Promises the compiler that the source files specified on the command line form the whole program.
The compiler is then able to apply optimizations based on the knowledge that the source code visible to it is the complete set of source code for the program being compiled. Without this knowledge, the compiler is more conservative when applying optimizations to the code.
Use this option to gain maximum performance from a small program.
Do not use this option if you do not have all of the source code to give to the compiler.