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======Compiling Tips and Tricks======

> A //technique// is what you call any trick you use more than once.

=====Compilers=====

The three dominant compiled languages used in HPC are C, Fortran and C++.

> Many people write the first and last as C/C++.  That's misleading.  C and C++ are separate languages.  Confusing them is responsible for many problems.

The compilers found at the CHPC fall into three families: GCC, Intel and [[guide:m9000|Sun Studio]].

> 'GCC' stands for GNU Compiler Collection; 'gcc' stands for GNU C Compiler; try not to confuse them 8-)

>> There's a fourth family, IBM XL, but we won't worry about it since the P690s are gone and BG/P is in hibernation.

====Table of Compilers and their Command Names====

| Language ^  GCC         ^  Intel     |
^ C        | ''gcc''      | ''icc''    |
^ Fortran  | ''gfortran'' | ''ifort''  |
^ C++      | ''g++''      | ''icpc''   |

So to actually compile a C program with the GNU C compiler you would type at the shell command line:

<code bash>
gcc -o hello hello.c
</code>

And for a Fortran 90 program using Intel's compiler:

<code bash>
ifort -o hello hello.f90
</code>

====Table of Compiler Variables====

By convention, there are variables that define the actual command invoked (see above) and which can be used in the shell, with make files and ''configure''.

^ Variable ^ Compiler ^ Example |
^ ''CC'' | C | ''CC=gcc'' |
^ ''FC'' | Fortran | ''FC=ifort'' |
^ ''CXX'' | C++ | ''CXX=/opt/gridware/intel/comp12/composerxe-2011.0.084/bin/intel64/icpc'' |

> The last example shows that you can include the full path to a specific compiler command --- usually the current [[guide:paths|PATH]] will be searched.

Fortran comes in two main flavours, Fortran 77 and Fortran 90 (also called 9x since it includes Fortran 95).  By default ''FC'' refers to the Fortran 9x compiler and when it is necessary to also point to a Fortran 77 compiler:

^ ''F77'' | Fortran 77 | ''F77=g77'' |

====Example====

When building an MPI library --- for example, OpenMPI --- one should set all four variables since MPI provides MPI compilers for:

| ''mpicc'' | MPI C compiler ^
| ''mpicxx'' | MPI C++ compiler |
| ''mpif77'' | MPI Fortran 77 compiler |
| ''mpif90'' | MPI Fortran 90 compiler |

When building OpenMPI then, before the usual ''make; make install'' command, tell ''configure'' which compilers to use:

<code bash>
./configure --prefix=$HOME/mpi/intel/ \ 
CC=icc CXX=icpc F77=ifort FC=ifort \
--with-openib \ 
--with-openib-libdir=/usr/lib64/
</code>

> The above example uses Intel compilers and also builds in InfiniBand support.


=====Which Compilers am I Using?=====

That's a very good question.

====Table of Compiler Commands for Version====

| Language ^  GCC         ^  Intel     |
^ C        | ''%%gcc --version%%''      | ''icc -v''    |
^ Fortran  | ''%%gfortran --version%%'' | ''ifort -v''  |
^ C++      | ''%%g++ --version%%''      | ''icpc -v''   |

The ''-v'' options stands for 'verbose' and it produces additional messages when used with any compiler command.  For the Intel compilers, ''-v'' by itself just produces the version:

<code bash>
login01:~$ icc -v
Version 12.0.0
</code>

For the GCC compilers, ''-v'' produces a lot more information:

<code>
login01:~$ gfortran -v
Using built-in specs.
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info 
--enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib 
--enable-__cxa_atexit --disable-libunwind-exceptions --enable-libgcj-multifile --enable-
languages=c,c++,objc,obj-c++,java,fortran,ada --enable-java-awt=gtk --disable-dssi 
--disable-plugin --with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre --with-cpu=generic 
--host=x86_64-redhat-linux
Thread model: posix
gcc version 4.1.2 20080704 (Red Hat 4.1.2-52)
</code>

Which is why GCC compilers provide the long option ''%%--version%%'' to just obtain the version:

<code>
login01:~$ gfortran --version
GNU Fortran (GCC) 4.1.2 20080704 (Red Hat 4.1.2-52)
Copyright (C) 2007 Free Software Foundation, Inc.

GNU Fortran comes with NO WARRANTY, to the extent permitted by law.
You may redistribute copies of GNU Fortran
under the terms of the GNU General Public License.
For more information about these matters, see the file named COPYING

</code>

> You also get a copyright message but the version is in the first line.
> <code>login01:~$ gfortran --version | head -1
GNU Fortran (GCC) 4.1.2 20080704 (Red Hat 4.1.2-52)</code>

Using a different, newer, version of GCC is easy through the use of GNU [[guide:compiling|modules]]:

<code bash>
login01:~$ module load gcc
login01:~$ gfortran --version | head -1
GNU Fortran (GCC) 4.7.2
</code>
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