NERSCPowering Scientific Discovery Since 1974

Gnu Compiler Peformance on Edison


By default, the Gnu compilers do not provide any optimization.

These are the Gnu optimization options we compared.  The quotations are from the gcc compiler on-line man page.

Gnu compiler options tested
-O3 -ffast-math In addition to the -O3 optimizations, this performs optimizations at the expense of an exact implementation of IEEE or ISO rules/specifications for math functions.
-O3 This is the highest numerical level of optimization.
-O3 -funroll-loops This unrolls loops whose number of iterations can be determined at compile time or upon entry to the loop. It also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations).
-O3 -ffast-math -funroll-loops See above.
-Ofast "Disregard strict standards compliance.  -Ofast enables all -O3 optimizations.  It also enables optimizations that are not valid for all standard compliant programs."


Gnu compiler on Edison


-O3 gives a comparatively good level of optimization.  If you can relax IEEE compliance, adding -ffast-math or using -Ofast can improve performance even more.