Performance Tips for NAG Fortran Compiler

  • Use -O3 or -O4 instead of just -O. This will lengthen compile time (sometimes substantially with -O4), but runtime performance is usually improved.
  • If you use assumed-shape arrays and you know that the actual arguments are always contiguous (i.e. you do not pass array slices using section notation), use -Oassumed=always_contig. With this option, a runtime error occurs if a non-contiguous actual argument is detected (so it is also useful for discovering whether you use such array sections).

    If you are not 100% sure, but you think that this is true all or almost all of the time, use -Oassumed. With this option, non-contiguous actual arguments will be accepted though access to them will be slow.