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[libcxx] Make stdatomic.h work when included from a C source file

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If a C source file includes the libc++ stdatomic.h, compilation will
break because (a) the C++ standard check will fail (which is expected),
and (b) `_LIBCPP_COMPILER_CLANG_BASED` won't be defined because the
logic defining it in `__config` is guarded by a `__cplusplus` check, so
we'll end up with a blank header. Move the detection logic outside of
the `__cplusplus` check to make the second check pass even in a C context
when you're using Clang. Note that `_LIBCPP_STD_VER` is not defined when
in C mode, hence stdatomic.h needs to check if in C++ mode before using
that macro to avoid a warning.

In an ideal world, a C source file wouldn't be including the libc++
header directory in its search path, so we'd never have this issue.
Unfortunately, certain build environments make this hard to guarantee,
and in this case it's easy to tweak this header to make it work in a C
context, so I'm hoping this is acceptable.

Fixes https://github.com/llvm/llvm-project/issues/57710.

Differential Revision: https://reviews.llvm.org/D134591
This commit is contained in:
Gergely Nagy 2022-09-27 07:59:55 -04:00 committed by Louis Dionne
parent 4c6683cd68
commit afec0f0ec3
3 changed files with 13 additions and 12 deletions
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20
libcxx/include/__config
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@ -22,6 +22,16 @@
# pragma GCC system_header
#endif
#if defined(__apple_build_version__)
# define _LIBCPP_COMPILER_CLANG_BASED
# define _LIBCPP_APPLE_CLANG_VER (__apple_build_version__ / 10000)
#elif defined(__clang__)
# define _LIBCPP_COMPILER_CLANG_BASED
# define _LIBCPP_CLANG_VER (__clang_major__ * 100 + __clang_minor__)
#elif defined(__GNUC__)
# define _LIBCPP_COMPILER_GCC
#endif
#ifdef __cplusplus
// _LIBCPP_VERSION represents the version of libc++, which matches the version of LLVM.
@ -202,16 +212,6 @@
# define __has_include(...) 0
# endif
# if defined(__apple_build_version__)
# define _LIBCPP_COMPILER_CLANG_BASED
# define _LIBCPP_APPLE_CLANG_VER (__apple_build_version__ / 10000)
# elif defined(__clang__)
# define _LIBCPP_COMPILER_CLANG_BASED
# define _LIBCPP_CLANG_VER (__clang_major__ * 100 + __clang_minor__)
# elif defined(__GNUC__)
# define _LIBCPP_COMPILER_GCC
# endif
# if !defined(_LIBCPP_COMPILER_CLANG_BASED) && __cplusplus < 201103L
# error "libc++ only supports C++03 with Clang-based compilers. Please enable C++11"
# endif

4
libcxx/include/stdatomic.h
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@ -121,7 +121,7 @@ using std::atomic_signal_fence // see below
# pragma GCC system_header
#endif
#if _LIBCPP_STD_VER > 20
#if defined(__cplusplus) && _LIBCPP_STD_VER > 20
#include <atomic>
#include <version>
@ -230,6 +230,6 @@ using std::atomic_thread_fence _LIBCPP_USING_IF_EXISTS;
# include_next <stdatomic.h>
# endif
#endif // _LIBCPP_STD_VER > 20
#endif // defined(__cplusplus) && _LIBCPP_STD_VER > 20
#endif // _LIBCPP_STDATOMIC_H

1
libcxx/test/libcxx/include_as_c.sh.cpp
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@ -34,6 +34,7 @@
#endif
#include <math.h>
#include <setjmp.h>
#include <stdatomic.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>