[libc++] Remove _LIBCPP_HAS_NO_LONG_LONG in favour of using_if_exists

[ldionne]

Summary

_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.

[ldionne]

libcxx/include/stdlib.h

153

@emaste @dim

What does !defined(__LONG_LONG_SUPPORTED) mean exactly? It doesn't mean that the long long type isn't provided by the compiler, because there are several other uses of long long in the library that are not guarded by that check. Is it only about the C Standard Library on FreeBSD not providing ::lldiv? What about __builtin_llabs() -- I would that if long long is a valid type, the compiler does implement __builtin_llabs as used above and we don't need to guard the above use of it?

[dim]

libcxx/include/stdlib.h

153

@emaste @dim

What does !defined(__LONG_LONG_SUPPORTED) mean exactly? It doesn't mean that the long long type isn't provided by the compiler, because there are several other uses of long long in the library that are not guarded by that check. Is it only about the C Standard Library on FreeBSD not providing ::lldiv? What about __builtin_llabs() -- I would that if long long is a valid type, the compiler does implement __builtin_llabs as used above and we don't need to guard the above use of it?

It's a historic thing in our https://github.com/freebsd/freebsd-src/blob/main/include/stdlib.h#L126 and various other standard headers:

/*
 * Functions added in C99 which we make conditionally available in the
 * BSD^C89 namespace if the compiler supports `long long'.
 * The #if test is more complicated than it ought to be because
 * __BSD_VISIBLE implies __ISO_C_VISIBLE == 1999 *even if* `long long'
 * is not supported in the compilation environment (which therefore means
 * that it can't really be ISO C99).
 *
 * (The only other extension made by C99 in thie header is _Exit().)
 */

The definition of __LONG_LONG_SUPPORTED comes from https://github.com/freebsd/freebsd-src/blob/main/sys/sys/cdefs.h#L391 :

#if (defined(__GNUC__) && __GNUC__ >= 2) && !defined(__STRICT_ANSI__) || __STDC_VERSION__ >= 199901
#define	__LONG_LONG_SUPPORTED
#endif

/* C++11 exposes a load of C99 stuff */
#if defined(__cplusplus) && __cplusplus >= 201103L
#define	__LONG_LONG_SUPPORTED
#ifndef	__STDC_LIMIT_MACROS
#define	__STDC_LIMIT_MACROS
#endif
#ifndef	__STDC_CONSTANT_MACROS
#define	__STDC_CONSTANT_MACROS
#endif
#endif

Ergo, C++ standards before C++11 have __LONG_LONG_SUPPORTED undefined. Unfortunately there are still quite a few programs requiring -std=c++98 (or more usually -std=gnu++98)...

[ldionne]

libcxx/include/stdlib.h

153

Okay, thanks a lot for the background. In that case, I think we can even remove the guard from around the use of __builtin_llabs and this patch should be non-breaking.

Would it be possible to add build bots for FreeBSD so we can test changes like those automatically?

[ldionne]

Remove #if around the use of __builtin_llabs

[emaste]

Would it be possible to add build bots for FreeBSD so we can test changes like those automatically?

Yes, this is on my TODO list. I cannot get to it this week but expect to work on it next week (Aug 30 2021-)

[ldionne]

In D108630#2962828, @emaste wrote:

Would it be possible to add build bots for FreeBSD so we can test changes like those automatically?

Yes, this is on my TODO list. I cannot get to it this week but expect to work on it next week (Aug 30 2021-)

Awesome, thanks! In the meantime, does this patch look good to you? I don't think it should break anything if you're compiling with a Clang that supports using_if_exists.

[ldionne]

Gentle ping.

[dim]

Hmm I have applied this on top of our existing libc++ headers, but if I compile a simple program using e.g. -std=c++98 I immediately get errors due to missing declarations:

% cat helloworld.cpp
#include <iostream>

int main(void)
{
  std::cout << "Hello World!" << std::endl;
  return 0;
}

% clang++ -std=c++98 -c helloworld.cpp
In file included from helloworld.cpp:1:
In file included from /usr/include/c++/v1/iostream:37:
In file included from /usr/include/c++/v1/ios:215:
In file included from /usr/include/c++/v1/__locale:15:
In file included from /usr/include/c++/v1/string:511:
In file included from /usr/include/c++/v1/string_view:179:
In file included from /usr/include/c++/v1/__string:57:
In file included from /usr/include/c++/v1/algorithm:653:
In file included from /usr/include/c++/v1/memory:670:
In file included from /usr/include/c++/v1/typeinfo:61:
In file included from /usr/include/c++/v1/exception:83:
/usr/include/c++/v1/cstdlib:102:9: error: no member named 'lldiv_t' in the global namespace; did you mean 'ldiv_t'?
using ::lldiv_t _LIBCPP_USING_IF_EXISTS;
      ~~^
/usr/include/stdlib.h:71:3: note: 'ldiv_t' declared here
} ldiv_t;
  ^
In file included from helloworld.cpp:1:
In file included from /usr/include/c++/v1/iostream:37:
In file included from /usr/include/c++/v1/ios:215:
In file included from /usr/include/c++/v1/__locale:15:
In file included from /usr/include/c++/v1/string:511:
In file included from /usr/include/c++/v1/string_view:179:
In file included from /usr/include/c++/v1/__string:57:
In file included from /usr/include/c++/v1/algorithm:653:
In file included from /usr/include/c++/v1/memory:670:
In file included from /usr/include/c++/v1/typeinfo:61:
In file included from /usr/include/c++/v1/exception:83:
/usr/include/c++/v1/cstdlib:106:9: error: no member named 'atoll' in the global namespace; did you mean 'atol'?
using ::atoll _LIBCPP_USING_IF_EXISTS;
      ~~^
/usr/include/stdlib.h:95:7: note: 'atol' declared here
long     atol(const char *);
         ^
In file included from helloworld.cpp:1:
In file included from /usr/include/c++/v1/iostream:37:
In file included from /usr/include/c++/v1/ios:215:
In file included from /usr/include/c++/v1/__locale:15:
In file included from /usr/include/c++/v1/string:511:
In file included from /usr/include/c++/v1/string_view:179:
In file included from /usr/include/c++/v1/__string:57:
In file included from /usr/include/c++/v1/algorithm:653:
In file included from /usr/include/c++/v1/memory:670:
In file included from /usr/include/c++/v1/typeinfo:61:
In file included from /usr/include/c++/v1/exception:83:
/usr/include/c++/v1/cstdlib:111:9: error: no member named 'strtoll' in the global namespace
using ::strtoll _LIBCPP_USING_IF_EXISTS;
      ~~^
/usr/include/c++/v1/cstdlib:113:9: error: no member named 'strtoull' in the global namespace; did you mean 'strtoul'?
using ::strtoull _LIBCPP_USING_IF_EXISTS;
      ~~^
/usr/include/stdlib.h:121:3: note: 'strtoul' declared here
         strtoul(const char * __restrict, char ** __restrict, int);
         ^
In file included from helloworld.cpp:1:
In file included from /usr/include/c++/v1/iostream:37:
In file included from /usr/include/c++/v1/ios:215:
In file included from /usr/include/c++/v1/__locale:15:
In file included from /usr/include/c++/v1/string:511:
In file included from /usr/include/c++/v1/string_view:179:
In file included from /usr/include/c++/v1/__string:57:
In file included from /usr/include/c++/v1/algorithm:653:
In file included from /usr/include/c++/v1/memory:670:
In file included from /usr/include/c++/v1/typeinfo:61:
In file included from /usr/include/c++/v1/exception:83:
/usr/include/c++/v1/cstdlib:132:9: error: no member named 'llabs' in the global namespace; did you mean 'labs'?
using ::llabs _LIBCPP_USING_IF_EXISTS;
      ~~^
/usr/include/stdlib.h:104:7: note: 'labs' declared here
long     labs(long) __pure2;
         ^
In file included from helloworld.cpp:1:
In file included from /usr/include/c++/v1/iostream:37:
In file included from /usr/include/c++/v1/ios:215:
In file included from /usr/include/c++/v1/__locale:15:
In file included from /usr/include/c++/v1/string:511:
In file included from /usr/include/c++/v1/string_view:179:
In file included from /usr/include/c++/v1/__string:57:
In file included from /usr/include/c++/v1/algorithm:653:
In file included from /usr/include/c++/v1/memory:670:
In file included from /usr/include/c++/v1/typeinfo:61:
In file included from /usr/include/c++/v1/exception:83:
/usr/include/c++/v1/cstdlib:135:9: error: no member named 'lldiv' in the global namespace; did you mean 'ldiv'?
using ::lldiv _LIBCPP_USING_IF_EXISTS;
      ~~^
/usr/include/stdlib.h:105:9: note: 'ldiv' declared here
ldiv_t   ldiv(long, long) __pure2;
         ^
In file included from helloworld.cpp:1:
In file included from /usr/include/c++/v1/iostream:37:
In file included from /usr/include/c++/v1/ios:215:
In file included from /usr/include/c++/v1/__locale:15:
In file included from /usr/include/c++/v1/string:511:
In file included from /usr/include/c++/v1/string_view:179:
In file included from /usr/include/c++/v1/__string:60:
/usr/include/c++/v1/cwchar:140:9: error: no member named 'wcstoll' in the global namespace
using ::wcstoll _LIBCPP_USING_IF_EXISTS;
      ~~^
/usr/include/c++/v1/cwchar:142:9: error: no member named 'wcstoull' in the global namespace; did you mean 'wcstoul'?
using ::wcstoull _LIBCPP_USING_IF_EXISTS;
      ~~^
/usr/include/wchar.h:170:3: note: 'wcstoul' declared here
         wcstoul(const wchar_t * __restrict, wchar_t ** __restrict, int);
         ^
8 errors generated.

But I think this is because my current clang is still 12.0.1? Was using_if_exists only added after 13.0.0 ?

[dim]

In D108630#2979220, @dim wrote:

Hmm I have applied this on top of our existing libc++ headers, but if I compile a simple program using e.g. -std=c++98 I immediately get errors due to missing declarations:

...

But I think this is because my current clang is still 12.0.1? Was using_if_exists only added after 13.0.0 ?

Ah yes indeed, with clang 13 it does work (it's a pity there's no fallback attribute). In any case, if FreeBSD updates to the next version of libc++ it will always be in combination with the corresponding version of clang (and the rest of llvm-project), so in practice it shouldn't be problem, at least for clang users.

Maybe this could give issues for gcc users though, as we sometimes tell people to use libc++ headers so as to avoid incompatibilities between linking libstdc++ and libc++ compiled code (which is isn't supported). Does gcc have any using_if_exists attribute? @emaste is the gcc user case important enough? (I can imagine ports maintainers complaining about this)

[dim]

Ah, gcc doesn't work at all with libc++ before C+11 :) I tried with gcc 12.0.0 20210711, and it spews hundreds of errors, starting off with:

In file included from /usr/include/c++/v1/iostream:36,
                 from helloworld.cpp:1:
/usr/include/c++/v1/__config:238:2: error: #error "libc++ does not support using GCC with C++03. Please enable C++11"
  238 | #error "libc++ does not support using GCC with C++03. Please enable C++11"
      |  ^~~~~
In file included from /usr/include/c++/v1/iostream:36,
                 from helloworld.cpp:1:
/usr/include/c++/v1/__config:445:3: error: #error "We don't know a correct way to implement alignof(T) in C++03 outside of Clang"
  445 | # error "We don't know a correct way to implement alignof(T) in C++03 outside of Clang"
      |   ^~~~~
In file included from /usr/include/c++/v1/iostream:36,
                 from helloworld.cpp:1:

so I guess that use case is out anyway... And in case of >= C++11, we can just assume long long is available, and __LONG_LONG_SUPPORTED will be defined.

[ldionne]

Does gcc have any using_if_exists attribute?

Nope, it doesn't. The idea is that if folks want to use GCC, they need to provide a compliant C Standard Library, or else we have to jump through a bunch of hoops (basically the world as it were before using_if_exists).

So if you update libc++ and Clang in lockstep, I don't think this is going to cause you any trouble.

libcxx/include/__config

 #ifdef __FreeBSD__
 #  include <sys/endian.h>
 #  include <osreldate.h>
 #  if _BYTE_ORDER == _LITTLE_ENDIAN
 #    define _LIBCPP_LITTLE_ENDIAN
 #  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__)
 #  include <sys/endian.h>
 #  if _BYTE_ORDER == _LITTLE_ENDIAN
 #    define _LIBCPP_LITTLE_ENDIAN
 #  else  // _BYTE_ORDER == _LITTLE_ENDIAN
 #    define _LIBCPP_BIG_ENDIAN

libcxx/include/cstdlib

 #define _LIBCPP_UNREACHABLE() _VSTD::abort()
 #endif
 
 _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;
 using ::free _LIBCPP_USING_IF_EXISTS;
 using ::malloc _LIBCPP_USING_IF_EXISTS;
 using ::realloc _LIBCPP_USING_IF_EXISTS;
 using ::abort _LIBCPP_USING_IF_EXISTS;
 using ::atexit _LIBCPP_USING_IF_EXISTS;
 using ::exit _LIBCPP_USING_IF_EXISTS;
 using ::_Exit _LIBCPP_USING_IF_EXISTS;
 #ifndef _LIBCPP_WINDOWS_STORE_APP
 using ::getenv _LIBCPP_USING_IF_EXISTS;
 using ::system _LIBCPP_USING_IF_EXISTS;
 #endif
 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;
 using ::mbstowcs _LIBCPP_USING_IF_EXISTS;
 using ::wcstombs _LIBCPP_USING_IF_EXISTS;
 #if !defined(_LIBCPP_CXX03_LANG) && defined(_LIBCPP_HAS_QUICK_EXIT)
 using ::at_quick_exit _LIBCPP_USING_IF_EXISTS;
 using ::quick_exit _LIBCPP_USING_IF_EXISTS;
 #endif
 #if _LIBCPP_STD_VER > 14 && defined(_LIBCPP_HAS_ALIGNED_ALLOC)
 using ::aligned_alloc _LIBCPP_USING_IF_EXISTS;
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_CSTDLIB

libcxx/include/cwchar

 using ::fwide _LIBCPP_USING_IF_EXISTS;
 using ::getwc _LIBCPP_USING_IF_EXISTS;
 using ::putwc _LIBCPP_USING_IF_EXISTS;
 using ::ungetwc _LIBCPP_USING_IF_EXISTS;
 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;
 using ::wcsncat _LIBCPP_USING_IF_EXISTS;
 using ::wcscmp _LIBCPP_USING_IF_EXISTS;
 using ::wcscoll _LIBCPP_USING_IF_EXISTS;
 using ::wcsncmp _LIBCPP_USING_IF_EXISTS;
 using ::wcsxfrm _LIBCPP_USING_IF_EXISTS;

libcxx/include/stdlib.h

 #endif
 
 #include_next <stdlib.h>
 
 #ifdef __cplusplus
 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
 #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
+#if !(defined(__FreeBSD__) && !defined(__LONG_LONG_SUPPORTED))
 inline _LIBCPP_INLINE_VISIBILITY long long abs(long long __x) _NOEXCEPT {
   return __builtin_llabs(__x);
 }
-#endif // _LIBCPP_HAS_NO_LONG_LONG
+#endif
 #endif // !defined(_LIBCPP_MSVCRT) && !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(__sun__)
 
 // 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
 #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
+#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
 
 #endif // _LIBCPP_STDLIB_H