threads/170073: stdatomic.h doesn't use clang builtings for C

[John Baldwin]

The following reply was made to PR threads/170073; it has been noted by GNATS.

From: John Baldwin <jhb at freebsd.org>
To: freebsd-threads at freebsd.org
Cc: Ariane van der Steldt <ariane at stack.nl>,
 freebsd-gnats-submit at freebsd.org,
 David Chisnall <theraven at freebsd.org>,
 Ed Schouten <ed at freebsd.org>
Subject: Re: threads/170073: stdatomic.h doesn't use clang builtings for C
Date: Mon, 30 Jul 2012 14:15:59 -0400

 On Sunday, July 22, 2012 3:51:56 pm Ariane van der Steldt wrote:
 > 
 > >Number:         170073
 > >Category:       threads
 > >Synopsis:       stdatomic.h doesn't use clang builtings for C
 > >Confidential:   no
 > >Severity:       non-critical
 > >Priority:       low
 > >Responsible:    freebsd-threads
 > >State:          open
 > >Quarter:        
 > >Keywords:       
 > >Date-Required:
 > >Class:          sw-bug
 > >Submitter-Id:   current-users
 > >Arrival-Date:   Sun Jul 22 20:00:18 UTC 2012
 > >Closed-Date:
 > >Last-Modified:
 > >Originator:     Ariane van der Steldt
 > >Release:        freebsd HEAD
 > >Organization:
 > >Environment:
 > FreeBSD mud.stack.nl 9.0-STABLE FreeBSD 9.0-STABLE #0: Thu Jun 14 13:51:30 
 CEST 2012     root at mud.stack.nl:/usr/obj/usr/src/sys/GENERIC  amd64
 > >Description:
 > First a little disclaimer: I tested this with clang-3.1 on bitrig. The 
 freebsd I ran this on is using clang-3.0 which does returns 0 for the 
 c{,xx}_atomic feature/extension.
 > 
 > stdatomic.h tests for __has_feature(cxx_atomic) which is only true when 
 compiling in c++11 mode.
 > However since stdatomic.h is a C header file, it should also work in c11 
 mode.
 > 
 > Running:
 >   echo '__has_feature(cxx_atomic)' | clang-3.1 -x c -std=c11 -E -
 > yields 0, while running:
 >   echo '__has_feature(c_atomic)' | clang-3.1 -x c -std=c11 -E -
 > yields 1.
 > 
 > In addition, I'm contemplating if __has_feature should be replaced by 
 __has_extension, so that non-c11 code can also include stdatomic.h.
 > >How-To-Repeat:
 > Run the clang 3.1 preprocessor on stdatomic.h for C code and it will use the 
 GNU counterparts of the contained code. Running the same code through the 
 clang++ 3.1 preprocessor yields the clang counterpart of the code.
 > >Fix:
 > See provided diff below, using __has_extension instead of __has_feature.
 > This also requires a compatibility define in sys/cdefs.h:
 > #ifndef __has_extension
 > #define __has_extension(x) 0
 > #endif
 > 
 > 
 > diff --git a/include/stdatomic.h b/include/stdatomic.h
 > index b0d1ea9..89f77dd 100644
 > --- a/include/stdatomic.h
 > +++ b/include/stdatomic.h
 > @@ -33,7 +33,7 @@
 >  #include <sys/cdefs.h>
 >  #include <sys/_types.h>
 >  
 > -#if __has_feature(cxx_atomic)
 > +#if __has_extension(c_atomic) || __has_extension(cxx_atomic)
 >  #define        __CLANG_ATOMICS
 >  #elif __GNUC_PREREQ__(4, 7)
 >  #define        __GNUC_ATOMICS
 
 Kicking this over to David and Ed who have worked on stdatomic.h.
 
 -- 
 John Baldwin