[libc++][AIX] Fix abs and div overload issue for compilers on AIX

[jasonliu]

Summary

AIX system's stdlib.h provide different overload of abs and div depending on compiler versions.
For example, std::div(long, long) and std::abs(long) are not available from OS's stdlib.h when building with clang, but they are available when building with xlclang compiler.
Therefore, we need to provide those extra overloads in libc++'s stdlib.h when OS's stdlib.h does not.

[• Quuxplusone]

libcxx/include/stdlib.h

105–115

I think this is the equivalent of what you have (just significantly disentangled, and making it more obvious that we don't know why __sun__ is in there 😛). What do you think?
I'd also want to see a similar disentangling performed for the floating-point abses.

[jasonliu]

Remove extra conditions because we only intended to target the headers for clang compilers.

[jasonliu]

libcxx/include/stdlib.h

105–115

Thanks Author.
I think we could actually remove the extra conditions on AIX because we are only going to compile this header with the clang compiler. And we don't really need to consider xlclang/xlc because those compilers do not have enough language features to compile the latest libc++ trunk anyway. (Those changes are originally from our downstream libc++ which is a fairly old version)
If you still want that disentangled version as a drive-by fix, let me know and I could do that.

[ldionne]

Please rebase onto main to re-trigger CI. This LGTM if CI passes.

[hubert.reinterpretcast]

Double checked re: scope of __ABS_LONG__. Confirming LGTM re: AIX context.

[jasonliu]

Update diff to trigger pre-CI rebuild.

libcxx/include/cmath

 using ::islessequal;
 using ::islessgreater;
 using ::isunordered;
 using ::isunordered;
 
 using ::float_t;
 using ::double_t;
 
-#ifndef _AIX
 using ::abs;
-#endif
 
 using ::acos;
 using ::acosf;
 using ::asin;
 using ::asinf;
 using ::atan;
 using ::atanf;
 using ::atan2;

libcxx/include/stdlib.h

 // abs
 
 #undef abs
 #undef labs
 #ifndef _LIBCPP_HAS_NO_LONG_LONG
 #undef llabs
 #endif
 
 // MSVCRT already has the correct prototype in <stdlib.h> if __cplusplus is defined
-#if !defined(_LIBCPP_MSVCRT) && !defined(__sun__) && !defined(_AIX)
+#if !defined(_LIBCPP_MSVCRT) && !defined(__sun__)
 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__) && !defined(_AIX)
+#endif // !defined(_LIBCPP_MSVCRT) && !defined(__sun__)
 
-#if !(defined(_AIX) || defined(__sun__))
+#if !defined(__sun__)
 inline _LIBCPP_INLINE_VISIBILITY float abs(float __lcpp_x) _NOEXCEPT {
   return __builtin_fabsf(__lcpp_x); // Use builtins to prevent needing math.h
 }
 
 inline _LIBCPP_INLINE_VISIBILITY double abs(double __lcpp_x) _NOEXCEPT {
   return __builtin_fabs(__lcpp_x);
 }
 
 inline _LIBCPP_INLINE_VISIBILITY long double
 abs(long double __lcpp_x) _NOEXCEPT {
   return __builtin_fabsl(__lcpp_x);
 }
-#endif // !(defined(_AIX) || defined(__sun__))
+#endif // !defined(__sun__)
 
 // div
 
 #undef div
 #undef ldiv
 #ifndef _LIBCPP_HAS_NO_LONG_LONG
 #undef lldiv
 #endif
 
 // MSVCRT already has the correct prototype in <stdlib.h> if __cplusplus is defined
-#if !defined(_LIBCPP_MSVCRT) && !defined(__sun__) && !defined(_AIX)
+#if !defined(_LIBCPP_MSVCRT) && !defined(__sun__)
 inline _LIBCPP_INLINE_VISIBILITY ldiv_t div(long __x, long __y) _NOEXCEPT {
   return ::ldiv(__x, __y);
 }
 #ifndef _LIBCPP_HAS_NO_LONG_LONG
 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 // _LIBCPP_MSVCRT / __sun__ / _AIX
+#endif // _LIBCPP_MSVCRT / __sun__
 } // extern "C++"
 #endif  // __cplusplus
 
 #endif  // _LIBCPP_STDLIB_H