[libcxx-commits] [PATCH] D108630: [libc++] Remove _LIBCPP_HAS_NO_LONG_LONG in favour of using_if_exists

[Dimitry Andric via Phabricator via libcxx-commits]

dim added inline comments.


================
Comment at: libcxx/include/stdlib.h:153
 }
-#ifndef _LIBCPP_HAS_NO_LONG_LONG
+#if !(defined(__FreeBSD__) && !defined(__LONG_LONG_SUPPORTED))
 inline _LIBCPP_INLINE_VISIBILITY lldiv_t div(long long __x,
----------------
ldionne wrote:
> @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?
> @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`)...



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https://reviews.llvm.org/D108630