine symbol export macros to enable default visibility. Without this, the advice to use hidden visibility would have resulted in test failures and unusable execution engine support libraries.
Differential Revision: https://reviews.llvm.org/D113731
We were discussing on discord regarding the need for extension-based systems like Python to dynamically link against MLIR (or else you can only have one extension that depends on it). Currently, when I set that up, I piggy-backed off of the flag that enables build libLLVM.so and libMLIR.so and depended on libMLIR.so from the python extension if shared library building was enabled. However, this is less than ideal.
In the current setup, libMLIR.so exports both all symbols from the C++ API and the C-API. The former is a kitchen sink and the latter is curated. We should be splitting them and for things that are properly factored to depend on the C-API, they should have the option to *only* depend on the C-API, and we should build that shared library no matter what. Its presence isn't just an optimization: it is a key part of the system.
To do this right, I needed to:
* Introduce visibility macros into mlir-c/Support.h. These should work on both *nix and windows as-is.
* Create a new libMLIRPublicAPI.so with just the mlir-c object files.
* Compile the C-API with -fvisibility=hidden.
* Conditionally depend on the libMLIR.so from libMLIRPublicAPI.so if building libMLIR.so (otherwise, also links against the static libs and will produce a mondo libMLIRPublicAPI.so).
* Disable re-exporting of static library symbols that come in as transitive deps.
This gives us a dynamic linked C-API layer that is minimal and should work as-is on all platforms. Since we don't support libMLIR.so building on Windows yet (and it is not very DLL friendly), this will fall back to a mondo build of libMLIRPublicAPI.so, which has its uses (it is also the most size conscious way to go if you happen to know exactly what you need).
Sizes (release/stripped, Ubuntu 20.04):
Shared library build:
libMLIRPublicAPI.so: 121Kb
_mlir.cpython-38-x86_64-linux-gnu.so: 1.4Mb
mlir-capi-ir-test: 135Kb
libMLIR.so: 21Mb
Static build:
libMLIRPublicAPI.so: 5.5Mb (since this is a "static" build, this includes the MLIR implementation as non-exported code).
_mlir.cpython-38-x86_64-linux-gnu.so: 1.4Mb
mlir-capi-ir-test: 44Kb
Things like npcomp and circt which bring their own dialects/transforms/etc would still need the shared library build and code that links against libMLIR.so (since it is all C++ interop stuff), but hopefully things that only depend on the public C-API can just have the one narrow dep.
I spot checked everything with nm, and it looks good in terms of what is exporting/importing from each layer.
I'm not in a hurry to land this, but if it is controversial, I'll probably split off the Support.h and API visibility macro changes, since we should set that pattern regardless.
Reviewed By: mehdi_amini, benvanik
Differential Revision: https://reviews.llvm.org/D90824
This is exposing the basic functionalities (create, nest, addPass, run) of
the PassManager through the C API in the new header: `include/mlir-c/Pass.h`.
In order to exercise it in the unit-test, a basic TableGen backend is
also provided to generate a simple C wrapper around the pass
constructor. It is used to expose the libTransforms passes to the C API.
Reviewed By: stellaraccident, ftynse
Differential Revision: https://reviews.llvm.org/D90667
Stella Laurenzo