don't set CMAKE_TUNE_FLAGS to anything by default

cmake/CMakeLists.txt

@@ -69,14 +69,6 @@ if(${CMAKE_CXX_COMPILER_ID} STREQUAL "Intel")
   endif()
 endif()
 
-if(${CMAKE_CXX_COMPILER_ID} STREQUAL "GNU")
-  set(CMAKE_TUNE_DEFAULT "-march=native")
-endif()
-
-if(${CMAKE_CXX_COMPILER_ID} STREQUAL "Clang")
-  set(CMAKE_TUNE_DEFAULT "-march=native")
-endif()
-
 # we require C++11 without extensions
 set(CMAKE_CXX_STANDARD 11)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
@@ -366,7 +358,7 @@ foreach(PKG_WITH_INCL KSPACE PYTHON VORONOI USER-COLVARS USER-MOLFILE USER-NETCD
   endif()
 endforeach()
 
-set(CMAKE_TUNE_FLAGS "${CMAKE_TUNE_DEFAULT}" CACHE STRING "Compiler specific optimization or instrumentation")
+set(CMAKE_TUNE_FLAGS "${CMAKE_TUNE_DEFAULT}" CACHE STRING "Compiler and machine specific optimization flags (compilation only)")
 separate_arguments(CMAKE_TUNE_FLAGS)
 foreach(_FLAG ${CMAKE_TUNE_FLAGS})
   target_compile_options(lammps PRIVATE ${_FLAG})

doc/src/Build_basics.rst

@@ -193,14 +193,17 @@ compiler and any :doc:`accelerator packages <Speed_packages>` you have
 included in the build.
 
 You can tell CMake to look for a specific compiler with setting CMake
-variable during configuration.  For a few common choices, there are also
+variables (listed below) during configuration.  For a few common
-presets in the ``cmake/presets`` folder.  For convenience, there is a
+choices, there are also presets in the ``cmake/presets`` folder.  For
-``CMAKE_TUNE_FLAGS`` variable that can be set to apply global compiler
+convenience, there is a ``CMAKE_TUNE_FLAGS`` variable that can be set to
-options.  More on that below, but you can also specify the corresponding
+apply global compiler options (applied to compilation only), to be used
-``CMAKE_*_FLAGS`` variables individually if you want to experiment with
+for adding compiler or host specific optimization flags in addition to
-alternate optimization flags.  You should specify all 3 compilers, so
+the "flags" variables listed below. You may also specify the
-that the (few) LAMMPS source files written in C or Fortran are built
+corresponding ``CMAKE_*_FLAGS`` variables individually, if you want to
-with a compiler consistent with the one used for the C++ files:
+experiment with alternate optimization flags.  You should specify all 3
+compilers, so that the (few) LAMMPS source files written in C or Fortran
+are built with a compiler consistent with the one used for the C++
+files:
 
 .. code-block:: bash
 
@@ -229,11 +232,7 @@ can be loaded with `-C ../cmake/presets/clang.cmake`.  Similarly,
 
 In addition you can set ``CMAKE_TUNE_FLAGS`` to specifically add
 compiler flags to tune for optimal performance on given hosts. By
-default these are initialized to some compiler specific flags, to
+default this variable is empty.
-optimize the LAMMPS executable with optimizations and instructions
-available on the host where LAMMPS is compiled. For example, for Intel
-compilers this would be ``-xHost`` and for GNU compilers this would be
-``-march=native``. To turn these flags off, do ``-D CMAKE_TUNE_FLAGS=``.
 
 .. note::
 

doc/src/Howto_cmake.rst

@@ -331,7 +331,7 @@ Some common LAMMPS specific variables
    * - ``BUILD_DOC``
      - include building the HTML format documentation for packaging/installing (default: ``off``)
    * - ``CMAKE_TUNE_FLAGS``
-     - common compiler flags, for optimization or instrumentation (default: compiler specific)
+     - common compiler flags, for optimization or instrumentation (default:)
    * - ``LAMMPS_MACHINE``
      - when set to ``name`` the LAMMPS executable and library will be called ``lmp_name`` and ``liblammps_name.a``
    * - ``LAMMPS_EXCEPTIONS``