[libcxx-commits] [libcxx] [libcxxabi] [libc++] Fix broken configuration system-libcxxabi on Apple (PR #110920)

[Louis Dionne via libcxx-commits]

https://github.com/ldionne created https://github.com/llvm/llvm-project/pull/110920

On Apple platforms, using system-libcxxabi as an ABI library wouldn't work because we'd try to re-export symbols from libc++abi that the system libc++abi.dylib might not have. Instead, only re-export those symbols when we're using the in-tree libc++abi.

This does mean that libc++.dylib won't re-export any libc++abi symbols when building against the system libc++abi, which could be fixed in various ways. However, the best solution really depends on the intended use case, so this patch doesn't try to solve that problem.

As a drive-by, also improve the diagnostic message when the user forgets to set the LIBCXX_CXX_ABI_INCLUDE_PATHS variable, which would previously lead to a confusing error.

Closes #104672

>From 5d452ab25d4e18a8b6e17a2c64bd4c30c53fcb13 Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2 at gmail.com>
Date: Wed, 2 Oct 2024 15:50:21 -0400
Subject: [PATCH] [libc++] Fix broken configuration system-libcxxabi on Apple

On Apple platforms, using system-libcxxabi as an ABI library wouldn't
work because we'd try to re-export symbols from libc++abi that the
system libc++abi.dylib might not have. Instead, only re-export those
symbols when we're using the in-tree libc++abi.

This does mean that libc++.dylib won't re-export any libc++abi symbols
when building against the system libc++abi, which could be fixed in
various ways. However, the best solution really depends on the intended
use case, so this patch doesn't try to solve that problem.

As a drive-by, also improve the diagnostic message when the user forgets
to set the LIBCXX_CXX_ABI_INCLUDE_PATHS variable, which would previously
lead to a confusing error.

Closes #104672
---
 libcxx/cmake/Modules/HandleLibCXXABI.cmake | 20 +++++++++++++++++++-
 libcxx/src/CMakeLists.txt                  |  8 ++------
 libcxxabi/src/CMakeLists.txt               |  2 ++
 3 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/libcxx/cmake/Modules/HandleLibCXXABI.cmake b/libcxx/cmake/Modules/HandleLibCXXABI.cmake
index 34e9a672a960f9..e7caf3044b8bcf 100644
--- a/libcxx/cmake/Modules/HandleLibCXXABI.cmake
+++ b/libcxx/cmake/Modules/HandleLibCXXABI.cmake
@@ -83,6 +83,10 @@ endfunction()
 
 # Link against a system-provided libstdc++
 if ("${LIBCXX_CXX_ABI}" STREQUAL "libstdc++")
+  if(NOT LIBCXX_CXX_ABI_INCLUDE_PATHS)
+    message(FATAL_ERROR "LIBCXX_CXX_ABI_INCLUDE_PATHS must be set when selecting libstdc++ as an ABI library")
+  endif()
+
   add_library(libcxx-abi-headers INTERFACE)
   import_private_headers(libcxx-abi-headers "${LIBCXX_CXX_ABI_INCLUDE_PATHS}"
     "cxxabi.h;bits/c++config.h;bits/os_defines.h;bits/cpu_defines.h;bits/cxxabi_tweaks.h;bits/cxxabi_forced.h")
@@ -96,6 +100,10 @@ if ("${LIBCXX_CXX_ABI}" STREQUAL "libstdc++")
 
 # Link against a system-provided libsupc++
 elseif ("${LIBCXX_CXX_ABI}" STREQUAL "libsupc++")
+  if(NOT LIBCXX_CXX_ABI_INCLUDE_PATHS)
+    message(FATAL_ERROR "LIBCXX_CXX_ABI_INCLUDE_PATHS must be set when selecting libsupc++ as an ABI library")
+  endif()
+
   add_library(libcxx-abi-headers INTERFACE)
   import_private_headers(libcxx-abi-headers "${LIBCXX_CXX_ABI_INCLUDE_PATHS}"
     "cxxabi.h;bits/c++config.h;bits/os_defines.h;bits/cpu_defines.h;bits/cxxabi_tweaks.h;bits/cxxabi_forced.h")
@@ -114,7 +122,13 @@ elseif ("${LIBCXX_CXX_ABI}" STREQUAL "libcxxabi")
   target_compile_definitions(libcxx-abi-headers INTERFACE "-DLIBCXX_BUILDING_LIBCXXABI")
 
   if (TARGET cxxabi_shared)
-    add_library(libcxx-abi-shared ALIAS cxxabi_shared)
+    add_library(libcxx-abi-shared INTERFACE)
+    target_link_libraries(libcxx-abi-shared INTERFACE cxxabi_shared)
+
+    # When using the in-tree libc++abi as an ABI library, libc++ re-exports the
+    # libc++abi symbols (on platforms where it can) because libc++abi is only an
+    # implementation detail of libc++.
+    target_link_libraries(libcxx-abi-shared INTERFACE cxxabi-reexports)
   endif()
 
   if (TARGET cxxabi_static)
@@ -131,6 +145,10 @@ elseif ("${LIBCXX_CXX_ABI}" STREQUAL "libcxxabi")
 
 # Link against a system-provided libc++abi
 elseif ("${LIBCXX_CXX_ABI}" STREQUAL "system-libcxxabi")
+  if(NOT LIBCXX_CXX_ABI_INCLUDE_PATHS)
+    message(FATAL_ERROR "LIBCXX_CXX_ABI_INCLUDE_PATHS must be set when selecting system-libcxxabi as an ABI library")
+  endif()
+
   add_library(libcxx-abi-headers INTERFACE)
   import_private_headers(libcxx-abi-headers "${LIBCXX_CXX_ABI_INCLUDE_PATHS}" "cxxabi.h;__cxxabi_config.h")
   target_compile_definitions(libcxx-abi-headers INTERFACE "-DLIBCXX_BUILDING_LIBCXXABI")
diff --git a/libcxx/src/CMakeLists.txt b/libcxx/src/CMakeLists.txt
index 48c5111a0acbf6..6b928ebeeeb221 100644
--- a/libcxx/src/CMakeLists.txt
+++ b/libcxx/src/CMakeLists.txt
@@ -210,14 +210,10 @@ if (LIBCXX_ENABLE_SHARED)
     target_link_libraries(cxx_shared PUBLIC libcxx-abi-shared)
   endif()
 
-  # Maybe re-export symbols from libc++abi
-  # In particular, we don't re-export the symbols if libc++abi is merged statically
-  # into libc++ because in that case there's no dylib to re-export from.
+  # Maybe force some symbols to be weak, not weak or not exported.
+  # TODO: This shouldn't depend on the platform, and ideally it should be done in the sources.
   if (APPLE AND LIBCXX_CXX_ABI MATCHES "libcxxabi$"
             AND NOT LIBCXX_STATICALLY_LINK_ABI_IN_SHARED_LIBRARY)
-    target_link_libraries(cxx_shared PRIVATE cxxabi-reexports)
-
-    # TODO: These exports controls should not be tied to whether we re-export libc++abi symbols
     target_link_libraries(cxx_shared PRIVATE
       "-Wl,-unexported_symbols_list,${CMAKE_CURRENT_SOURCE_DIR}/../lib/libc++unexp.exp"
       "-Wl,-force_symbols_not_weak_list,${CMAKE_CURRENT_SOURCE_DIR}/../lib/notweak.exp"
diff --git a/libcxxabi/src/CMakeLists.txt b/libcxxabi/src/CMakeLists.txt
index 6f16c614212ef1..480e528b819bb9 100644
--- a/libcxxabi/src/CMakeLists.txt
+++ b/libcxxabi/src/CMakeLists.txt
@@ -213,6 +213,8 @@ if (LIBCXXABI_ENABLE_SHARED)
     list(APPEND LIBCXXABI_INSTALL_TARGETS "cxxabi_shared")
   endif()
 
+  # TODO: Move this to libc++'s HandleLibCXXABI.cmake since this is effectively trying to control
+  #       what libc++ re-exports.
   add_library(cxxabi-reexports INTERFACE)
   function(export_symbols file)
     # -exported_symbols_list is only available on Apple platforms