[libcxx] Link against android_support when needed

[smeenai]

Summary

libc++ on Android needs to be linked against libandroid_support on API
levels less than 21 to provide needed functions that aren't in the libc
on those platforms (e.g. posix_memalign for libcxxabi). libc++ from the
NDK is a linker script that pulls in libandroid_support, but for
building libc++ itself, we need to explicitly add libandroid_support as
a dependency. Moreover, libc++ headers reference the functions provided
by libandroid_support, so it needs to be added as a public dependency.

[danalbert]

We don't build the NDK's libc++ with CMake. What's the motivation for this?

[smeenai]

In D73516#1844922, @danalbert wrote:

We don't build the NDK's libc++ with CMake. What's the motivation for this?

I'm experimenting with building my own libc++ for Android (using LLVM's runtimes build setup) for testing purposes. I know the NDK's libc++ is built with ndk-build, but having (best-effort) support for Android in libc++'s CMake seems reasonable too.

[danalbert]

libcxx/CMakeLists.txt

784

This should probably be PRIVATE rather than PUBLIC. The consumer doesn't need it if their minSdkVersion is 21+. The real constraint might be describable with a generator expression, but I'm not sure. (if we don't generate config files for this it doesn't matter anyway)

[smeenai]

libcxx/CMakeLists.txt

784

You're right ... I was assuming that ANDROID_PLATFORM_LEVEL would be consistent across the build of libc++ and the build of clients, but that's not necessarily true. With that being said, we don't generate config files for this as far as I'm aware, so the only place this PUBLIC will affect is other parts of LLVM's build that depend on libc++, and I think it's fair to assume the entirety of LLVM is being built with the same ANDROID_PLATFORM_LEVEL.

[danalbert]

libcxx/CMakeLists.txt

784

Okay, works for me.

libcxx/CMakeLists.txt

if (LIBCXX_TARGETING_MSVC)
     target_link_libraries(${target} PRIVATE ucrt${LIB_SUFFIX}) # Universal C runtime
     target_link_libraries(${target} PRIVATE vcruntime${LIB_SUFFIX}) # C++ runtime
     target_link_libraries(${target} PRIVATE msvcrt${LIB_SUFFIX}) # C runtime startup files
     target_link_libraries(${target} PRIVATE msvcprt${LIB_SUFFIX}) # C++ standard library. Required for exception_ptr internals.
     # Required for standards-complaint wide character formatting functions
     # (e.g. `printfw`/`scanfw`)
     target_link_libraries(${target} PRIVATE iso_stdio_wide_specifiers)
   endif()
+
+  if (ANDROID AND ANDROID_PLATFORM_LEVEL LESS 21)
+    target_link_libraries(${target} PUBLIC android_support)
+  endif()
 endfunction()
 
 # Windows-related flags =======================================================
 function(cxx_add_windows_flags target)
   if(WIN32 AND NOT MINGW)
     target_compile_definitions(${target} PRIVATE
                                  # Ignore the -MSC_VER mismatch, as we may build
                                  # with a different compatibility version.

libcxxabi/src/CMakeLists.txt

if (LIBCXXABI_USE_LLVM_UNWINDER)
   endif()
 else()
   add_library_flags_if(LIBCXXABI_HAS_GCC_S_LIB gcc_s)
 endif()
 if (MINGW)
   # MINGW_LIBRARIES is defined in config-ix.cmake
   list(APPEND LIBCXXABI_LIBRARIES ${MINGW_LIBRARIES})
 endif()
+if (ANDROID AND ANDROID_PLATFORM_LEVEL LESS 21)
+  list(APPEND LIBCXXABI_LIBRARIES android_support)
+endif()
 
 if (NOT LIBCXXABI_USE_COMPILER_RT)
   add_library_flags_if(LIBCXXABI_HAS_GCC_LIB gcc)
 endif ()
 
 # Setup flags.
 add_link_flags_if_supported(-nodefaultlibs)