-O[level]: Optimisation

The -O[level] flag indicates the optimisation level of the compiled program. A level of 1 gives the least optimisation, whereas a level of 3 (some compilers go higher) gives the most optimisation.

NOTES:

• Certain types of program, such as device drivers, should never be compiled with an optimisation of more than level 2.
• Optimisation re-arranges the original source code. It is strongly recommended the program be first compiled with NO optimisation, then ran with test files. The program could then be compiled with higher optimisation, and the results compared.

  This would ensure that the compiler did not introduce errors by re-arranging the code.

On some systems -O is equivalent to -O1; on others, it is equivalent to -O2. Check the local compiler man page.