Introduction

IBM Fortran, C, and C++ compiles are done without any optimization by default. Any level of optimization done by the compiler must be explicitly specified by means of flags to the compiler at compile and link time. This is different from the situation on the previous Cray platforms at NERSC, whose compilers provided a fairly high level of optimization by default, and it was necessary to ask explicitly for an unoptimized compile if that was what was wanted.

For the most part, the IBM Fortran, C, and C++ compilers all have the same optimization arguments. The description of these arguments below applies to all three compilers unless otherwise stated.

These are the currently recommended optimization options for compiling on the machine on which you will be running the code:

	-O3 -qstrict -qarch=auto-qtune=auto

These options provide a compromise between minimizing compilation time and maximizing the performance of the compiled code. All of these options as well as several other useful optimization options will be described below.

Some specific examples of the compile time and run time impact of several different sets of optimization arguments on several public benchmarks are given at Compiler Optimization Argument Examples.