[cfe-dev] The Conditional Operator and Unused Results

[Pierre Habouzit]

On Sun, Oct 03, 2010 at 01:44:22AM +0200, Pierre Habouzit wrote:
> On Sat, Oct 02, 2010 at 02:51:36PM -0600, Justin Bogner wrote:
> > Chris Lattner <clattner at apple.com> writes:
> > > What is the real-world code you're seeing this on, and what diagnostic are you getting?
> > 
> > There are two cases in the code base I was compiling. The first is
> > setting up a threading domain, of which the relevent bits are simply:
> > 
> >     #define _GNU_SOURCE
> >     #include <sched.h>
> >     extern cpu_set_t cpuMask;
> >     
> >     void setCpuAffinity(const int cpu)
> >     {
> >         CPU_ZERO( &cpuMask );
> >         CPU_SET( cpu, &cpuMask );  
> >     }
> > 
> > This gives me:
> > 
> >     $ ./Debug+Asserts/bin/clang -fsyntax-only ~/tmp/cpuset.c 
> >     /home/bogner/tmp/cpuset.c:8:5: warning: expression result unused [-Wunused-value]
> >         CPU_SET( cpu, &cpuMask );  
> >         ^~~~~~~~~~~~~~~~~~~~~~~~
> >     In file included from /home/bogner/tmp/cpuset.c:2:
> >     /usr/include/sched.h:72:33: note: instantiated from:
> >     # define CPU_SET(cpu, cpusetp)   __CPU_SET_S (cpu, sizeof (cpu_set_t), cpusetp)
> >                                      ^
> >     In file included from /home/bogner/tmp/cpuset.c:2:
> >     In file included from /usr/include/sched.h:35:
> >     /usr/include/bits/sched.h:145:9: note: instantiated from:
> >           : 0; }))
> >             ^
> >     1 warning generated.
> > 
> > This is the more serious case, because It's not easy to suppress
> > warnings from standard headers. The other case involved an iterator
> > foreach macro, which has something like:
> > 
> >     for (...; ...; doNextIteration ? ++i : i)
> > 
> > This one's not so bad, as I can change it to:
> > 
> >     for (...; ...; (void)(doNextIteration ? ++i : i))
> 
> I opened http://llvm.org/bugs/show_bug.cgi?id=8218 about this a few days
> ago. GCC heuristic seems that the return value of an expression
> statement not being used is just fine.
> 
> And like I say in the bug report it makes sense because expression
> statements are usually used through a macro, that looks like a funcall,
> and well, if clang doesn't warn if you don't catch the return value of a
> function call, then it shouldn't warn if you don't catch the return
> value of an expression statement.

And I even provided a patch now for that. It fixes the CPU_SET case.

-- 
·O·  Pierre Habouzit
··O                                                madcoder at debian.org
OOO                                                http://www.madism.org