Scientific Computing

CMake Find modules are by their nature a little aggressive about finding libraries and executables. This becomes a factor on Windows in particular when Anaconda Python is not active in the current Terminal. CMake find_package(Python) by default prefers Anaconda Python over system Python unless overridden as below. Anaconda Python won’t operate correctly without conda activate, which presumably the user has either forgotten to do or doesn’t desire at the moment. To decrease the aggressiveness and find Windows Store Python etc. when conda isn’t activated on Windows, add to the project CMakeLists.txt before find_package(Python):

set(Python_FIND_REGISTRY LAST)
# this avoids non-active conda from getting picked anyway on Windows

set(Python_FIND_VIRTUALENV STANDARD)
# Use environment variable PATH to decide preference for Python

find_package(Python)

Anaconda Python by default auto-activates the “base” environment each time a new Terminal is opened. This slows opening new Terminal, particularly on systems with slow disks or virtual disks. Particularly if the user isn’t constantly using Python, it can be beneficial to make conda only active when specified. To disable conda auto-activation on new Terminal, type:

conda config --set auto_activate_base false

This setting takes effect in .condarc

Fortran compilers typically have options for enforcing Fortran standards. The compilers raise additional warnings or errors for code that is not deemed compliant. Fortran standard options can make false warnings, so we generally do not enable standards checking for user defaults. However, we do enforce implicit none as a quality measure.

It’s also important to use implicit none so that each variable must be assigned beforehand. We recommend the Fortran 2018 statement:

implicit none (type, external)

which requires explicitly defined procedure interfaces as well.

type

the traditional implicit none default

external

new for Fortran 2018, requires explicit interface for external procedures.

GCC Gfortran -std=f2018 enforces Fortran 2018 standard. Consider these Gfortran options:

gfortran -Wall -fimplicit-none

Intel oneAPI -stand f18 enforces Fortran 2018 standard. Consider these options that also enforce implicit none:

ifx -warn

Cray Fortran compiler enforces implicit none via option:

ftn -eI

note that’s a capital “I” not a lowercase “ell”.

Nvidia HPC Fortran compiler enforces implicit none via:

nvfortran -Mdclchk

NAG Fortran has -f2018 Fortran 2018 flag. Enforce implicit none by:

nagfor -u

CMake logic to enforce these standards:

if(CMAKE_Fortran_COMPILER_ID STREQUAL "Cray")
  add_compile_options("$<$<COMPILE_LANGUAGE:Fortran>:-eI>")
elseif(CMAKE_Fortran_COMPILER_ID STREQUAL "GNU")
  add_compile_options(-Wall "$<$<COMPILE_LANGUAGE:Fortran>:-fimplicit-none>")
elseif(CMAKE_Fortran_COMPILER_ID MATCHES "^Intel")
  add_compile_options("$<$<COMPILE_LANGUAGE:Fortran>:-warn>")
elseif(CMAKE_Fortran_COMPILER_ID STREQUAL "NVHPC")
  add_compile_options("$<$<COMPILE_LANGUAGE:Fortran>:-Mdclchk>")
elseif(CMAKE_Fortran_COMPILER_ID STREQUAL "NAG")
  add_compile_options("$<$<COMPILE_LANGUAGE:Fortran>:-u>")
endif()

GCC Gfortran and Intel oneAPI are the most advanced, widely available modern Fortran compilers. Useful Fortran 2018 enhancements include: select rank assumed array rank, error stop within pure procedures, random_init to initialize random number seed, and implicit none (type, external) to require external procedures to be explicitly declared. GCC 10 is the oldest version currently maintained. Intel oneAPI has full Fortran 2018 support.

To get recent GCC is usually straightforward. Red Hat should use GCC Toolset. macOS Homebrew quickly adds the latest GCC version. If Ubuntu gfortran repo defaults aren’t adequate, get recent Gfortran via PPA.

Here are some of the major changes in Gfortran by version:

• Gfortran 12 enhances OpenMP 5 and OpenACC 2.6 support. Numerous bugfixes. bind(C) with character length greater than one.
• Gfortran 11 completed OpenMP 4.5 support
• Gfortran 10 added select rank
• Gfortran 9 added random_init() to initialize the random generator seed.
• Gfortran 8 added automatic nested loop exchange with do concurrent, actual argument array with too few elements for dummy argument now errors, initial support for parameterized derived types (simply define kind at initialization) and coarray support for teams. Standard flag -std=f2018 added and deprecated -std=f2008ts.
• Gfortran 7 added derived type IO select type. Complete Fortran 2003 support, Fortran 2018 non-constant stop and error stop codes, and -fdec- options to help compile very old non-standard code.

Gfortran 6 added Fortran 2008 submodule support, useful for large projects to save compilation time and allow powerful use scenarios. Fortran 2003 deferred-length character are useful for avoiding bothersome trim() everywhere.

GCC 5 added full support for OpenMP 4.0, Fortran 2003 ieee_ intrinsics, Fortran 2008 error stop in pure procedures with constant error code. GCC 4.9 added Fortran 2003 deferred-length character variables in derived types. GCC 4.8 supported Fortran 2008 polymorphism, including select type, class(*), type(*), and Fortran 2018 assumed rank dimension(..). GCC 4.6 was the first version of Gfortran reaching beyond Fortran 95, with Fortran 2003 deferred-length character variable and Fortran 2008 impure elemental support. GCC 4.5 added Fortran 2008 iso_fortran_env. GCC 4.4 added initial support for polymorphism and OpenMP 3.

CMake allows switching parameters based on compiler version. This is very useful for modern Fortran programs.

Example CMakeLists.txt for Fortran compiler version dependent options.

if(CMAKE_Fortran_COMPILER_ID STREQUAL "GNU")
  add_compile_options(
  -Wall
  $<$<COMPILE_LANGUAGE:Fortran>:-fimplicit-none>
  )
endif()

add_executable(myprog main.f90)

Reference: Gfortran changelog

Major projects with CPU arch-specific code may distribute binaries for multiple CPU architectures for operating systems. Apple Silicon is a key example. For example, Anaconda Python has Intel and Apple Silicon installers. Check if a C++, GNU Octave, Matlab, or Python executable on Apple Silicon is using native or a Rosetta CPU arch translated binary.

CMake can detect the host CPU arch to use with Intel compilers. We discourage package maintainers from settings flags like “-march=native” (GCC, Clang) and “-xHost” (Intel oneAPI) because they may break on user systems such as ARM or HPC. However, the user can set flags for a project by setting environment variables like CFLAGS before the first project configure in CMake. This allows optimizing for a target compiler while compiling from a different host. Or, the user on an appropriate system may simply set their ~/.profile to have CFLAGS=-march=native or similar.

Projects use a variety of methods to detect which compiler is being used to thereby set compilation options. Although this discussion focuses on Fortran, it is easily and equally applicable to other languages such as C and C++.

Robustly detect compiler in CMake CMAKE_Fortran_COMPILER_ID. This tells the compiler vendor (GNU, Intel, Clang, etc.) Don’t use CMAKE_Fortran_COMPILER because there are several compiler executables per vendor and this will not be robust over time. To get the compiler version, CMAKE_Fortran_COMPILER_VERSION allows version comparisons.

Example:

if(CMAKE_Fortran_COMPILER_ID MATCHES "^Intel")
  add_compile_options("$<$<COMPILE_LANGUAGE:Fortran>:-check;-warn;-traceback>")
elseif(CMAKE_Fortran_COMPILER_ID STREQUAL "GNU")
  # Gfortran
  add_compile_options("$<$<COMPILE_LANGUAGE:Fortran>:-Wall;-Werror=array-bounds;-Wextra;-Wpedantic>")
endif()

_set_printf_count_output is a Visual-Studio only function. Therefore, C++ programs that call it should use a preprocessor conditional:

#ifdef _MSC_VER
  _set_printf_count_output();
#endif

C++ / C programs using POSIX write or read need a preprocessor include stack:

#if defined(_WIN32)
#include <io.h>
#else
#include <unistd.h>
#endif

Cygwin startup configuration is controlled by Windows environment variable CYGWIN. Not populating Cygwin PATH from Windows PATH avoids confusion from Windows programs being used instead of Cygwin programs. Set Windows environment variable:

CYGWIN_NOWINPATH=1

will result in a basic Cygwin PATH like:

/usr/local/bin:/usr/bin

Look inside Cygwin “/etc/profile” for more environment variables that may be of interest.

NOTE: all Windows environment variables are imported by Cygwin. Override these variables in ~/.profile or scripts if desired.

Related: don’t populate WSL PATH from Windows PATH.