[llvm] [llvm] annotate interfaces in AsmParser, BinaryFormat, Bitcode, and Bitstream libraries for DLL export (PR #141794)

[Andrew Rogers via llvm-commits]

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@@ -120,7 +121,7 @@ namespace llvm {
 extern FunctionSummary::ForceSummaryHotnessType ForceSummaryEdgesCold;
 }
 
-extern llvm::cl::opt<bool> UseNewDbgInfoFormat;
+LLVM_ABI extern llvm::cl::opt<bool> UseNewDbgInfoFormat;
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andrurogerz wrote:

> Should extern precede LLVM_ABI?

There is actually a problem with moving the `LLVM_ABI` annotation after the `extern` keyword. When we're building with gcc, `Compiler.h` currently defines `LLVM_ABI` as a C++ GNU visibility attribute: `[[gnu::visibility("default")]]`. When this attribute follows `extern` on a variable declaration, gcc produces `error: standard attributes in middle of decl-specifiers` errors.

I think there are two ways to proceed:
1. Leave things as-is and require `LLVM_ABI` before `extern`. This is counter to MSDN documentation for `__declspec(dllimport)` when building for Windows, but behaves correctly and does not generate any warnings.
2. Always define `LLVM_ABI` to `__attribute__(visibility(("default")))` on non-Windows platforms. This is how it is currently defined when compiling with Clang on non-Windows.

Option 2 is arguably the better choice because placement of `__attribute__` attributes is more flexible (compiler-dependent). I think it might also allow us to eliminate the `LLVM_ABI_FRIEND` macro in favor of `LLVM_ABI`. @compnerd  do you have any thoughts?

https://github.com/llvm/llvm-project/pull/141794