[lttng-dev] [PATCH 1/3] urcu: avoid "expression result unused" from clang

[Mathieu Desnoyers]

* Lai Jiangshan (eag0628 at gmail.com) wrote:
> How about the one in yy.diff?
> 
> xx.diff fixes the one what I had sent in 1/3 patch, but it may still
> cause warning in future if the complier become more strict.

Hrm. In any cases, I think that even with the 2 patches you propose, the
warning "unused expression return" would still be valid (because we end
up not using the result of the outer statement-expression). We might
want to consider simpler approaches:

1 - Add the missing (void) casts before each CMM_STORE_SHARED() in the
    code. I don't like this because it makes the CMM_STORE_SHARED common
    case more cumbersome to use.
2 - Change the CMM_STORE_SHARED() API so it does not return anything.
    I think most use-cases don't involve using its return value anyway.

Thoughts ?

Thanks,

Mathieu

> 
> On Sat, Dec 8, 2012 at 1:27 AM, Lai Jiangshan <eag0628 at gmail.com> wrote:
> >
> >
> > On Saturday, December 8, 2012, Mathieu Desnoyers wrote:
> >>
> >> * Lai Jiangshan (laijs at cn.fujitsu.com) wrote:
> >> > The last expression result is unused and clang will complain.
> >> > The trick in the patch supresses the complaint
> >>
> >> Hrm, but with this patch, gcc complains:
> >>
> >> rculfhash.c:1921:3: warning: variable '_w' set but not used
> >> [-Wunused-but-set-variable]
> >>
> >> using
> >>
> >> gcc version 4.7.2 (Debian 4.7.2-4)
> >
> >
> > Ouch, my gcc is too old
> >
> >
> >>
> >>
> >> Thoughts ?
> >>
> >> Thanks,
> >>
> >> Mathieu
> >>
> >> >
> >> > Signed-off-by: Lai Jiangshan <laijs at cn.fujitsu.com>
> >> > ---
> >> >  urcu/system.h        |    5 ++++-
> >> >  2 files changed, 4 insertions(+), 1 deletions(-)
> >> >
> >> > diff --git a/urcu/system.h b/urcu/system.h
> >> > index 2a45f22..6b7b0af 100644
> >> > --- a/urcu/system.h
> >> > +++ b/urcu/system.h
> >> > @@ -46,12 +46,15 @@
> >> >  /*
> >> >   * Store v into x, where x is located in shared memory. Performs the
> >> >   * required cache flush after writing. Returns v.
> >> > + * "_w" here avoids the warning from clang:
> >> > + *   warning: expression result unused [-Wunused-value]
> >> >   */
> >> >  #define CMM_STORE_SHARED(x, v)               \
> >> >       ({                              \
> >> >               __typeof__(x) _v = _CMM_STORE_SHARED(x, v);     \
> >> > +             __typeof__(x) _w;       \
> >> >               cmm_smp_wmc();          \
> >> > -             _v;                     \
> >> > +             _w = _v;                \
> >> >       })
> >> >
> >> >  #endif /* _URCU_SYSTEM_H */
> >> > --
> >> > 1.7.4.4
> >> >
> >> >
> >> > _______________________________________________
> >> > lttng-dev mailing list
> >> > lttng-dev at lists.lttng.org
> >> > http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
> >>
> >>
> >> --
> >> Mathieu Desnoyers
> >> Operating System Efficiency R&D Consultant
> >> EfficiOS Inc.
> >> http://www.efficios.com
> >>
> >> _______________________________________________
> >> lttng-dev mailing list
> >> lttng-dev at lists.lttng.org
> >> http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev




-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com