[libc++] Remove _LIBCPP_HAS_NO_LONG_LONG in favour of using_if_exists

_LIBCPP_HAS_NO_LONG_LONG was only defined on FreeBSD. Instead, use the
using_if_exists attribute to skip over declarations that are not available
on the base system. Note that there's an annoying limitation that we can't
conditionally define a function based on whether the base system provides
a function, so for example we still need preprocessor logic to define the
abs() and div() overloads.

Differential Revision: https://reviews.llvm.org/D108630

libcxx/include/__config

@@ -255,9 +255,6 @@
 #  else  // _BYTE_ORDER == _LITTLE_ENDIAN
 #    define _LIBCPP_BIG_ENDIAN
 #  endif  // _BYTE_ORDER == _LITTLE_ENDIAN
-#  ifndef __LONG_LONG_SUPPORTED
-#    define _LIBCPP_HAS_NO_LONG_LONG
-#  endif  // __LONG_LONG_SUPPORTED
 #endif // __FreeBSD__
 
 #if defined(__NetBSD__) || defined(__OpenBSD__)

libcxx/include/cstdlib

@@ -99,26 +99,18 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 using ::size_t _LIBCPP_USING_IF_EXISTS;
 using ::div_t _LIBCPP_USING_IF_EXISTS;
 using ::ldiv_t _LIBCPP_USING_IF_EXISTS;
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
 using ::lldiv_t _LIBCPP_USING_IF_EXISTS;
-#endif // _LIBCPP_HAS_NO_LONG_LONG
 using ::atof _LIBCPP_USING_IF_EXISTS;
 using ::atoi _LIBCPP_USING_IF_EXISTS;
 using ::atol _LIBCPP_USING_IF_EXISTS;
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
 using ::atoll _LIBCPP_USING_IF_EXISTS;
-#endif // _LIBCPP_HAS_NO_LONG_LONG
 using ::strtod _LIBCPP_USING_IF_EXISTS;
 using ::strtof _LIBCPP_USING_IF_EXISTS;
 using ::strtold _LIBCPP_USING_IF_EXISTS;
 using ::strtol _LIBCPP_USING_IF_EXISTS;
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
 using ::strtoll _LIBCPP_USING_IF_EXISTS;
-#endif // _LIBCPP_HAS_NO_LONG_LONG
 using ::strtoul _LIBCPP_USING_IF_EXISTS;
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
 using ::strtoull _LIBCPP_USING_IF_EXISTS;
-#endif // _LIBCPP_HAS_NO_LONG_LONG
 using ::rand _LIBCPP_USING_IF_EXISTS;
 using ::srand _LIBCPP_USING_IF_EXISTS;
 using ::calloc _LIBCPP_USING_IF_EXISTS;
@@ -137,14 +129,10 @@ using ::bsearch _LIBCPP_USING_IF_EXISTS;
 using ::qsort _LIBCPP_USING_IF_EXISTS;
 using ::abs _LIBCPP_USING_IF_EXISTS;
 using ::labs _LIBCPP_USING_IF_EXISTS;
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
 using ::llabs _LIBCPP_USING_IF_EXISTS;
-#endif // _LIBCPP_HAS_NO_LONG_LONG
 using ::div _LIBCPP_USING_IF_EXISTS;
 using ::ldiv _LIBCPP_USING_IF_EXISTS;
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
 using ::lldiv _LIBCPP_USING_IF_EXISTS;
-#endif // _LIBCPP_HAS_NO_LONG_LONG
 using ::mblen _LIBCPP_USING_IF_EXISTS;
 using ::mbtowc _LIBCPP_USING_IF_EXISTS;
 using ::wctomb _LIBCPP_USING_IF_EXISTS;

libcxx/include/cwchar

@@ -137,13 +137,9 @@ using ::wcstod _LIBCPP_USING_IF_EXISTS;
 using ::wcstof _LIBCPP_USING_IF_EXISTS;
 using ::wcstold _LIBCPP_USING_IF_EXISTS;
 using ::wcstol _LIBCPP_USING_IF_EXISTS;
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
 using ::wcstoll _LIBCPP_USING_IF_EXISTS;
-#endif // _LIBCPP_HAS_NO_LONG_LONG
 using ::wcstoul _LIBCPP_USING_IF_EXISTS;
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
 using ::wcstoull _LIBCPP_USING_IF_EXISTS;
-#endif // _LIBCPP_HAS_NO_LONG_LONG
 using ::wcscpy _LIBCPP_USING_IF_EXISTS;
 using ::wcsncpy _LIBCPP_USING_IF_EXISTS;
 using ::wcscat _LIBCPP_USING_IF_EXISTS;

libcxx/include/stdlib.h

@@ -96,10 +96,14 @@ void *aligned_alloc(size_t alignment, size_t size);                       // C11
 extern "C++" {
 // abs
 
-#undef abs
-#undef labs
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
-#undef llabs
+#ifdef abs
+# undef abs
+#endif
+#ifdef labs
+# undef labs
+#endif
+#ifdef llabs
+# undef llabs
 #endif
 
 // MSVCRT already has the correct prototype in <stdlib.h> if __cplusplus is defined
@@ -107,11 +111,9 @@ extern "C++" {
 inline _LIBCPP_INLINE_VISIBILITY long abs(long __x) _NOEXCEPT {
   return __builtin_labs(__x);
 }
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
 inline _LIBCPP_INLINE_VISIBILITY long long abs(long long __x) _NOEXCEPT {
   return __builtin_llabs(__x);
 }
-#endif // _LIBCPP_HAS_NO_LONG_LONG
 #endif // !defined(_LIBCPP_MSVCRT) && !defined(__sun__)
 
 #if !defined(__sun__)
@@ -131,10 +133,14 @@ abs(long double __lcpp_x) _NOEXCEPT {
 
 // div
 
-#undef div
-#undef ldiv
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
-#undef lldiv
+#ifdef div
+# undef div
+#endif
+#ifdef ldiv
+# undef ldiv
+#endif
+#ifdef lldiv
+# undef lldiv
 #endif
 
 // MSVCRT already has the correct prototype in <stdlib.h> if __cplusplus is defined
@@ -142,12 +148,12 @@ abs(long double __lcpp_x) _NOEXCEPT {
 inline _LIBCPP_INLINE_VISIBILITY ldiv_t div(long __x, long __y) _NOEXCEPT {
   return ::ldiv(__x, __y);
 }
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
+#if !(defined(__FreeBSD__) && !defined(__LONG_LONG_SUPPORTED))
 inline _LIBCPP_INLINE_VISIBILITY lldiv_t div(long long __x,
                                              long long __y) _NOEXCEPT {
   return ::lldiv(__x, __y);
 }
-#endif // _LIBCPP_HAS_NO_LONG_LONG
+#endif
 #endif // _LIBCPP_MSVCRT / __sun__
 } // extern "C++"
 #endif // __cplusplus