[dpdk-dev] [PATCH] eal/common: better likely() and unlikely()

[Aleksey Baulin]

Sorry for late response. Jim had given the correct answer already.
You won't get an extra instruction with compiler optimization turned on.

Aleksey.

On Mon, Nov 20, 2017 at 4:36 PM, Wiles, Keith <keith.wiles at intel.com> wrote:

>
>
> > On Nov 19, 2017, at 4:16 PM, Aleksey Baulin <aleksey.baulin at gmail.com>
> wrote:
> >
> > A warning is issued when using an argument to likely() or unlikely()
> > builtins which is evaluated to a pointer value, as __builtin_expect()
> > expects a 'long int' type for its first argument. With this fix
> > a pointer value is converted to an integer with the value of 0 or 1.
> >
> > Signed-off-by: Aleksey Baulin <Aleksey.Baulin at gmail.com>
> > ---
> > lib/librte_eal/common/include/rte_branch_prediction.h | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/lib/librte_eal/common/include/rte_branch_prediction.h
> b/lib/librte_eal/common/include/rte_branch_prediction.h
> > index a6a56d1..2e7dc69 100644
> > --- a/lib/librte_eal/common/include/rte_branch_prediction.h
> > +++ b/lib/librte_eal/common/include/rte_branch_prediction.h
> > @@ -50,7 +50,7 @@
> >  *
> >  */
> > #ifndef likely
> > -#define likely(x)  __builtin_expect((x),1)
> > +#define likely(x)    __builtin_expect(!!(x), 1)
> > #endif /* likely */
> >
> > /**
> > @@ -64,7 +64,7 @@
> >  *
> >  */
> > #ifndef unlikely
> > -#define unlikely(x)  __builtin_expect((x),0)
> > +#define unlikely(x)  __builtin_expect(!!(x), 0)
>
> I have not looked at the generated code, but does this add some extra
> instruction now to do the !!(x) ?
>
> > #endif /* unlikely */
> >
> > #endif /* _RTE_BRANCH_PREDICTION_H_ */
> > --
> > 2.7.4
> >
>
> Regards,
> Keith
>
>
-- 
Aleksey Baulin