libcxx: Define __STDCPP_THREADS__ to 1, not to __cplusplus.

[thakis]

Summary

[cpp.predefined]p2:

__STDCPP_THREADS__
 Defined, and has the value integer literal 1, if and only if a program
 can have more than one thread of execution .

Also define it only if it's not defined already, since it's supposed
to be defined by the compiler.

Part of PR33230.

[EricWF]

libcxx/include/thread

17

This doesn't actually appear in the header synopsis, since it's supposed to be predefined by the compiler.
And if it's supposed to be predefined, I think __config may be a more appropriate place for it to live.

110

This should be defined by the compiler, not the library. So we should only define it if it hasn't already been defined.

libcxx/test/std/thread/macro.pass.cpp

21

Can you add a test that it's defined to 1 specifically?

[EricWF]

This really should be defined by the compiler.

[thakis]

remove from synopsis, define conditional, add test for value

[thakis]

Did most things, except that I kept it in thread for now. PTAL.

libcxx/include/thread

110

Currently it's only defined if thread is included; in __config it'd always be defined. Is the latter what we want? If the compiler doesn't define it, if <thread> is included seems like an ok signal.

[EricWF]

LGTM after moving the define to <__config>.

libcxx/include/thread

110

What if you want to use __STDCPP_THREADS__ to determine if you can include thread?
Defining it in <__config> best matches the correct behavior of having the compiler predefine it.

[thakis]

I started moving this into __config, but that raises several questions:

1. We probably only want to set it there if _LIBCPP_HAS_NO_THREADS is not defined:

#if !defined(_LIBCPP_HAS_NO_THREADS) && !defined(__STDCPP_THREADS__)
#define __STDCPP_THREADS__ 1
#endif

2. If the compiler does set __STDCPP_THREADS__ but _LIBCPP_HAS_NO_THREADS is set, that should probably be an error:

#if defined(__STDCPP_THREADS__) && defined(_LIBCPP_HAS_NO_THREADS)
#error _LIBCPP_HAS_NO_THREADS cannot be set when __STDCPP_THREADS__ is.
#endif

But now if a compiler does set __STDCPP_THREADS__ it's kind of out of libc++'s control. Once the clang half is implemented, maybe there's going to be some flag to cause it to not set that define and we could mention that here – but then again putting compiler-specific diags in libc++ is a bit strange as well.

So I guess I'll land this with those two tweaks and then maybe adjust when I move the define into clang.

[thakis]

Landed in r367316.

libcxx/include/thread

 // -*- C++ -*-
 //===--------------------------- thread -----------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP_THREAD
 #define _LIBCPP_THREAD
 
 /*
 
     thread synopsis
 
-#define __STDCPP_THREADS__ __cplusplus
+#define __STDCPP_THREADS__ 1
 
 namespace std
 {
 
 class thread
 {
 public:
     class id;

 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
-#define __STDCPP_THREADS__ __cplusplus
+#define __STDCPP_THREADS__ 1
 
 #ifdef _LIBCPP_HAS_NO_THREADS
 #error <thread> is not supported on this single threaded system
 #else // !_LIBCPP_HAS_NO_THREADS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp> class __thread_specific_ptr;

libcxx/test/std/thread/macro.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: libcpp-has-no-threads
 
 // <thread>
 
-// #define __STDCPP_THREADS__ __cplusplus
+// #define __STDCPP_THREADS__ 1
 
 #include <thread>
 
 #include "test_macros.h"
 
 int main(int, char**)
 {
 #ifndef __STDCPP_THREADS__
 #error __STDCPP_THREADS__ is not defined
 #endif
 
   return 0;
 }