During optimization, the compiler changes the unoptimized code sequences, derived from the source code, into equivalent code sequences that execute faster and usually require less memory space. It is also possible for an expression that would normally cause an exception to be removed by optimization, thus preventing the exception.
Because the optimization is achieved by transforming the code using knowledge obtained from a larger program context, the direct correspondence between source and object code is often lost. Optimized code is also more sensitive to subtle coding errors.
One example of a subtle coding error is to type cast a pointer variable incorrectly. The compiler assumes ISO conformance when doing optimization. If your program does not conform, you may receive undefined results. For more information, see ANSI aliasing rules and Using ANSI aliasing rules.