[libunwind] Add _LIBUNWIND_DISABLE_VISIBILITY_ANNOTATIONS

[thomasanderson]

Summary

It's useful to be able to disable visibility annotations entirely; for
example, if we're building libunwind static to include in another library,
and we don't want any libunwind functions getting exported out of that
library.

A macro to indicate static builds might make more sense, but for now
we want to be consistent with libc++/libc++abi. Reevaluate in the
future.

[compnerd]

src/config.h

47

If the intent is to prevent the symbols from being exported, then hidden visibility is still desirable. I think that having something like a check for a static build makes more sense rather than just disabling the visibility annotations.

[thomasanderson]

src/config.h

47

This is what libc++ and libc++abi do already. Search for _LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS and _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS

[thomasanderson]

src/config.h

47

Forgot to say, for this use case, you're expected to link with -fvisibility=hidden.

[compnerd]

src/config.h

47

I assume you mean compile libunwind with -fvisibility=hidden, which makes the condition even weirder, because you really do want the visibility, just dont want to mark functions for export (basically, you want to use the visibility similar to DLL storage). Indicating a static build and conditionalizing it on that seems more inline with the intent and the desired behaviour.

[compnerd]

Discussed with @thakis on IRC; we can go ahead with this, but this will change in the future to be conditionalised on whether you are building a static or shared library. The intention here is to build without the exports and maintain hidden visibility. Although this is different from libc++, it is due to the fact that the current libunwind interfaces dont make as much sense on Windows (the itanium model and the windows exception model are thoroughly different), so removing the visibility attributes doesn't make much sense.

[thakis]

r306442, thanks!

src/config.h

   #endif
 #endif
 
-// FIXME: these macros are not correct for COFF targets
-#define _LIBUNWIND_EXPORT __attribute__((visibility("default")))
-#define _LIBUNWIND_HIDDEN __attribute__((visibility("hidden")))
+#if defined(_LIBUNWIND_DISABLE_VISIBILITY_ANNOTATIONS)
+  #define _LIBUNWIND_EXPORT
+  #define _LIBUNWIND_HIDDEN
+#else
+  // FIXME: these macros are not correct for COFF targets
+  #define _LIBUNWIND_EXPORT __attribute__((visibility("default")))
+  #define _LIBUNWIND_HIDDEN __attribute__((visibility("hidden")))
+#endif
 
 #if (defined(__APPLE__) && defined(__arm__)) || defined(__USING_SJLJ_EXCEPTIONS__)
 #define _LIBUNWIND_BUILD_SJLJ_APIS