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Programming Environment

This page will give you an overview of the Cray programming environment. It starts with a presentation of the programming environments and compiler wrappers that you can use to compile your C, C++ or Fortran code. Finally, some basic information on how to compile an MPI or OpenMP programs are given.

Compiler Suites

On LUMI, the different compiler suites are accessible using module collections. These collections load the appropriates modules to use one of the supported programming environments for LUMI.

Switching compiler suites

The compiler collections are accessible through modules and in particular, the module load command:

$ module load PrgEnv-<name>

where <name> is the name of the compiler suite. There are 3 collections available on LUMI. The default collection is Cray.

Description Module collection
CCE Cray Compiling Environment PrgEnv-cray
GCC GNU Compiler Collection PrgEnv-gnu
AOCC AMD Optimizing C/C++ Compiler PrgEnv-aocc

For example, if you want to use the GNU’s compiler collection:

$ module load PrgEnv-gnu

After you have loaded a programming environment, the compiler wrappers (cc, CC and ftn) are available.

Changing compiler versions

If the default compiler version does not suit you, you can change the version after having a loaded a programming environment. This operation is performed using the module swap command.

$ module swap <compiler> <compiler>/<version>

Where <compiler> is the name of the compiler module for the loaded programming environment and <compiler> the version you want to use. For example

$ module swap cce cce/11.0.2
$ module swap gcc gcc/10.2.0

Compiler Wrappers

The module collection provides wrappers to the C, C++ and Fortran compilers. The command used to invoke these wrappers are listed below.

  • cc: C compiler
  • CC: C++ compiler
  • ftn: Fortran compiler

No matter which vendor's compiler module is loaded, always use one of the above commands to invoke the compiler. Using these wrappers will invoke the underlying compiler according to the compiler suite that is loaded in the environment. For some libraries, the appropriate option for the linking will also be included. See here for more information.

About MPI Wrappers

The Cray compiler wrappers replace other wrappers commonly found on HPC systems like the mpicc, mpic++ and mpif90 wrappers. You don't need to use these wrappers to compile an MPI code on LUMI. See here.

Below are examples on how to use the wrappers for the different programming languages.

cc -c source1.c
cc -c source2.c
cc source1.o source2.o -o myprogram 
CC -c source1.cpp
CC -c source2.cpp
CC source1.o source2.o -o myprogram 
ftn -c source1.f90
ftn -c source2.f90
ftn source1.o source2.o -o myprogram 

In the example above, no additional options are provided. However, in most cases this is not the case and the arguments used with the commands vary according to which compiler module is loaded. For example, the arguments and options supported by the GNU Fortran compiler are different from those supported by the Cray Fortran compiler.

Wrapper and compiler options

The following flags are a good starting point to achieved good performance:

Compilers Good performance Aggressive optimizations
Cray C/C++ -O2 -funroll-loops -ffast-math -Ofast -funroll-loops
Cray Fortran Default -O3 -hfp3
GCC -O2 -ftree-vectorize -funroll-loops -ffast-math -Ofast -funroll-loops

Detailed information about the available compiler options are available here:

The man pages of the wrappers and of the underlying compilers are also a good place to explore the options. The command to access the man pages are presented in the table below.

Language Wrapper CCE GNU
C man cc man craycc man gcc
C++ man CC man crayCC man g++
Fortran man ftn man crayftn man gfortran

Choosing the target architecture

When using the Cray programming environment, there is no need to specify compiler flags to target specific CPU architecture, like -march and -mtune in gcc. Instead, you load an appropriate combination of modules to choose the target architecture when compiling. These modules influence the optimizations performed by the compiler, as well as the libraries (e.g. which BLAS routines are used in Cray LibSci) used. Therefore, we recommend that you compile with craype-x86-milan for LUMI-C, even if the compiler optimizations for Zen 3 processors are immature at the moment.

The table below summarize the available modules.

Module Target
craype-x86-milan LUMI-C CPUs
craype-x86-rome LUMI-D CPUs, login nodes CPUs and EAP CPUs
craype-accel-amd-gfx908 EAP GPUs
craype-accel-nvidia75 LUMI-D GPUs

Libraries Linking

The wrapper will pass the appropriate linking information to the compiler and linker for libraries accessible via modules prefixed by cray-. These libraries don't require user-provided options in order to be linked. For other libraries, the user should provide the appropriate include (-I) and library (-L) search paths as well as linking command (-l).

If you have used a Cray system in the past, you may be familiar with the legacy linking behaviour of the Cray compiler wrappers. Historically, the wrappers built statically linked executables. In recent versions of the Cray programming environment, this not the case anymore, libraries are now dynamically linked. The following options are available to you to control the behaviour of your application

  • Follow the default Linux policy and at runtime use the system default version of the shared libraries (so may change as and when system is upgraded)
  • Hard code the path of each library into the binary at compile time so that a specific version is loaded when the application start (as long as the library is still installed). Set CRAY_ADD_RPATH=yes at compile time to use this mode.
  • Allow the currently loaded programming environment modules to select the library version at runtime. Applications must not be linked with CRAY_ADD_RPATH=yes and must add the following line to the Slurm script:

Static linking is unsupported by Cray at the moment.

Using the wrapper with a configure script

In order to compile an application that uses a series of ./configure, make, and make install commands, you can pass the compiler wrappers in the appropriate environment variables. This should be sufficient for a configure step to succeed.

./configure CC=cc CXX=CC FC=ftn

For other tools, you can try to export environment variables so that the tool you are using is aware of the wrappers.

export CC=cc 
export CXX=CC
export FC=ftn
export F77=ftn
export F90=ftn

Compile an MPI Program

When you load a programming environment, the appropriate MPI module is loaded in the environment (cray-mpich). In order to compile your MPI program, you should use the set of compiler wrappers (cc, CC, ftn). The wrappers will automatically link codes with the MPI libraries.

If you are using a build system that uses a configure script, you may need to provide the appropriate variables so that the correct wrapper is used. For example:

./configure MPICC=cc MPICXX=CC MPIF77=ftn MPIF90=ftn

Compile an OpenMP Program

The table below summarizes the compiler flags used to enable OpenMP for the different compilers.

C/C++ -fopenmp -fopenmp -fopenmp
Fortran -h omp -fopenmp n.a.

When using the OpenMP compiler flag, the wrapper will link to the multithreaded version of the Cray libraries.