3.4.70 –O[n]

Specify optimization level.

n can be 1, 2, 3, 4, or 5. No space is allowed between -O and n.

If -O[n] is not specified, only a very basic level of optimization limited to local common subexpression elimination and dead code analysis is performed. A program’s performance may be significantly improved when compiled with an optimization level than without optimization. Use of -O (which sets -O3) or -fast (which sets -O5) is recommended for most programs.

Each -On level includes the optimizations performed at the levels below it. Generally, the higher the level of optimization a program is compiled with, the better runtime performance obtained. However, higher optimization levels may result in increased compilation time and larger executable files.

Debugging with -g does not suppress -On, but -On limits -g in certain ways; see the dbx documentation.

The -O3 and -O4 options reduce the utility of debugging such that you cannot display variables from dbx, but you can still use the dbx where command to get a symbolic traceback.

If the optimizer runs out of memory, it attempts to proceed over again at a lower level of optimization, resuming compilation of subsequent routines at the original level.

For details on optimization, see the Fortran Programming Guide chapters “Performance Profiling” and “Performance and Optimization.”