[llvm] eliminate the need for a second preprocessor definition to gate `LLVM_ABI` macro definitions

[andrurogerz]

The LLVM_ABI and related macros defined in llvm/Support/Compiler.h are used to annotate LLVM's public symbols for export. They resolve to different annotations depending on the build target:

• When building LLVM as a Windows DLL, they are defined to either __declspec(dllexport) when building the DLL or __declspec(dllimport) when building clients.
• When building LLVM as a shared library on for ELF or dylib for Mach-O, they are defined to default visibility annotations __attribute__(visibility(("default"))) to ensure the symbols are exported when default visibility is set to hidden.
• When building LLVM as a static library, they are noops.

However, even when building LLVM as a DLL or shared library, a subset of libraries like tablegen always link against individual static libraries of LLVM components, such as the Support library. In order to make the LLVM_ABI macros apply correctly in this case (e.g. become noops), there is an override preprocessor define LLVM_BUILD_STATIC. This definition is enabled dynamically whenever an LLVM target defined with llvm_add_library or related CMake macros passes the DISABLE_LLVM_LINK_LLVM_DYLIB argument.

For correctness, the LLVM libraries should be built for either static or dynamic linking with one static set of LLVM_ABI definitions-- the definitions should not be controllable via a dynamic definition like LLVM_BUILD_STATIC. This means we need one set of libraries and llvm-config.h for targets that use DISABLE_LLVM_LINK_LLVM_DYLIB and another for those that link against the DLL or dynamic library.

Ideally, the LLVM_ABI definitions in Compiler.h would be simplified to the following:

#if defined(LLVM_BUILD_STATIC)
#define LLVM_ABI
#define LLVM_TEMPLATE_ABI
#define LLVM_EXPORT_TEMPLATE
#define LLVM_ABI_EXPORT
#elif defined(_WIN32) && !defined(__MINGW32__)
#if defined(LLVM_EXPORTS)
#define LLVM_ABI __declspec(dllexport)
#define LLVM_TEMPLATE_ABI
#define LLVM_EXPORT_TEMPLATE __declspec(dllexport)
#else
#define LLVM_ABI __declspec(dllimport)
#define LLVM_TEMPLATE_ABI __declspec(dllimport)
#define LLVM_EXPORT_TEMPLATE
#endif

LLVM_BUILD_STATIC would be defined (or not) in llvm-config.h based on CMake configuration. Other related definitions in llvm-config.h like LLVM_BUILD_LLVM_DYLIB and LLVM_BUILD_SHARED_LIBS could potentially be eliminated.

[gerekon]

Sorry for maybe stupid question. As far as I understand when building LLVM DLL LLVM_BUILD_STATIC is defined by providing DISABLE_LLVM_LINK_LLVM_DYLIB to add_llvm_library.
But in that case LLVM_ABI gets empty so API marked with it are not forced to be exported. But when DLL is build I assume it should export API used by other components.

Maybe I miss smth.

[gerekon]

Sorry for maybe stupid question. As far as I understand when building LLVM DLL LLVM_BUILD_STATIC is defined by providing DISABLE_LLVM_LINK_LLVM_DYLIB to add_llvm_library. But in that case LLVM_ABI gets empty so API marked with it are not forced to be exported. But when DLL is build I assume it should export API used by other components.

Maybe I miss smth.

Sorry guys. My bad.
Looks like LLVM_BUILD_STATIC is applied to libllvm.cpp only. The main code base in included into the lib using LLVM_DYLIB_COMPONENTS and those components libs are added by add_llvm_component_library.

Looks like the problem is that for MINGW
#define LLVM_ABI attribute((visibility("default")))

but that is not an equivalent for dllexport