[PATCH] D50652: [libcxx] By default, do not use internal_linkage to hide symbols from the ABI

[Duncan P. N. Exon Smith via Phabricator via cfe-commits]

dexonsmith added inline comments.


================
Comment at: libcxx/include/__config:798-804
 #ifndef _LIBCPP_HIDE_FROM_ABI
-#  define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_INTERNAL_LINKAGE
+#  ifdef _LIBCPP_ABI_HIDDEN_USE_INTERNAL_LINKAGE
+#    define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_INTERNAL_LINKAGE
+#  else
+#    define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_ALWAYS_INLINE
+#  endif
 #endif
----------------
This doesn't leave a user-friendly way of opting out of internal linkage if a vendor decides to turn it on by default.  I suggest adding a layer of indirection:
```
// Elsewhere.
#cmakedefine _LIBCPP_ABI_HIDDEN_USE_INTERNAL_LINKAGE_BY_DEFAULT

// Here.
#ifndef _LIBCPP_ABI_HIDDEN_USE_INTERNAL_LINKAGE
#  ifdef _LIBCPP_ABI_HIDDEN_USE_INTERNAL_LINKAGE_BY_DEFAULT
#    define _LIBCPP_ABI_HIDDEN_USE_INTERNAL_LINKAGE 1
#  else
#    define _LIBCPP_ABI_HIDDEN_USE_INTERNAL_LINKAGE 0
#  endif
#endif

#ifndef _LIBCPP_HIDE_FROM_ABI
#  if _LIBCPP_ABI_HIDDEN_USE_INTERNAL_LINKAGE
#    define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_INTERNAL_LINKAGE
#  else
#    define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_ALWAYS_INLINE
#  endif
#endif
```
Then users can set the behaviour with `-D_LIBCPP_ABI_HIDDEN_USE_INTERNAL_LINKAGE=1` or `-D_LIBCPP_ABI_HIDDEN_USE_INTERNAL_LINKAGE=0`, overriding the vendor default.


Repository:
  rCXX libc++

https://reviews.llvm.org/D50652