[libc++] Rely solely on using_if_exists for ::aligned_alloc, ::quick_exit and ::timespec_get

[ldionne]

Summary

Instead of carrying around #ifdefs to determine whether those functions
are available on the platform, use the using_if_exists attribute (with
Clang) to import it into namespace std only when available. That was the
purpose of this attribute from the start.

This change means that trying to use libc++ with an old SDK (or on an
old platform for platforms that ship system headers in /usr/include)
will require a recent Clang that supports the using_if_exists attribute.
When using an older Clang or GCC, the underlying platform has to support
a C11 standard library.

The motivation for this change is that our logic for determining whether
those functions are provided was wrong in at least some cases (some Apple
platforms). Instead of fixing that logic, the goal has always been to
remove it entirely in favour of the using_if_exists attribute, so that's
what this patch does.

[ldionne]

Pinging a few vendors to make sure this change isn't an issue for them (please read the change description to understand the impact).

[ldionne]

Expand the change to timespec_get and quick_exit.

Note that we'll have to wait for a while before we can submit this change, since we need to drop support
for Clang 12 (which doesn't support the attribute). I'm putting this patch up for review to give a heads
up to vendors and to show that this is the ideal direction for the library, but I'll also have to fix the
incorrect logic separately on Apple platforms.

[phosek]

We always build libc++ with the just built Clang so from our perspective this change should have no impact but thanks for checking.

[dim]

In D108203#2949462, @ldionne wrote:

Expand the change to timespec_get and quick_exit.

Note that we'll have to wait for a while before we can submit this change, since we need to drop support
for Clang 12 (which doesn't support the attribute).

Hm, that's a pity, I wasn't aware that this attribute was introduced so relatively recently. We have clang (and libc++) 11.0.1 in FreeBSD 12.x, which is going to be supported for a while, still. FreeBSD 13.0 and later have clang/libc++ 12.0.1.

I'm putting this patch up for review to give a heads
up to vendors and to show that this is the ideal direction for the library, but I'll also have to fix the
incorrect logic separately on Apple platforms.

In FreeBSD we always import mostly complete copies of the llvm-project tree (except for the parts we don't use), so clang and libc++ are always upgraded at the same time. That said, we also support building the base system (including libc++) using other toolchains such as external gcc's, but we will be able to add version checks for those.

@emaste I think FreeBSD has had aligned_alloc, quick_exit and timespec_get for a very long time now? At least for all supported versions?

[ldionne]

In D108203#2950077, @dim wrote:

In D108203#2949462, @ldionne wrote:

Expand the change to timespec_get and quick_exit.

Note that we'll have to wait for a while before we can submit this change, since we need to drop support
for Clang 12 (which doesn't support the attribute).

Hm, that's a pity, I wasn't aware that this attribute was introduced so relatively recently. We have clang (and libc++) 11.0.1 in FreeBSD 12.x, which is going to be supported for a while, still. FreeBSD 13.0 and later have clang/libc++ 12.0.1.

FWIW, I think that's fine. We have a compiler support policy and we should stand by it. We negotiated that policy so it would be reasonable for both users of libc++ and the libc++ project itself, waiting approximately a year before ripping out this workaround is really not a big deal. What's important is that we can do it in a reasonable time frame, and I think that's enough.

For the time being, I'm about to check-in a patch that bypasses this whole logic whenever the compiler supports using_if_exists, so from our perspective, this code won't be used anymore anyways.

[ldionne]

Rebase onto D108297. I'll land this once we drop support for Clang 12.

[ldionne]

Rebase onto main

[ldionne]

Rebase onto main. This should be ready to land since we don't support compilers without using_if_exists (except GCC, but we assume a proper C11 implementation on that compiler).

libcxx/include/__config

 // a MS compatibility version is specified.
 #  if defined(_MSC_VER) && !defined(__MINGW32__)
 #    define _LIBCPP_MSVCRT // Using Microsoft's C Runtime library
 #  endif
 #  if (defined(_M_AMD64) || defined(__x86_64__)) || (defined(_M_ARM) || defined(__arm__))
 #    define _LIBCPP_HAS_BITSCAN64
 #  endif
 #  define _LIBCPP_HAS_OPEN_WITH_WCHAR
-#  if defined(_LIBCPP_MSVCRT)
-#    define _LIBCPP_HAS_QUICK_EXIT
-#  endif
 
 // Some CRT APIs are unavailable to store apps
 #  if defined(WINAPI_FAMILY)
 #    include <winapifamily.h>
 #    if !WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP) &&                  \
         (!defined(WINAPI_PARTITION_SYSTEM) ||                                  \
          !WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_SYSTEM))
 #      define _LIBCPP_WINDOWS_STORE_APP

 #endif // !defined(_LIBCPP_LITTLE_ENDIAN) && !defined(_LIBCPP_BIG_ENDIAN)
 
 #if __has_attribute(__no_sanitize__) && !defined(_LIBCPP_COMPILER_GCC)
 #  define _LIBCPP_NO_CFI __attribute__((__no_sanitize__("cfi")))
 #else
 #  define _LIBCPP_NO_CFI
 #endif
 
-// If the compiler supports using_if_exists, pretend we have those functions and they'll
-// be picked up if the C library provides them.
-//
-// TODO: Once we drop support for Clang 12, we can assume the compiler supports using_if_exists
-//       for platforms that don't have a conforming C11 library, so we can drop this whole thing.
-#if __has_attribute(using_if_exists)
-# define _LIBCPP_HAS_TIMESPEC_GET
-# define _LIBCPP_HAS_QUICK_EXIT
-# define _LIBCPP_HAS_ALIGNED_ALLOC
-#else
-#if (defined(__ISO_C_VISIBLE) && (__ISO_C_VISIBLE >= 2011)) || __cplusplus >= 201103L
-#  if defined(__FreeBSD__)
-#    define _LIBCPP_HAS_ALIGNED_ALLOC
-#    define _LIBCPP_HAS_QUICK_EXIT
-#    if __FreeBSD_version >= 1300064 || \
-       (__FreeBSD_version >= 1201504 && __FreeBSD_version < 1300000)
-#      define _LIBCPP_HAS_TIMESPEC_GET
-#    endif
-#  elif defined(__BIONIC__)
-#    if __ANDROID_API__ >= 21
-#      define _LIBCPP_HAS_QUICK_EXIT
-#    endif
-#    if __ANDROID_API__ >= 28
-#      define _LIBCPP_HAS_ALIGNED_ALLOC
-#    endif
-#    if __ANDROID_API__ >= 29
-#      define _LIBCPP_HAS_TIMESPEC_GET
-#    endif
-#  elif defined(__Fuchsia__) || defined(__wasi__) || defined(__NetBSD__)
-#    define _LIBCPP_HAS_ALIGNED_ALLOC
-#    define _LIBCPP_HAS_QUICK_EXIT
-#    define _LIBCPP_HAS_TIMESPEC_GET
-#  elif defined(__OpenBSD__)
-#    define _LIBCPP_HAS_ALIGNED_ALLOC
-#    define _LIBCPP_HAS_TIMESPEC_GET
-#  elif defined(__linux__)
-#    if !defined(_LIBCPP_HAS_MUSL_LIBC)
-#      if _LIBCPP_GLIBC_PREREQ(2, 15) || defined(__BIONIC__)
-#        define _LIBCPP_HAS_QUICK_EXIT
-#      endif
-#      if _LIBCPP_GLIBC_PREREQ(2, 17)
-#        define _LIBCPP_HAS_ALIGNED_ALLOC
-#        define _LIBCPP_HAS_TIMESPEC_GET
-#      endif
-#    else // defined(_LIBCPP_HAS_MUSL_LIBC)
-#      define _LIBCPP_HAS_ALIGNED_ALLOC
-#      define _LIBCPP_HAS_QUICK_EXIT
-#      define _LIBCPP_HAS_TIMESPEC_GET
-#    endif
-#  elif defined(_LIBCPP_MSVCRT)
-     // Using Microsoft's C Runtime library, not MinGW
-#    define _LIBCPP_HAS_TIMESPEC_GET
-#  elif defined(__APPLE__)
-     // timespec_get and aligned_alloc were introduced in macOS 10.15 and
-     // aligned releases
-#    if ((defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ >= 101500) || \
-         (defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ >= 130000) || \
-         (defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__ >= 130000) || \
-         (defined(__ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__ >= 60000))
-#      define _LIBCPP_HAS_ALIGNED_ALLOC
-#      define _LIBCPP_HAS_TIMESPEC_GET
-#    endif
-#  endif // __APPLE__
-#endif
-#endif // __has_attribute(using_if_exists)
-
 #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
 

libcxx/include/cstdlib

 using ::div _LIBCPP_USING_IF_EXISTS;
 using ::ldiv _LIBCPP_USING_IF_EXISTS;
 using ::lldiv _LIBCPP_USING_IF_EXISTS;
 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)
+#if !defined(_LIBCPP_CXX03_LANG)
 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)
+#if _LIBCPP_STD_VER > 14
 using ::aligned_alloc _LIBCPP_USING_IF_EXISTS;
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_CSTDLIB

libcxx/include/ctime

 
 #include <__config>
 #include <time.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
-// FIXME:
-// Apple SDKs don't define ::timespec_get unconditionally in C++ mode. This
-// should be fixed in future SDKs, but for the time being we need to avoid
-// trying to use that declaration when the SDK doesn't provide it. Note that
-// we're detecting this here instead of in <__config> because we can't include
-// system headers from <__config>, since it leads to circular module dependencies.
-// This is also meant to be a very temporary workaround until the SDKs are fixed.
-#if defined(__APPLE__) && !__has_attribute(using_if_exists)
-#   include <sys/cdefs.h>
-#   if defined(_LIBCPP_HAS_TIMESPEC_GET) && (__DARWIN_C_LEVEL < __DARWIN_C_FULL)
-#       define _LIBCPP_HAS_TIMESPEC_GET_NOT_ACTUALLY_PROVIDED
-#   endif
-#endif
-
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 using ::clock_t _LIBCPP_USING_IF_EXISTS;
 using ::size_t _LIBCPP_USING_IF_EXISTS;
 using ::time_t _LIBCPP_USING_IF_EXISTS;
 using ::tm _LIBCPP_USING_IF_EXISTS;
-#if _LIBCPP_STD_VER > 14 && defined(_LIBCPP_HAS_TIMESPEC_GET)
+#if _LIBCPP_STD_VER > 14
 using ::timespec _LIBCPP_USING_IF_EXISTS;
 #endif
 using ::clock _LIBCPP_USING_IF_EXISTS;
 using ::difftime _LIBCPP_USING_IF_EXISTS;
 using ::mktime _LIBCPP_USING_IF_EXISTS;
 using ::time _LIBCPP_USING_IF_EXISTS;
 using ::asctime _LIBCPP_USING_IF_EXISTS;
 using ::ctime _LIBCPP_USING_IF_EXISTS;
 using ::gmtime _LIBCPP_USING_IF_EXISTS;
 using ::localtime _LIBCPP_USING_IF_EXISTS;
 using ::strftime _LIBCPP_USING_IF_EXISTS;
-#if _LIBCPP_STD_VER > 14 && defined(_LIBCPP_HAS_TIMESPEC_GET) && !defined(_LIBCPP_HAS_TIMESPEC_GET_NOT_ACTUALLY_PROVIDED)
+#if _LIBCPP_STD_VER > 14
 using ::timespec_get _LIBCPP_USING_IF_EXISTS;
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_CTIME