To answer my question, I have made some conclusions and assumptions: So let me say that compiling with O0 does not mean that no optimizations will be applied. Options that reduce compilation time and make debugging better will be turned on. In other words, O0 is optimizing in the level of compilation. Produced binaries are not optimized in order to ease debugging process. I give an example: This option is enabled at O0 level -faggressive-loop-optimizations In GCC documentation: This option tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop. This assumes that loop code does not invoke undefined behavior by for example causing signed integer overflows or out-of-bound array accesses. The bounds for the number of iterations of a loop are used to guide loop unrolling and peeling and loop exit test optimizations. This option is enabled by default. So for GCC 4.8.x, there are almost 50 options turned on by default. Can someone confirm that?
