Re: detection and support of OpenMP

[Bruno Haible]

Paul Eggert wrote:
> In reference to:
> 
> http://lists.gnu.org/archive/html/autoconf-patches/2007-10/msg00017.html
> 
> Bruno contributed that patch to Autoconf, dated 2007-05-21.
> Bruno, was that patch derived from Steven's work?

It was derived from these two comment lines found in Steven's macro:

  # Flags to try:  -fopenmp (gcc), -openmp (icc), -mp (SGI & PGI),
  #                -xopenmp (Sun), -omp (Tru64), -qsmp=omp (AIX), none

You find the AC_C_OPENMP history here:
  
http://cvs.savannah.gnu.org/viewvc/gettext/gettext-tools/m4/openmp.m4?hideattic=0&root=gettext&view=log
  http://cvs.savannah.gnu.org/viewvc/gnulib/gnulib/m4/openmp.m4?view=log
  http://lists.gnu.org/archive/html/autoconf/2007-05/msg00029.html

Other than the list of compilers and options to try, there are no commonalities
between http://lists.gnu.org/archive/html/autoconf/2007-05/msg00029.html
and the code Steven's AX_OPENMP macro, because

  - I didn't want to try arbitrary options on arbitrary compilers, because
    the option "-openmp" causes the creation of a file "penmp" with most
    compilers.

  - I wanted the OPENMP_FLAGS to be empty if $CC $CFLAGS already supports
    OpenMP without any additional flags.

  - I wanted to check the _OPENMP macro, because according to the OpenMP spec,
    it is this macro which indicates OpenMP support in the compiler, not some
    library function.

  - The _AC_LANG_PREFIX business made Steven's code unintelligible to me
    (and the quotation rules around _AC_LANG_PREFIX are still black magic to
    me).

> If so, how about the following patch to the Autoconf changelog?
> 
> diff --git a/ChangeLog b/ChangeLog
> index a5b43ae..0aff913 100644
> --- a/ChangeLog
> +++ b/ChangeLog
> @@ -626,6 +626,8 @@
>       * NEWS: Mention AC_C_OPENMP.
>       * lib/autoconf/c.m4 (AC_C_OPENMP): New macro.
>       * doc/autoconf.texi (C Compiler): Document AC_C_OPENMP.
> +     Based in part on Steven G. Johnson's AX_OPENMP macro in the
> +     Autoconf macro archive.

This is inaccurate. But you can write:

        Based in part on Steven G. Johnson's investigations for the AX_OPENMP
        macro in the Autoconf macro archive.

Bruno