[SystemZ][ZOS] libcxx - no posix memalign

[zibi]

Summary

The unavailability of posix_memalign on z/OS forces us to define _LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION'. The use of posix_memalign is being used in libcxx/src/new.cpp.

[zibi]

Unfortunately we need one more change on z/OS specially for the shared build.
The -fno-aligned-allocation is required to avoid using aligned allocations otherwise, __cpp_aligned_new is defined to 201606L and we result with unresolved symbols.

[zibi]

@ldionne Louis, are you ok, with new addition added after your approval?

[zibi]

resubmitting to get CI result and revalidating acceptance from @ldionne

[ldionne]

In D90178#2376073, @zibi wrote:

@ldionne Louis, are you ok, with new addition added after your approval?

Sorry, but no I'm not. Let's not revert and talk.

libcxx/CMakeLists.txt

525

Can you explain why you need this? IOW, what issue are you seeing if you use -faligned-allocation?

[zibi]

libcxx/CMakeLists.txt

525

I'm seeing unresolved symbols to align version of operator new preventing from linking.

[zibi]

My understanding is that we don't want to disable -faligned-allocation since users might provide the required operator new/delete.
There is another related option -faligned-alloc-unavailable which we made active for z/OS but the library explicit tramps it in cmake configuration. .

[zibi]

This already landed on Nov 12, 2020.

libcxx/CMakeLists.txt

function(cxx_add_basic_build_flags target)
   # in the dylib.
   set_target_properties(${target} PROPERTIES
     CXX_STANDARD 17
     CXX_STANDARD_REQUIRED YES
     CXX_EXTENSIONS NO)
 
   # When building the dylib, don't warn for unavailable aligned allocation
   # functions based on the deployment target -- they are always available
-  # because they are provided by the dylib itself.
+  # because they are provided by the dylib itself with the excepton of z/OS.
+  if (ZOS)
+    target_add_compile_flags_if_supported(${target} PRIVATE -fno-aligned-allocation)
+  else()
     target_add_compile_flags_if_supported(${target} PRIVATE -faligned-allocation)
+  endif()
 
   # On all systems the system c++ standard library headers need to be excluded.
   # MSVC only has -X, which disables all default includes; including the crt.
   # Thus, we do nothing and hope we don't accidentally include any of the C++
   # headers
   target_add_compile_flags_if_supported(${target} PUBLIC -nostdinc++)
 
   # Hide all inline function definitions which have not explicitly been marked

libcxx/include/__config

 // If we are getting operator new from the MSVC CRT, then allocation overloads
 // for align_val_t were added in 19.12, aka VS 2017 version 15.3.
 #if defined(_LIBCPP_MSVCRT) && defined(_MSC_VER) && _MSC_VER < 1912
 #  define _LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION
 #elif defined(_LIBCPP_ABI_VCRUNTIME) && !defined(__cpp_aligned_new)
    // We're deferring to Microsoft's STL to provide aligned new et al. We don't
    // have it unless the language feature test macro is defined.
 #  define _LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION
+#elif defined(__MVS__)
+#  define _LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION
 #endif
 
 #if defined(__APPLE__)
 #  if !defined(__MAC_OS_X_VERSION_MIN_REQUIRED) && \
       defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__)
 #    define __MAC_OS_X_VERSION_MIN_REQUIRED __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__
 #  endif
 #endif // defined(__APPLE__)