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GNU compilers

The GNU Compiler Collection (GCC) includes front ends for the C (gcc), C++ (g++), and Fortran (gfortran) programming languages. Invoking these compilers is done through the ftn, cc and CC compilers wrappers.

Choose a version

The GNU Compiler Collection is available from the PrgEnv-gnu module. This module loads the default version of the compiler.

$ module load PrgEnv-gnu

If you wish to use an older or newer version, you can list the available version with

$ module avail gcc

and then switch to the desired version using

$ module swap gcc gcc/<version>

As of April 2022, GCC versions 9.3.0, 10.3.0, and 11.2.0 are available on LUMI. We recommend using GCC 11, if possible, because it can optimize code specifically for the Zen 3 processors in LUMI (with the craype-x86-milan module or the compiler flag -march=znver3). GCC 9 and 10 can only optimize for Zen 2 processors. The performance difference is quite small, though, on average ca 1%, see for example these tests from Phoronix.

OpenMP Support

OpenMP is turned off by default, it's turned on using the -fopenmp flag.

Optimization options

man gcc - man gfortran

The default optimization level of the GNU compiler is -O0, it's therefore necessary to add additional optimization flags. A good starting point is

-O2 -ftree-vectorize -funroll-loops -ffast-math
  • the -O2 option performs nearly all supported optimizations
  • the -ffast-math relax the IEEE specifications for math functions. This option can produce incorrect results, don't use this flag if you code is sensitive to floating-point optimizations.
  • the -funroll-loops option allows the compiler to unroll loops

A more aggressive option might be

-O3 -funroll-loops

or for even more aggressive optimization

-Ofast -funroll-loops

The -Ofast enables all -O3 optimizations and disregards strict standards compliance.

Legacy Fortran codes

It is common to experience problems when compiling older Fortran codes with GCC 10 and newer versions. Typically, these codes are not fully compliant with the Fortran standard. The most common error message is Error: Type mismatch ... in connection with MPI calls. In those cases, a less strict compiler mode can be activated with the extra flags:

-fallow-argument-mismatch

or

-std=legacy

Compiler Feedback

Information about the optimizations and transformations performed by the compiler can be obtained using the -fopt-info option.

Debugging

To ease a debugging process, it's useful to generate an executable containing debugging information. For this purpose, you can use the -g option.

Most of the time, the debug information works best at low levels of code optimization, so consider using the -O0 level. The -g options can be specified on a per-file basis so that only a small part of your application incur the debugging penalty.