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Compilers and Flags

The following compilers are available on the ZIH system:

GNU Compiler Collection Clang Compiler Intel Compiler PGI Compiler (Nvidia HPC SDK)
Further information GCC website Clang documentation C/C++, Fortran PGI website
Module name GCC Clang iccifort PGI
C Compiler gcc clang icc pgcc
C++ Compiler g++ clang++ icpc pgc++
Fortran Compiler gfortran - ifort pgfortran

For an overview of the installed compiler versions, please use module spider <module name> on the ZIH systems. Additionally you can use module av and look below "compilers" to see all available compiler modules.

All compilers support various language standards, at least up to ISO C11, ISO C++ 2014, and Fortran 2003. Please check the man pages to verify that your code can be compiled.

Please note that the linking of C++ files normally requires the C++ version of the compiler to link the correct libraries.

Compiler Flags

Common options are:

  • -g to include information required for debugging
  • -pg to generate gprof-like sample-based profiling information during the run
  • -O0, -O1, -O2, -O3 to customize the optimization level from no (-O0) to aggressive (-O3) optimization
  • -I to set search path for header files
  • -L to set search path for libraries

Please note that aggressive optimization allows deviation from the strict IEEE arithmetic. Since the performance impact of options like -fp-model strict is very hard you have to balance speed and desired accuracy of your application yourself.

The user benefits from the (nearly) same set of compiler flags for optimization for the C, C++, and Fortran-compilers. In the following table, only a couple of important compiler-dependent options are listed. For more detailed information about these and further flags, the user should refer to the man pages or use the option --help to list all options of the compiler.

GCC Intel PGI Description
-fopenmp -fopenmp -mp turn on OpenMP support
-std=c99, -std=c++11, -std=f2018 -std=c99, -std=c++11, -std18 -c99, --c++11, n/a set language standard, for example C99, C++11, Fortran 2018
-mieee-fp -frounding-math -fp-model precise or -fp-model strict -Kieee limit floating-point optimizations and maintain declared precision
-ffast-math -mp1 or -fp-model fast -Mfprelaxed allow floating-point optimizations, may violate IEEE conformance
-Ofast -fast -fast Maximize performance, implies a couple of other flags
-fsignaling-nans -fno-trapping-math C/C++: -fpe-trap, Fortran: -fpe-all -Ktrap controls the behavior when floating-point exceptions occur
-mavx -msse4.2 -mavx -msse4.2 -fastsse "generally optimal flags" for supporting SSE instructions
-flto -ipo -Mipa interprocedural / link-time optimization (across source files)
-floop-parallelize-all -ftree-parallelize-loops=<numthreads> -parallel -Mconcur auto-parallelizer
-fprofile-generate -prof-gen -Mpfi create instrumented code to generate profile in file
-fprofile-use -prof-use -Mpfo use profile data for optimization


We can not generally give advice as to which option should be used. To gain maximum performance please test the compilers and a few combinations of optimization flags. In case of doubt, you can also contact HPC support and ask the staff for help.

Architecture-specific Optimizations

Different architectures of CPUs feature different vector extensions (like SSE4.2 and AVX) to accelerate computations. The following matrix shows proper compiler flags for the architectures at the ZIH:

HPC System Architecture GCC Intel Nvidia HPC
Alpha Centauri AMD Rome -march=znver2 -march=core-avx2 -tp=zen2
Barnard AMD Sapphire Rapids -march=sapphirerapids -march=core-sapphirerapids
Julia Intel Cascade Lake -march=cascadelake -march=cascadelake -tp=cascadelake
Romeo AMD Rome -march=znver2 -march=core-avx2 -tp=zen2
All x86 Host's architecture -march=native -xHost or -march=native -tp=host
Power9 IBM Power9 -mcpu=power9 or -mcpu=native -tp=pwr9 or -tp=host

To build an executable for different node types with the Intel compiler, use -axcode, where code is to be replaced with one or more target architectures. For Cascade Lake and Sapphire Rapids. the option -axcascadelake,sapphirerapids (for Intel compilers) instructs the compiler to optimized code paths for the specified architecture(s), if possible. If the application is executed on one of these architectures, the optimized code path will be chosen. A baseline code path will also be generated. This path is used on other architectures than the specified ones and is used in code sections that were not optimized by the compiler for a specific architecture. Other optimization flags can be used as well for, e.g. -O3. However, the -march option cannot be used here, as this will overwrite the -axcode option. This increases the size of the program code (might result in poorer L1 instruction cache hits) but enables to run the same program on different hardware types with compiler optimizations.