Re: With and Without gcc OMP

[Orna Agmon Ben-Yehuda]

On Mon, 24 Sep 2007, Oleg Goldshmidt wrote:

Date: Mon, 24 Sep 2007 19:35:57 +0200
From: Oleg Goldshmidt <[EMAIL PROTECTED]>
Reply-To: linux-il@cs.huji.ac.il
To: Orna Agmon Ben-Yehuda <[EMAIL PROTECTED]>
Cc: linux-il@cs.huji.ac.il
Subject: Re: With and Without gcc OMP

Orna Agmon Ben-Yehuda <[EMAIL PROTECTED]> writes:

Hi All,

I am trying to compile the same code using gcc 4.2, which has gomp
(gnu open mp - a library for automatic thread paralelization) already
in it. I want the code to still compile cleanly (using -Werror)
without openMP at all, because it is said that there is a performance
penalty to compiling with openMP, even if not using it (that is,
defining the number of threads at runtime to be 1).

I would also like to be able to still compile the code cleanly on
prior versions of gcc, specifically on 3.4.* which I currently use.

The problem is that in order to use OMP I need to insert pragmas such as

#pragma omp for

And both gcc 3.4.4 and gcc 4.2 without the -fopenmp flag complain
about ignoring a pragma for each such line.

I tried hiding the pragma using a construct such as

#ifdef HAVE_OMP
#define MYOMP(C) C
#else
#define MYOMP(C)
#endif

Hi Orna,

Have you tried

#ifdef _OPENMP
#pragma omp for
#endif

?

According to the OpenMP spec (http://www.openmp.org) v2.5 _OPENMP must
be defined when the compiler implementation supports preprocessing and
OpenMP. It seems to work for me with gcc 4.1.1 (yes, with -W -Wall
-Werror :) on CentOS 5.

If you also need to base your preprocessing logic on the gcc version use
__GNUC__, __GNUC_MINOR__, and __GNUC_PREREQ() - see features.h. However,
if you don't need that then I think that _OPENMP is the cleanest solution.

Enjoy,



Hi Guys,

Thank you for all your suggestions.

Oleg - I did not try your specific solution, but a similar thing, and as 
Shachar mentioned, indeed #if did the trick where #ifdef did not.
So my current (ugly) solution is:
#if (HAVE_OMP)
#prgama omp for
#endif

I define HAVE_OMP=1 according to the compiler's abilities in the 
configure.in (yes, I do use it, Omer) and change it to HAVE_OMP=0 if the 
current target is not an openmp target. If _OPENMP is not defined when 
-fopenmp is not used, than I can use it to replace my overcomplicated 
makefile bit. But it seems to me that it is intended to hide c code, and 
not pragmas. The whole point of pragmas is that the code can compile even 
if they are not defined.

It is ugly because omp applies to the next code block, which is visually 
intuitive using a one-liner, but currently looks like a mess. It also 
means that the more code I import, the more changes I need to 
make to it.

Omer - when I considered writing a CPPP (C pre pre processor) I also 
considered the poor people who will have to live with the compilation 
system I maintain. I already support *.[fdijh] *.[df]90 *.com *.inc *.c++ 
and various others, with two preprocessors one over the other (fortran's 
built in, different in each fortran compiler, and CPP). "Getting out of 
the box" would make the package unfit to get into any reasonable luggage 
compartment.

Orna.
--
Orna Agmon Ben-Yehuda http://ladypine.org/
ICQ: 348759096


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