[llvm] [llvm] annotate interfaces in llvm-c for DLL export (PR #141701)

[Saleem Abdulrasool via llvm-commits]

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@@ -0,0 +1,40 @@
+/*===-- llvm-c/Visibility.h - Visibility macros for llvm-c ------*- C++ -*-===*\
+|*                                                                            *|
+|* Part of the LLVM Project, under the Apache License v2.0 with LLVM          *|
+|* Exceptions.                                                                *|
+|* See https://llvm.org/LICENSE.txt for license information.                  *|
+|* SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception                    *|
+|*                                                                            *|
+|*===----------------------------------------------------------------------===*|
+|*                                                                            *|
+|* This header defines visibility macros used for the LLVM C interface. These *|
+|* macros are used to annotate C functions that should be exported as part of *|
+|*                                                                            *|
+\*===----------------------------------------------------------------------===*/
+
+#ifndef LLVM_C_VISIBILITY_H
+#define LLVM_C_VISIBILITY_H
+
+/// LLVM_C_ABI is the export/visibility macro used to mark symbols declared in
+/// llvm-c as exported when llvm is built as a shared library.
+
+#if defined(LLVM_BUILD_LLVM_DYLIB) || defined(LLVM_BUILD_SHARED_LIBS) ||       \
+    defined(LLVM_ENABLE_PLUGINS)
+#if defined(LLVM_BUILD_STATIC)
+#define LLVM_C_ABI
+#elif defined(_WIN32) && !defined(__MINGW32__)
+#if defined(LLVM_EXPORTS)
+#define LLVM_C_ABI __declspec(dllexport)
+#else
+#define LLVM_C_ABI __declspec(dllimport)
+#endif
+#elif defined(__has_attribute) && __has_attribute(visibility)
+#define LLVM_C_ABI __attribute__((visibility("default")))
+#else
+#define LLVM_C_ABI
+#endif
+#else
+#define LLVM_C_ABI
+#endif
----------------
compnerd wrote:

I really would like to see this be simplified for LLVM-C, perhaps something like:

```c
#if defined(LLVM_C_STATIC)
# define LLVM_C_ABI
#else
# if defined(_WIN32) && !defined(__MINGW32__)
#   if defined(LLVM_C_EXPORTS)
#     define LLVM_C_ABI __declspec(dllexport)
#   else
#     define LLVM_C_ABI __declspec(dllimport)
#   endif
# else
#   if defined(__has_attribute) && __has_attribute(visibility)
#     define LLVM_C_ABI __attribute__((__visibility__("default")))
#   else
#     define LLVM_C_ABI
#   endif
# endif
#endif
```

This way we can independently control whether LLVM-C is static or dynamic.

https://github.com/llvm/llvm-project/pull/141701