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

[Lai Jiangshan]

On Saturday, December 8, 2012, Mathieu Desnoyers wrote:

> * Lai Jiangshan (eag0628 at gmail.com <javascript:;>) 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.


I don't like it either, it is not friendly.


> 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.



It will break the original goal:
CMM_STORE_SHARED() can be used as chained assignment:
tmp = CMM_STORE_SHARED(x, v)
CMM_STORE_SHARED(y, CMM_STORE_SHARED(x, v))
func(CMM_STORE_SHARED(x,v))



> Thoughts ?
>
> Thanks,
>
> Mathieu
>
> >
> > On Sat, Dec 8, 2012 at 1:27 AM, Lai Jiangshan <eag0628 at gmail.com<javascript:;>>
> wrote:
> > >
> > >
> > > On Saturday, December 8, 2012, Mathieu Desnoyers wrote:
> > >>
> > >> * Lai Jiangshan (laijs at cn.fujitsu.com <javascript:;>) 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 <javascript:;>>
> > >> > ---
> > >> >  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 <javascript:;>
> > >> > 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 <javascript:;>
> > >> http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
>
>
>
>
> --
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com
>
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