G++ Compiler Optimization Levels Explained: From -O0 to -Os

The g++ compiler offers multiple optimization levels when compiling C++ programs, ranging from -O0 (no optimization) to -O3 (aggressive optimization). The default -O0 level prioritizes fast compile times with no code reordering, while -O1 introduces basic improvements like dead code elimination and register allocation. The -O2 level is considered the standard choice among developers, enabling inlining, loop unrolling, and constant folding without sacrificing accuracy or safety. -O3 goes further than -O2 but can sometimes hurt cache performance, making it less predictably beneficial. An additional -Os flag is available for cases where reducing binary size is preferred over raw execution speed.