This is definitely untrue. The difference between -O1, -O2, -O3, and -Os is as follows: - O1 includes optimizations that only require a small amount of additio

This is definitely untrue. The difference between -O1, -O2, -O3, and -Os is as follows: - O1 includes optimizations that only require a small amount of additional compilation time. (Fast compilation, code is much faster than -O0.) - O2 includes O1, and additional optimizations that use a medium amount of compilation time. (Medium compilation, reasonably fast code.) - O3 includes O2, and additional optimizations that take more compilation time. (Slower compilation, faster, larger code. Unrolled loops and the like.) It often makes sense to compile hot inner loops with O3 because the compiler can do some fairly aggressive vectorization, if you know what you are doing... and the code remains correct in either case, if it was correct in the first place. It's basically "free" vectorization, when it works. - Os includes O2, except flags that increase code size. (Medium compilation, code is slightly slower and smaller than O2.) The only optimization level that actually changes the meaning of correct code is -Ofast, which is rarely used. It's described in the manual with the phrase "Disregard strict standards compliance". ALL levels of optimization will change the meaning of your code if your code relies on undefined behavior. For example, https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html > -faggressive-loop-optimizations > 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. It's always enabled.