[libcxx-commits] [libcxxabi] 5343b20 - [libcxxabi] Don't enable -fvisibility=hidden on Windows (#207943)

[via libcxx-commits]

Author: Martin Storsjö
Date: 2026-07-07T16:16:36+03:00
New Revision: 5343b204252d7bf5bdfcacd664e78e9f7f9fd745

URL: https://github.com/llvm/llvm-project/commit/5343b204252d7bf5bdfcacd664e78e9f7f9fd745
DIFF: https://github.com/llvm/llvm-project/commit/5343b204252d7bf5bdfcacd664e78e9f7f9fd745.diff

LOG: [libcxxabi] Don't enable -fvisibility=hidden on Windows (#207943)

This option is supported (and hence gets added by the
target_add_compile_flags_if_supported function) on Windows, but if using
dllexport, there's no point in using hidden visibility. (However if
building both static and shared libraries at the same time, it could be
beneficial to have hidden visibilty in the static libraries.)

In asserts enabled builds of Clang, building with -fvisibility=hidden
caused errors like this:

    dllexport GlobalValue must have default or protected visibility
    ptr @_ZTIv
    [...]
    fatal error: error in backend: Broken module found, compilation aborted!

Normally, combining -fvisibility=hidden with dllexport is not a problem,
but specifically for `__cxxabiv1:: __fundamental_type_info` it does make
Clang trigger an inconsistency, visible in asserts enabled builds of
Clang - see https://github.com/llvm/llvm-project/issues/207963.

This fixes Windows builds with Clang with asserts enabled, after
cb942d009a5ed029ce20d9b75b3604f551f0cb56.

Added: 
    

Modified: 
    libcxxabi/src/CMakeLists.txt

Removed: 
    


################################################################################
diff  --git a/libcxxabi/src/CMakeLists.txt b/libcxxabi/src/CMakeLists.txt
index fc66ce7187170..dd23e9bb5bf79 100644
--- a/libcxxabi/src/CMakeLists.txt
+++ b/libcxxabi/src/CMakeLists.txt
@@ -195,7 +195,14 @@ if (LIBCXXABI_SUPPORTS_SHARED_LIBRARY)
   # Build with -fsized-deallocation, which is default in recent versions of Clang.
   # TODO(LLVM 21): This can be dropped once we only support Clang >= 19.
   target_add_compile_flags_if_supported(cxxabi_shared_objects PRIVATE -fsized-deallocation)
-  target_add_compile_flags_if_supported(cxxabi_shared_objects PRIVATE -fvisibility=hidden)
+  if (NOT WIN32)
+    # Building libcxxabi with -fvisibility=hidden on Windows, if dllexport is
+    # used, can trigger a Clang error (if Clang is built with asserts enabled),
+    # https://github.com/llvm/llvm-project/issues/207963
+    # Once fixed, we could consider allowing -fvisibility=hidden on
+    # Windows too (as it can be beneficial for static libraries).
+    target_add_compile_flags_if_supported(cxxabi_shared_objects PRIVATE -fvisibility=hidden)
+  endif()
 
   add_library(cxxabi_shared SHARED)
   set_target_properties(cxxabi_shared
@@ -286,7 +293,14 @@ target_compile_options(cxxabi_static_objects PRIVATE "${LIBCXXABI_ADDITIONAL_COM
 # Build with -fsized-deallocation, which is default in recent versions of Clang.
 # TODO(LLVM 21): This can be dropped once we only support Clang >= 19.
 target_add_compile_flags_if_supported(cxxabi_static_objects PRIVATE -fsized-deallocation)
-target_add_compile_flags_if_supported(cxxabi_static_objects PRIVATE -fvisibility=hidden)
+if (NOT WIN32)
+  # Building libcxxabi with -fvisibility=hidden on Windows, if dllexport is
+  # used, can trigger a Clang error (if Clang is built with asserts enabled),
+  # https://github.com/llvm/llvm-project/issues/207963
+  # Once fixed, we could consider allowing -fvisibility=hidden on
+  # Windows too (as it can be beneficial for static libraries).
+  target_add_compile_flags_if_supported(cxxabi_static_objects PRIVATE -fvisibility=hidden)
+endif()
 
 if(LIBCXXABI_HERMETIC_STATIC_LIBRARY)
   # If the hermetic library doesn't define the operator new/delete functions