On Mon, Nov 30, 2009 at 9:49 AM, Jed Brown <[email protected]> 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 --------------- Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC) Instituto de Desarrollo Tecnológico para la Industria Química (INTEC) Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) PTLC - Güemes 3450, (3000) Santa Fe, Argentina Tel/Fax: +54-(0)342-451.1594