__builtin_expect with other compilers

[Lisandro Dalcin]

On Mon, Nov 30, 2009 at 9:49 AM, Jed Brown <jed at 59a2.org> wrote:
> I noticed that GCC mispredicts the conditional in CHKERRQ.  At high
> optimization levels it still relocates most of the PetscError call, but
> the non-error path is cleaner with
>
>  #define CHKERRQ(n) if (UNLIKELY(n)) {return PetscError(__LINE__,__FUNCT__,__FILE__,__SDIR__,n,0," ");}
>

I wanted to comment on this from long time ago, but never got time to
investigate

> where the following definitions are typical
>
>  #define UNLIKELY(c) __builtin_expect(!!(c),0)
>  #define LIKELY(c)   __builtin_expect(!!(c),1)
>

Indeed. Cython (used in petsc4py) uses these definitions everywhere.

>
> I'm not familiar with the analogues for other compilers.
>

IIRC, Intel compilers do support __builtin_expect, too.

>  I don't
> suggest cluttering the code with such hints, but just putting it in
> CHKERRQ would decrease code size for the non-error path which seems like
> a good thing.
>

A big +1 from my side. This will alleviate (at least with gcc and icc)
some Matt's concerns about miss-predictions we discussed long ago. At
that time, I was asking for optimized builds to "#define CHKERRQ(ierr)
if (ierr) return ierr;" instead of empty.




-- 
Lisandro Dalcín
---------------
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