Add infrastructure for openmp4 compat

cmake/CMakeLists.txt

@@ -229,16 +229,7 @@ pkg_depends(USER-SCAFACOS MPI)
 
 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)
@@ -248,6 +239,13 @@ if(BUILD_OMP)
   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)

doc/src/Build_basics.rst

@@ -145,11 +145,9 @@ Some compilers do not fully support the ``default(none)`` directive
 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
+LAMMPS will try to detect compilers that use OpenMP 4.0 semantics and
-incorrect and unwanted sharing) can be replaced with ``default(shared)``
+change the directives accordingly, but if your compiler is not
-while dropping all ``shared()`` directives. The script
+detected, you may set the CMake variable ``-D LMP_OMP_COMPAT=4``.
-'src/USER-OMP/hack_openmp_for_pgi_gcc9.sh' can be used to automate
-this conversion.
 
 ----------
 

src/USER-OMP/README

@@ -9,11 +9,3 @@ doc/Section_accelerate.html, sub-section 5.2
 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.

src/USER-OMP/hack_openmp_for_pgi_gcc9.sh

@@ -1,12 +0,0 @@
-#!/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

src/omp_compat.h

@@ -0,0 +1,35 @@
+/* -*- 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
+