build: add a heuristic to determine the C++ ABI

[compnerd]

Summary

We need to have a more principled method to detect the C++ ABI we are
targeting. Add a heuristic to determine that and pass it to the build.

[majnemer]

Why isn't this equivalent to _MSC_VER ?

[smeenai]

In D28383#637570, @majnemer wrote:

Why isn't this equivalent to _MSC_VER ?

You can have scenarios where you're targeting the Itanium ABI but still have _MSC_VER defined, e.g.

% clang -target i686-windows-itanium -fmsc-version=1900 -E -dM - < /dev/null | grep _MSC_VER
#define _MSC_VER 1900

[rnk]

In D28383#637570, @majnemer wrote:

Why isn't this equivalent to _MSC_VER ?

It might also be nice for clang to define some macro to indicate which C++ ABI is in use.

All that said, I think it makes sense to encode this in libc++ include/__config_site because you don't want people to build libc++ for the MS ABI and then try to use it with a mingw triple. It'd be nice if we could diagnose that at compile time instead of link time.

[compnerd]

@rnk happy to add such a macro. Would __cpp_abi_itanium and __cpp_abi_microsoft be palatable?

This actually does get encoded into __config albeit in a round-about way. The __config_site gets concatenated with __config into __generated_config which is installed as __config.

[EricWF]

In D28383#637672, @compnerd wrote:

@rnk happy to add such a macro. Would __cpp_abi_itanium and __cpp_abi_microsoft be palatable?

I would love these macros. I would prefer if the stock __config worked for all but the rarest of configurations.

[compnerd]

We have a temporary workaround while a proper macro is considered by SD-6.

CMakeLists.txt

 config_define_if_not(LIBCXX_ENABLE_THREADS _LIBCPP_HAS_NO_THREADS)
 config_define_if_not(LIBCXX_ENABLE_MONOTONIC_CLOCK _LIBCPP_HAS_NO_MONOTONIC_CLOCK)
 config_define_if_not(LIBCXX_ENABLE_THREAD_UNSAFE_C_FUNCTIONS _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS)
 
 config_define_if(LIBCXX_HAS_PTHREAD_API _LIBCPP_HAS_THREAD_API_PTHREAD)
 config_define_if(LIBCXX_HAS_EXTERNAL_THREAD_API _LIBCPP_HAS_THREAD_API_EXTERNAL)
 config_define_if(LIBCXX_HAS_MUSL_LIBC _LIBCPP_HAS_MUSL_LIBC)
 
+# Indicate the ABI that we are targeting.  If the c++abi library is none and we
+# are building for windows, assume that we want the Microsoft C++ ABI.  Note
+# that MinGW uses the itanium ABI and still reports as WIN32 under CMake.  If we
+# are targeting the itanium ABI, then we must have one of the itanium ABI
+# support libraries (libc++abi, libsupc++, or libcxxrt).  Windows itanium and
+# MinGW both will use one of those to provide the support routines which
+# differentiates it from MS ABI.
+if(WIN32 AND NOT MINGW AND "${LIBCXX_CXX_ABI}" STREQUAL "none")
+  config_define_if(TRUE _LIBCPP_USES_MS_ABI)
+endif()
+
 # By default libc++ on Windows expects to use a shared library, which requires
 # the headers to use DLL import/export semantics. However when building a
 # static library only we modify the headers to disable DLL import/export.
 if (DEFINED WIN32 AND LIBCXX_ENABLE_STATIC AND NOT LIBCXX_ENABLE_SHARED)
   message(STATUS "Generating custom __config for non-DLL Windows build")
   config_define(ON _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
 endif()
 

include/__config

 #endif
 
 #define _LIBCPP_VERSION 4000
 
 #ifndef _LIBCPP_ABI_VERSION
 #define _LIBCPP_ABI_VERSION 1
 #endif
 
+#if defined(_LIBCPP_USES_MS_ABI)
+#define _LIBCPP_ABI_MS
+#else
+#define _LIBCPP_ABI_IA
+#endif
+
 #if defined(_LIBCPP_ABI_UNSTABLE) || _LIBCPP_ABI_VERSION >= 2
 // Change short string representation so that string data starts at offset 0,
 // improving its alignment in some cases.
 #define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 // Fix deque iterator type in order to support incomplete types.
 #define _LIBCPP_ABI_INCOMPLETE_TYPES_IN_DEQUE
 // Fix undefined behavior in how std::list stores it's linked nodes.
 #define _LIBCPP_ABI_LIST_REMOVE_NODE_POINTER_UB

include/__config_site.in

 #cmakedefine _LIBCPP_HAS_NO_STDOUT
 #cmakedefine _LIBCPP_HAS_NO_THREADS
 #cmakedefine _LIBCPP_HAS_NO_MONOTONIC_CLOCK
 #cmakedefine _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
 #cmakedefine _LIBCPP_HAS_MUSL_LIBC
 #cmakedefine _LIBCPP_HAS_THREAD_API_PTHREAD
 #cmakedefine _LIBCPP_HAS_THREAD_API_EXTERNAL
 #cmakedefine _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS
+#cmakedefine _LIBCPP_USES_MS_ABI
 
 #endif // _LIBCPP_CONFIG_SITE