Add infrastructure for openmp4 compat

find_package(OpenMP QUIET)

# TODO: this is a temporary workaround until a better solution is found. AK 2019-05-30

# GNU GCC 9.x uses settings incompatible with our use of 'default(none)' in OpenMP pragmas

# where we assume older GCC semantics. For the time being, we disable OpenMP by default

# for GCC 9.x and beyond. People may manually turn it on, but need to run the script

# src/USER-OMP/hack_openmp_for_pgi_gcc9.sh on all sources to make it compatible with gcc 9.

if ((CMAKE_CXX_COMPILER_ID STREQUAL "GNU") AND (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 8.99.9))

  option(BUILD_OMP "Build with OpenMP support" OFF)

else()

option(BUILD_OMP "Build with OpenMP support" ${OpenMP_FOUND})

endif()

if(BUILD_OMP)

  find_package(OpenMP REQUIRED)

  endif()

  set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")

  set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")

  if ((CMAKE_CXX_COMPILER_ID STREQUAL "GNU") AND (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 8.99.9))

    # GCC 9.x strictly implements OpenMP 4.0 semantics for consts.

    add_definitions(-DLMP_OMP_COMPAT=4)

  else()

    add_definitions(-DLMP_OMP_COMPAT=3)

  endif()

endif()

if(PKG_MSCG OR PKG_USER-ATC OR PKG_USER-AWPMD OR PKG_USER-QUIP OR PKG_LATTE)

and others (e.g. GCC version 9 and beyond) may implement OpenMP 4.0

semantics, which are incompatible with the OpenMP 3.1 semantics used

in LAMMPS (for maximal compatibility with compiler versions in use).

In those case, all ``default(none)`` directives (which aid in detecting

incorrect and unwanted sharing) can be replaced with ``default(shared)``

while dropping all ``shared()`` directives. The script

'src/USER-OMP/hack_openmp_for_pgi_gcc9.sh' can be used to automate

this conversion.

LAMMPS will try to detect compilers that use OpenMP 4.0 semantics and

change the directives accordingly, but if your compiler is not

detected, you may set the CMake variable ``-D LMP_OMP_COMPAT=4``.

----------

The person who created this package is Axel Kohlmeyer at Temple U

(akohlmey at gmail.com).  Contact him directly if you have questions.

--------------------------

This directory also contains a shell script:

hack_openmp_for_pgi.sh

which will convert OpenMP directives in src files

into a form compatible with the PGI compiler.

#!/bin/sh

# convert default(none) directives for OpenMP pragmas to default(shared) and remove shared() directive

# this allows compiling OpenMP pragmas in LAMMPS with compilers that don't support default(none) properly

# or require backward incompatible OpenMP 4 and OpenMP 5 semantics

for f in *.h *.cpp

do \

   sed -e '/#pragma omp/s/^\(.*default\)(none)\(.*\)$/\1(shared)\2/' \

       -e '/#pragma omp/s/shared([a-z0-9,_]\+)//' \

       -i.bak $f

done

/* -*- c++ -*- ----------------------------------------------------------

   LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator

   http://lammps.sandia.gov, Sandia National Laboratories

   Steve Plimpton, sjplimp@sandia.gov

   Copyright (2020) Sandia Corporation.  Under the terms of Contract

   DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains

   certain rights in this software.  This software is distributed under

   the GNU General Public License.

   See the README file in the top-level LAMMPS directory.

------------------------------------------------------------------------- */

// There is no way to annotate an OpenMP construct that

// (a) accesses const variables, (b) has default(none),

// and (c) is valid in both OpenMP 3.0 and 4.0.

//

// (in OpenMP 3.0, the const variables have a predetermined

//  sharing attribute and are *forbidden* from being declared

//  in the omp construct.  In OpenMP 4.0, this predetermined

//  sharing attribute is removed, and thus they are *required*

//  to be declared in the omp construct)

//

// To date, most compilers still accept the OpenMP 3.0 form,

// so this is what LAMMPS primarily uses.  For those compilers

// that strictly implement OpenMP 4.0 (such as GCC 9.0), we

// give up default(none).

#if LMP_OMP_COMPAT == 4

#    define LMP_SHARED(...)

#    define LMP_DEFAULT_NONE default(shared)

#else

#    define LMP_SHARED(...) shared(__VA_ARGS__)

#    define LMP_DEFAULT_NONE default(none)

#endif