Re: detection and support of OpenMP

[Bruno Haible]

Noah Misch wrote on Tuesday:
> If we reach the stage of testing additional compiler options, the current $CC
> $CFLAGS failed to compile the test program.  If that compiler treats `-openmp'
> like `-o penmp', it will still fail to compile the test program.  Do we really
> need this special care?
> 
> Possible simplifying patch:
> 
> diff -Nurp -X dontdiff ac-clean/lib/autoconf/c.m4 
> ac-openmp_percomp/lib/autoconf/c.m4
> --- ac-clean/lib/autoconf/c.m4        2007-05-21 18:42:08.000000000 -0400
> +++ ac-openmp_percomp/lib/autoconf/c.m4       2007-05-21 18:54:19.000000000 
> -0400
> @@ -1906,48 +1906,13 @@ AC_DEFUN([AC_OPENMP],
>          dnl   SGI C, PGI C         -mp
>          dnl   Tru64 Compaq C       -omp
>          dnl   IBM C (AIX, Linux)   -qsmp=omp
> -        for ac_brand in GCC SunPRO Intel SGI/PGI Compaq IBM; do
> -          case $ac_brand in
> -            GCC)
> -              ac_conditional='defined __GNUC__'
> -              ac_option='-fopenmp' ;;
> -            SunPRO)
> -              ac_conditional='defined __SUNPRO_C || defined __SUNPRO_CC'
> -              ac_option='-xopenmp' ;;
> -            Intel)
> -              ac_conditional='defined __INTEL_COMPILER'
> -              ac_option='-openmp' ;;
> -            SGI/PGI)
> -              ac_conditional='defined __sgi || defined __PGI || defined 
> __PGIC__'
> -              ac_option='-mp' ;;
> -            Compaq)
> -              ac_conditional='defined __DECC || defined __DECCXX'
> -              ac_option='-omp' ;;
> -            IBM)
> -              ac_conditional='defined __xlc__ || defined __xlC__'
> -              ac_option='-qsmp=omp' ;;
> -          esac
> -          if test $ac_brand = GCC; then
> -            if test "$ac_compiler_gnu" = yes; then
> -              ac_openmp_result=yes
> -            else
> -              ac_openmp_result=no
> -            fi
> -          else
> -            AC_EGREP_CPP([Brand], [
> -              #if $ac_conditional
> -               Brand
> -              #endif
> -              ], [ac_openmp_result=yes], [ac_openmp_result=no])
> -          fi
> -          if test $ac_openmp_result = yes; then
> -            ac_save_[]_AC_LANG_PREFIX[]FLAGS=$[]_AC_LANG_PREFIX[]FLAGS
> -            _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $ac_option"
> -            AC_LINK_IFELSE([_AC_LANG_OPENMP],
> -              [ac_cv_prog_[]_AC_LANG_ABBREV[]_openmp=$ac_option])
> -            _AC_LANG_PREFIX[]FLAGS=$ac_save_[]_AC_LANG_PREFIX[]FLAGS
> -            break
> -          fi
> +        for ac_option in -fopenmp -xopenmp -openmp -mp -omp -qsmp=omp; do
> +          ac_save_[]_AC_LANG_PREFIX[]FLAGS=$[]_AC_LANG_PREFIX[]FLAGS
> +          _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $ac_option"
> +          AC_LINK_IFELSE([_AC_LANG_OPENMP],
> +            [ac_cv_prog_[]_AC_LANG_ABBREV[]_openmp=$ac_option])
> +          _AC_LANG_PREFIX[]FLAGS=$ac_save_[]_AC_LANG_PREFIX[]FLAGS
> +          break
>          done
>        fi
>        ])

Excellent observation. You must be a mathematician :-)

Well, your patch never got to trying the second, third, etc. possibility,
due to the unconditional 'break'. I have tested the appended modified patch:
it works with gcc (with result -fopenmp) and with Sun cc (with result -xopenmp),
therefore I believe the other compilers will work as well. Also I added
the necessary comments.

The good thing about your patch is that it gets rid of the compiler brand
tests, thus the Fortran support should work fine with this version (untested).


2007-05-27  Noah Misch  <address@hidden>
            Bruno Haible   <address@hidden>

        * lib/autoconf/c.m4 (AC_OPENMP): Use a simple loop instead of compiler
        brand tests.

*** lib/autoconf/c.m4.old       2007-05-27 18:16:13.000000000 +0200
--- lib/autoconf/c.m4   2007-05-27 22:45:43.000000000 +0200
***************
*** 1903,1951 ****
          dnl   SGI C, PGI C         -mp
          dnl   Tru64 Compaq C       -omp
          dnl   IBM C (AIX, Linux)   -qsmp=omp
!         for ac_brand in GCC SunPRO Intel SGI/PGI Compaq IBM; do
!           case $ac_brand in #(
!             GCC)
!               ac_conditional='defined __GNUC__'
!               ac_option='-fopenmp' ;; #(
!             SunPRO)
!               ac_conditional='defined __SUNPRO_C || defined __SUNPRO_CC'
!               ac_option='-xopenmp' ;; #(
!             Intel)
!               ac_conditional='defined __INTEL_COMPILER'
!               ac_option='-openmp' ;; #(
!             SGI/PGI)
!               ac_conditional='defined __sgi || defined __PGI || defined 
__PGIC__'
!               ac_option='-mp' ;; #(
!             Compaq)
!               ac_conditional='defined __DECC || defined __DECCXX'
!               ac_option='-omp' ;; #(
!             IBM)
!               ac_conditional='defined __xlc__ || defined __xlC__'
!               ac_option='-qsmp=omp' ;;
!           esac
!           if test $ac_brand = GCC; then
!             if test "$ac_compiler_gnu" = yes; then
!               ac_openmp_result=yes
!             else
!               ac_openmp_result=no
!             fi
!           else
!             AC_EGREP_CPP([Brand],
!               [
!                 #if $ac_conditional
!                  Brand
!                 #endif
!               ],
!               [ac_openmp_result=yes],
!               [ac_openmp_result=no])
!           fi
!           if test $ac_openmp_result = yes; then
!             ac_save_[]_AC_LANG_PREFIX[]FLAGS=$[]_AC_LANG_PREFIX[]FLAGS
!             _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $ac_option"
!             AC_LINK_IFELSE([_AC_LANG_OPENMP],
!               [ac_cv_prog_[]_AC_LANG_ABBREV[]_openmp=$ac_option])
!             _AC_LANG_PREFIX[]FLAGS=$ac_save_[]_AC_LANG_PREFIX[]FLAGS
              break
            fi
          done])])
--- 1903,1920 ----
          dnl   SGI C, PGI C         -mp
          dnl   Tru64 Compaq C       -omp
          dnl   IBM C (AIX, Linux)   -qsmp=omp
!         dnl If in this loop a compiler is passed an option that it doesn't
!         dnl understand or that it misinterprets, the AC_LINK_IFELSE test
!         dnl will fail (since we know that it failed without the option),
!         dnl therefore the loop will continue searching for an option, and
!         dnl no output file called 'penmp' or 'mp' is created.
!         for ac_option in -fopenmp -xopenmp -openmp -mp -omp -qsmp=omp; do
!           ac_save_[]_AC_LANG_PREFIX[]FLAGS=$[]_AC_LANG_PREFIX[]FLAGS
!           _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $ac_option"
!           AC_LINK_IFELSE([_AC_LANG_OPENMP],
!             [ac_cv_prog_[]_AC_LANG_ABBREV[]_openmp=$ac_option])
!           _AC_LANG_PREFIX[]FLAGS=$ac_save_[]_AC_LANG_PREFIX[]FLAGS
!           if test "$ac_cv_prog_[]_AC_LANG_ABBREV[]_openmp" != unsupported; 
then
              break
            fi
          done])])