[PATCH] c++: Predefine __STDCPP_THREADS__ in the compiler if thread model is not single

[Jakub Jelinek]

Hi!

The following patch predefines __STDCPP_THREADS__ macro to 1 if c++11 or
later and thread model (e.g. printed by gcc -v) is not single.
There are two targets not handled by this patch, those that define
THREAD_MODEL_SPEC.  In one case - QNX - it looks just like a mistake
to me, instead of setting thread_model=posix in config.gcc it uses
THREAD_MODEL_SPEC macro to set it unconditionally to posix.
The other is hpux10, which uses -threads option to decide if threads
are enabled or not, but that option isn't really passed to the compiler.
I think that is something that really should be solved in config/pa/
instead, e.g. in the config/xxx/xxx-c.c targets usually set their own
predefined macros and it could handle this, and either pass the option
also to the compiler, or say predefine __STDCPP_THREADS__ if _DCE_THREADS
macro is defined already (or -D_DCE_THREADS found on the command line),
or whatever else.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk? 

2020-11-13  Jakub Jelinek  <jakub@redhat.com>

	* c-cppbuiltin.c: Include configargs.h.
	(c_cpp_builtins): For C++11 and later if THREAD_MODEL_SPEC is not
	defined, predefine __STDCPP_THREADS__ to 1 unless thread_model is
	"single".

--- gcc/c-family/c-cppbuiltin.c.jj	2020-10-22 10:37:25.890800306 +0200
+++ gcc/c-family/c-cppbuiltin.c	2020-11-13 12:34:25.290963074 +0100
@@ -32,6 +32,7 @@ along with GCC; see the file COPYING3.
 #include "debug.h"		/* For dwarf2out_do_cfi_asm.  */
 #include "common/common-target.h"
 #include "cppbuiltin.h"
+#include "configargs.h"
 
 #ifndef TARGET_OS_CPP_BUILTINS
 # define TARGET_OS_CPP_BUILTINS()
@@ -1033,6 +1034,12 @@ c_cpp_builtins (cpp_reader *pfile)
 	cpp_define (pfile, "__cpp_threadsafe_static_init=200806L");
       if (flag_char8_t)
         cpp_define (pfile, "__cpp_char8_t=201811L");
+#ifndef THREAD_MODEL_SPEC
+      /* Targets that define THREAD_MODEL_SPEC need to define
+	 __STDCPP_THREADS__ in their config/XXX/XXX-c.c themselves.  */
+      if (cxx_dialect >= cxx11 && strcmp (thread_model, "single") != 0)
+	cpp_define (pfile, "__STDCPP_THREADS__=1");
+#endif
     }
   /* Note that we define this for C as well, so that we know if
      __attribute__((cleanup)) will interface with EH.  */

	Jakub