[libc++] Simplify __config a bit

[philnik]

Summary

Simplify logic in __config by assuming that we are using Clang in C++03 mode. Also, use standardized feature-test macros instead of compiler-specific checks (like __has_feature) in a couple of places.

[Mordante]

In general I like the approach. I want to have a closer look when the CI is green.

libcxx/include/__config

211

One of the things bothering about this file is the lack-of overview what indention level parts have.
After processing this patch would it make sense to format this file?
Then all # foo parts have the proper indention.

408–416

Can this be really removed?

428–429

nit: maybe use using instead?

902–903

This looks odd, 199711L refers to C++98. When RTTI is disable is it a "random" value or always 0L?

[philnik]

• Address comments
• Try to fix CI

libcxx/include/__config

211

I'm not sure. clang-format indents everything an unneeded level because we have #ifdef __cplusplus around the whole file. What I've formatted here was done mostly by hand. Although I think it still would be a major improvement over the status quo. I don't think anybody (other than Arthur maybe) would actually care if the default indentation is one to far in. So yeah, it's probably a good idea.

408–416

It looks weird in the diff, but the typedefs are part of the C++03 block above.

902–903

Neither Clang nor GCC define it actually if RTTI is disabled. The || __cpp_rtti < 199711L is more a technically-correct than anything else. I've been bitten by that before with other FTMs.

[philnik]

• Use attribute

[philnik]

• Try to fix GCC

[ldionne]

Can you please add a description for this change and avoid making formatting changes in the same patch? It seems interesting, but I can't quite figure out what's the essence of the change in its current state.

[Mordante]

SGTM, but I will need some time to have a good look at the changes made.

libcxx/include/__config

211

Mainly to make it easier to spot how deeply nested some parts are. Especially in the compiler specific parts.

[philnik]

Rebased and stuff

[ldionne]

I'd like the commit message to explain what's going on here since it's not obvious. From our discussion:

Simplify logic in __config by assuming that we are using Clang in C++03 mode. Also, use standardized feature-test macros instead of compiler-specific checks (like __has_feature) in a couple of places.

LGTM with:

• A more descriptive commit message
• CI passing
• The visibility changes split out.

Thanks a lot, this is actually a really nice refactoring. I am especially looking forward to the split-up patch refactoring visibility attributes.

libcxx/include/__config

535

Can you please split the visibility part of this change into another patch? It's really not related to the rest of the patch.

[philnik]

• Remove visibility changes

libcxx/include/__config

 #    define _LIBCPP_COMPILER_CLANG_BASED
 #    define _LIBCPP_CLANG_VER (__clang_major__ * 100 + __clang_minor__)
 #  elif defined(__GNUC__)
 #    define _LIBCPP_COMPILER_GCC
 #  elif defined(_MSC_VER)
 #    define _LIBCPP_COMPILER_MSVC
 #  endif
 
-#  if defined(_LIBCPP_COMPILER_GCC) && __cplusplus < 201103L
-#    error "libc++ does not support using GCC with C++03. Please enable C++11"
+#  if !defined(_LIBCPP_COMPILER_CLANG_BASED) && __cplusplus < 201103L
+#    error "libc++ only supports C++03 with Clang-based compilers. Please enable C++11"
 #  endif
 
 // FIXME: ABI detection should be done via compiler builtin macros. This
 // is just a placeholder until Clang implements such macros. For now assume
 // that Windows compilers pretending to be MSVC++ target the Microsoft ABI,
 // and allow the user to explicitly specify the ABI to handle cases where this
 // heuristic falls short.
 #  if defined(_LIBCPP_ABI_FORCE_ITANIUM) && defined(_LIBCPP_ABI_FORCE_MICROSOFT)

 
 #  if __has_attribute(__no_sanitize__) && !defined(_LIBCPP_COMPILER_GCC)
 #    define _LIBCPP_NO_CFI __attribute__((__no_sanitize__("cfi")))
 #  else
 #    define _LIBCPP_NO_CFI
 #  endif
 
 #  ifndef _LIBCPP_CXX03_LANG
-#    define _LIBCPP_ALIGNOF(_Tp) alignof(_Tp)
-#  elif defined(_LIBCPP_COMPILER_CLANG_BASED)
-#    define _LIBCPP_ALIGNOF(_Tp) _Alignof(_Tp)
-#  else
-#    error "We don't know a correct way to implement alignof(T) in C++03 outside of Clang"
-#  endif
-
-#  define _LIBCPP_PREFERRED_ALIGNOF(_Tp) __alignof(_Tp)
-
-#  if defined(_LIBCPP_COMPILER_CLANG_BASED)
 
-#    if defined(__APPLE__) && !defined(__i386__) && !defined(__x86_64__) && (!defined(__arm__) || __ARM_ARCH_7K__ >= 2)
-#      define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
-#    endif
-
-#    if __has_feature(cxx_alignas)
-#      define _ALIGNAS_TYPE(x) alignas(x)
-#      define _ALIGNAS(x) alignas(x)
-#    else
-#      define _ALIGNAS_TYPE(x) __attribute__((__aligned__(_LIBCPP_ALIGNOF(x))))
-#      define _ALIGNAS(x) __attribute__((__aligned__(x)))
-#    endif
+#    define _LIBCPP_ALIGNOF(_Tp) alignof(_Tp)
+#    define _ALIGNAS_TYPE(x) alignas(x)
+#    define _ALIGNAS(x) alignas(x)
+#    define _LIBCPP_NORETURN [[noreturn]]
+#    define _NOEXCEPT noexcept
+#    define _NOEXCEPT_(x) noexcept(x)
+
+#  else
+
+#    define _LIBCPP_ALIGNOF(_Tp) _Alignof(_Tp)
+#    define _ALIGNAS_TYPE(x) __attribute__((__aligned__(_LIBCPP_ALIGNOF(x))))
+#    define _ALIGNAS(x) __attribute__((__aligned__(x)))
+#    define _LIBCPP_NORETURN __attribute__((noreturn))
+#    define _LIBCPP_HAS_NO_NOEXCEPT
+#    define nullptr __nullptr
+#    define _NOEXCEPT throw()
+#    define _NOEXCEPT_(x)
 
-#    if __cplusplus < 201103L
 typedef __char16_t char16_t;
 typedef __char32_t char32_t;
-#    endif
+
+#  endif
 
-#    if !__has_feature(cxx_exceptions)
-#      define _LIBCPP_NO_EXCEPTIONS
-#    endif
+#  if !defined(__cpp_exceptions) || __cpp_exceptions < 199711L
+#    define _LIBCPP_NO_EXCEPTIONS
+#  endif
 
-#    if __has_feature(cxx_attributes)
-#      define _LIBCPP_NORETURN [[noreturn]]
-#    else
-#      define _LIBCPP_NORETURN __attribute__((noreturn))
-#    endif
+#  define _LIBCPP_PREFERRED_ALIGNOF(_Tp) __alignof(_Tp)
 
-#    ifdef _LIBCPP_CXX03_LANG
-#      define nullptr __nullptr
+#  if defined(_LIBCPP_COMPILER_CLANG_BASED)
+
+#    if defined(__APPLE__) && !defined(__i386__) && !defined(__x86_64__) && (!defined(__arm__) || __ARM_ARCH_7K__ >= 2)
+#      define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 #    endif
 
 // Objective-C++ features (opt-in)
 #    if __has_feature(objc_arc)
 #      define _LIBCPP_HAS_OBJC_ARC
 #    endif
 
 #    if __has_feature(objc_arc_weak)
 #      define _LIBCPP_HAS_OBJC_ARC_WEAK
 #    endif
 
 #    if __has_extension(blocks)
 #      define _LIBCPP_HAS_EXTENSION_BLOCKS
 #    endif
 
 #    if defined(_LIBCPP_HAS_EXTENSION_BLOCKS) && defined(__APPLE__)
 #      define _LIBCPP_HAS_BLOCKS_RUNTIME
 #    endif
 
-#    if !(__has_feature(cxx_noexcept))
-#      define _LIBCPP_HAS_NO_NOEXCEPT
-#    endif
-
 #    if !__has_feature(address_sanitizer)
 #      define _LIBCPP_HAS_NO_ASAN
 #    endif
 
 // Allow for build-time disabling of unsigned integer sanitization
 #    if __has_attribute(no_sanitize)
 #      define _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK __attribute__((__no_sanitize__("unsigned-integer-overflow")))
 #    endif
 
 #    define _LIBCPP_ALWAYS_INLINE __attribute__((__always_inline__))
 
 #    define _LIBCPP_DISABLE_EXTENSION_WARNING __extension__
 
 #  elif defined(_LIBCPP_COMPILER_GCC)
 
-#    define _ALIGNAS(x) __attribute__((__aligned__(x)))
-#    define _ALIGNAS_TYPE(x) __attribute__((__aligned__(_LIBCPP_ALIGNOF(x))))
-
-#    define _LIBCPP_NORETURN __attribute__((noreturn))
-
-#    if !defined(__EXCEPTIONS)
-#      define _LIBCPP_NO_EXCEPTIONS
-#    endif
-
 #    if !defined(__SANITIZE_ADDRESS__)
 #      define _LIBCPP_HAS_NO_ASAN
 #    endif
 
 #    define _LIBCPP_ALWAYS_INLINE __attribute__((__always_inline__))
 
 #    define _LIBCPP_DISABLE_EXTENSION_WARNING __extension__
 
 #  elif defined(_LIBCPP_COMPILER_MSVC)
 
 #    define _LIBCPP_WARNING(x) __pragma(message(__FILE__ "(" _LIBCPP_TOSTRING(__LINE__) ") : warning note: " x))
 
 #    if _MSC_VER < 1900
 #      error "MSVC versions prior to Visual Studio 2015 are not supported"
 #    endif
 
-#    define __alignof__ __alignof
 #    define _LIBCPP_NORETURN __declspec(noreturn)
-#    define _ALIGNAS(x) __declspec(align(x))
-#    define _ALIGNAS_TYPE(x) alignas(x)
 
 #    define _LIBCPP_WEAK
 
 #    define _LIBCPP_HAS_NO_ASAN
 
 #    define _LIBCPP_ALWAYS_INLINE __forceinline
 
 #    define _LIBCPP_HAS_NO_VECTOR_EXTENSION

 #    define _LIBCPP_HIDDEN
 #    define _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
 #    define _LIBCPP_TEMPLATE_VIS
 #    define _LIBCPP_TEMPLATE_DATA_VIS
 #    define _LIBCPP_ENUM_VIS
 
 #  endif // defined(_LIBCPP_OBJECT_FORMAT_COFF)
 
 #  ifndef _LIBCPP_HIDDEN
 #    if !defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
 #      define _LIBCPP_HIDDEN __attribute__((__visibility__("hidden")))
 #    else
 #      define _LIBCPP_HIDDEN
 #    endif
 #  endif
 
 #  ifndef _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS

 // clang-format on
 
 #  define _VSTD_FS std::__fs::filesystem
 
 #  if __has_attribute(__enable_if__)
 #    define _LIBCPP_PREFERRED_OVERLOAD __attribute__((__enable_if__(true, "")))
 #  endif
 
-#  ifndef _LIBCPP_HAS_NO_NOEXCEPT
-#    define _NOEXCEPT noexcept
-#    define _NOEXCEPT_(x) noexcept(x)
-#  else
-#    define _NOEXCEPT throw()
-#    define _NOEXCEPT_(x)
-#  endif
-
 #  ifndef __SIZEOF_INT128__
 #    define _LIBCPP_HAS_NO_INT128
 #  endif
 
 #  ifdef _LIBCPP_CXX03_LANG
 #    define static_assert(...) _Static_assert(__VA_ARGS__)
 #    define decltype(...) __decltype(__VA_ARGS__)
 #  endif // _LIBCPP_CXX03_LANG

 #    define _LIBCPP_NO_DESTROY
 #  endif
 
 #  ifndef _LIBCPP_HAS_NO_ASAN
 extern "C" _LIBCPP_FUNC_VIS void __sanitizer_annotate_contiguous_container(const void*, const void*, const void*,
                                                                            const void*);
 #  endif
 
 // Try to find out if RTTI is disabled.
-#  if defined(_LIBCPP_COMPILER_CLANG_BASED) && !__has_feature(cxx_rtti)
-#    define _LIBCPP_NO_RTTI
-#  elif defined(__GNUC__) && !defined(__GXX_RTTI)
-#    define _LIBCPP_NO_RTTI
-#  elif defined(_LIBCPP_COMPILER_MSVC) && !defined(_CPPRTTI)
+#  if !defined(__cpp_rtti) || __cpp_rtti < 199711L
 #    define _LIBCPP_NO_RTTI
 #  endif
 
 #  ifndef _LIBCPP_WEAK
 #    define _LIBCPP_WEAK __attribute__((__weak__))
 #  endif
 
 // Thread API

 #    define _LIBCPP_DIAGNOSE_WARNING(...) __attribute__((diagnose_if(__VA_ARGS__, "warning")))
 #    define _LIBCPP_DIAGNOSE_ERROR(...) __attribute__((diagnose_if(__VA_ARGS__, "error")))
 #  else
 #    define _LIBCPP_DIAGNOSE_WARNING(...)
 #    define _LIBCPP_DIAGNOSE_ERROR(...)
 #  endif
 
 // Use a function like macro to imply that it must be followed by a semicolon
-#  if _LIBCPP_STD_VER > 14 && __has_cpp_attribute(fallthrough)
+#  if __has_cpp_attribute(fallthrough)
 #    define _LIBCPP_FALLTHROUGH() [[fallthrough]]
-#  elif __has_cpp_attribute(clang::fallthrough)
-#    define _LIBCPP_FALLTHROUGH() [[clang::fallthrough]]
 #  elif __has_attribute(__fallthrough__)
 #    define _LIBCPP_FALLTHROUGH() __attribute__((__fallthrough__))
 #  else
 #    define _LIBCPP_FALLTHROUGH() ((void)0)
 #  endif
 
 #  if __has_attribute(__nodebug__)
 #    define _LIBCPP_NODEBUG __attribute__((__nodebug__))