GCC - bwHPC Wiki GCC - bwHPC Wiki


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Description Content
module load compiler/gnu/xxx
License GPL
Citing n/a
Links GNU-Homepage
Graphical Interface No
Included modules gcc | g++ | gfortran

1 Introduction

The GNU Compiler Collection (GCC) consists of tools to compile C, C++ and Fortran programs:

gcc GNU C compiler
g++ GNU C++ compiler
gfortran GNU Fortran compiler

More information about the MPI versions of the GCC Compiler is available here:

2 Versions and Availability

A list of versions currently available on all bwHPC-C5-Clusters can be obtained from the

Cluster Information System CIS

There is a version of GCC available on the system without loading a module but it may be outdated and it is recommended to load the GNU compiler module.
On the command line interface of any bwHPC cluster you'll get a list of available versions by using the command 'module avail compiler/gnu'.

$ : bwUniCluster
$ module avail compiler/gnu
-------------------- /opt/bwhpc/common/modulefiles ---------------------
compiler/gnu/4.5          compiler/gnu/4.8          compiler/gnu/5.2
compiler/gnu/4.7(default) compiler/gnu/4.9

3 Loading the module

3.1 Default Version

Load the version of GCC with the command 'module load compiler/gnu'.

$ module avail compiler/gnu
------------------ /opt/bwhpc/common/modulefiles -------------------
compiler/gnu/4.5          compiler/gnu/4.8          compiler/gnu/5.2
compiler/gnu/4.7(default) compiler/gnu/4.9
$ module load compiler/gnu
$ module list
Currently Loaded Modulefiles:
  1) compiler/gnu/4.7(default)

Here, we got the "default" version 4.7 (example).
The module will try to load modules it needs to function. If loading the module fails, check if you have already loaded the module with 'module list'.

3.1.1 Specific (newer or older) Version

If you wish to load a specific (older or newer) version (if available), you can do so using e.g. 'module load compiler/gnu/version' to load the version you desires.

$ module list
Currently Loaded Modulefiles:
  1) compiler/gnu/4.7(default)
$ module clear
Are you sure you want to clear all loaded modules!? [n] y
$ : New version will be used :
$ module load compiler/gnu/5.2
$ module list
Currently Loaded Modulefiles:
  1) compiler/gnu/5.2

GCC "version 5.2" is loaded now.

4 GCC-Specific Environments

To see a list of all GCC environments set by the 'module load'-command use 'env | grep GNU'.
Or use the command 'module show compiler/gnu/version'.
Example (Version 5.2)

GNU_VERSION = 5.2.0 
GNU_HOME = /opt/bwhpc/common/compiler/gnu/5.2.0 
GNU_BIN_DIR = /opt/bwhpc/common/compiler/gnu/5.2.0/bin 
GNU_MAN_DIR = /opt/bwhpc/common/compiler/gnu/5.2.0/man 
GNU_LIB_DIR = /opt/bwhpc/common/compiler/gnu/5.2.0/lib64 
PATH = $PATH:/opt/bwhpc/common/compiler/gnu/5.2.0/bin 
MANPATH = $MANPATH:/opt/bwhpc/common/compiler/gnu/5.2.0/man 
LD_LIBRARY_PATH = $LD_LIBRARY_PATH:/opt/bwhpc/common/compiler/gnu/5.2.0/lib64 
CC = gcc 
CXX = g++ 
F77 = gfortran 
FC = gfortran 
F90 = gfortran 

5 Documentation

5.1 Online documentation

GCC online documentation

5.2 Local documentation

For version specific documentation see the help page of the module.
Show informations for the default version by using the command 'module help compiler/gnu'.

$ module help compiler/gnu
----------- Module Specific Help for 'compiler/gnu/4.7' -----------
This module provides the GNU compiler suite version 4.7.3
via commands 'gcc', 'g++' and 'gfortran' (see also 'http://gcc.gnu.org/').
The GNU compiler has been build with gmp-4.3.2, mpfr-2.4.2 and mpc-0.8.1.

  cpp      - GNU pre processor
  gcc      - GNU C compiler
  g++      - GNU C++ compiler
  gfortran - GNU Fortran compiler (Fortran 77, 90 and 95)

Libraries can be found in
  $GNU_LIB_DIR = /opt/bwhpc/common/compiler/gnu/4.7.3/x86_64/lib64

Local documentation:
  See commands 'man cpp', 'man gcc', 'man g++' and 'man gfortran'.

Online documentation:

For details on library and include dirs please call
    module show compiler/gnu/4.7

Please do not add the gnu compiler module to any automatic environment setup
procedure (neither to ~/.profile nor to ~/.bashrc).

5.2.1 Manual Pages

For detailed lists of the different program options consult the particular man page.

$ man gcc
$ man g++
$ man gfortran

6 Optimizations

You can turn on various optimization options to enhance the performance of your program. Which options are the best depends on the specific program and can be determined by benchmarking your code. A command which gives good performance and a decent file size is

$ gcc -march=native -O2 ex.c -o ex

There are more aggressive optimization flags but the compiled programs can get quite large and the compilation process will probably take much longer. Moreover it can happen that the so compiled program is even slower. Such a command would be for example

$ gcc -march=native -O3 ex.c -o ex

For a complete list of all the optimization options execute

$ gcc --help=optimizers

6.1 Profiling with Gprof

Gprof is the profiler which belongs to the gcc compiler. Gprof is also installed on the system and can be used to profile your code. The profiler supports C, C++, Pascal and Fortran 77 program code. With this tool it is possible to analyse call times and time spent within program functions.
The first required step is to compile your program with the profiling flag "-pg".

$ gcc -pg ex.c -o ex

Compiled in this way your program will generate profiling data during execution. By default a file named gmon.out can be found in the work directory after running the program.

$ ./ex

The next step is to run the gprof program to analyse the gmon.out profiling data file. This file contains profiling data concerning the program execution, like an overview, time information or the call graph, in human readable format.

$ gprof gmon.out ex > outputfile.txt