GNU compilers

GNU compilers

The GNU Compiler Collection (GCC) includes front ends for the C (gcc), C++ (g++), and Fortran (gfortran) programming languages.

Choose a version

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

bash

$ module load foss

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

bash

$ module spider foss

and then switch to the desired version using

bash

$ module swap foss foss/<version>

OpenMP Support

OpenMP is turned off by default. You can turn it on using the -fopenmp flag.

Optimization options

:material-help-circle-outline: man gcc - man gfortran

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

bash

-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 your code is sensitive to floating-point optimizations.
• the -funroll-loops option allows the compiler to unroll loops

A more aggressive option might be

bash

-O3 -funroll-loops

or for even more aggressive optimization

bash

-Ofast -funroll-loops

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

• GCC documentation about optimization options

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:

bash

-fallow-argument-mismatch

or

bash

-std=legacy

Compiler Feedback

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

• GCC documentation about developer options

Debugging

To ease a debugging process, it is 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 incurs the debugging penalty.

• GCC documentation about debug options