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# Copyright (C) 2015-2016 R Core Team

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# Written by David MacKenzie, with help from
# Akim Demaille, Paul Eggert,
# Franc,ois Pinard, Karl Berry, Richard Pixley, Ian Lance Taylor,
# Roland McGrath, Noah Friedman, david d zuhn, and many others.

# [a small part, modified for clang and Intel in 2015,6.]


# R_OPENMP
# --------
# Check which options need to be passed to the C compiler to support OpenMP.
# Set the OPENMP_CFLAGS / OPENMP_CXXFLAGS / OPENMP_FFLAGS variable to these
# options.
# The options are necessary at compile time (so the #pragmas are understood)
# and at link time (so the appropriate library is linked with).
# This macro takes care to not produce redundant options if $CC $CFLAGS already
# supports OpenMP. It also is careful to not pass options to compilers that
# misinterpret them; for example, most compilers accept "-openmp" and create
# an output file called 'penmp' rather than activating OpenMP support.
AC_DEFUN([R_OPENMP],
[
  OPENMP_[]_AC_LANG_PREFIX[]FLAGS=
  AC_ARG_ENABLE([openmp],
    [AS_HELP_STRING([--disable-openmp], [do not use OpenMP])])
  if test "$enable_openmp" != no; then
    AC_CACHE_CHECK([for $[]_AC_CC[] option to support OpenMP],
      [ac_cv_prog_[]_AC_LANG_ABBREV[]_openmp],
      [AC_LINK_IFELSE([_AC_LANG_OPENMP],
     [ac_cv_prog_[]_AC_LANG_ABBREV[]_openmp='none needed'],
     [ac_cv_prog_[]_AC_LANG_ABBREV[]_openmp='unsupported'
      dnl Try these flags:
      dnl   GCC >= 4.2, clang 3.8 -fopenmp
      dnl   clang 3.7.x       -fopenmp=libomp
      dnl   (-fopenmp is accepted but does not work)
      dnl   Oracle C, Fortran     -xopenmp
          dnl   (also accepts -fopenmp as from 12.4)
      dnl   Intel C, Fortran      -qopenmp
      dnl   Intel                 -openmp (deprecated)
          dnl   (https://software.intel.com/en-us/node/581863,
      dnl    https://software.intel.com/en-us/node/525020)
      dnl   SGI C, PGI C          -mp
      dnl   Tru64 Compaq C        -omp
      dnl   IBM C (AIX, Linux)    -qsmp=omp
          dnl   Cray CCE              -homp
          dnl   NEC SX                -Popenmp
          dnl   Lahey Fortran (Linux) --openmp
      dnl If in this loop a compiler is passed an option that it doesn't
      dnl understand or that it misinterprets, the AC_LINK_IFELSE test
      dnl will fail (since we know that it failed without the option),
      dnl therefore the loop will continue searching for an option, and
      dnl no output file called 'penmp' or 'mp' is created.
      for ac_option in -fopenmp -xopenmp -qopenmp \
                           -openmp -mp -omp -qsmp=omp -homp \
               -fopenmp=libomp \
                           -Popenmp --openmp; do
        ac_save_[]_AC_LANG_PREFIX[]FLAGS=$[]_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])])
    case $ac_cv_prog_[]_AC_LANG_ABBREV[]_openmp in #(
      "none needed" | unsupported)
    ;; #(
      *)
    OPENMP_[]_AC_LANG_PREFIX[]FLAGS=$ac_cv_prog_[]_AC_LANG_ABBREV[]_openmp ;;
    esac
  fi
  AC_SUBST([OPENMP_]_AC_LANG_PREFIX[FLAGS])
])