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Asked: 2026-05-11T09:24:06+00:00

I see this flag a lot in the makefiles. What does it mean and

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I see this flag a lot in the makefiles. What does it mean and when should it be used?

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  1. Optimization level 2.

    From the GCC man page:

    -O1 Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.

    -O2 Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. The compiler does not perform loop unrolling or function inlining when you specify -O2. As compared to -O, this option increases both compilation time and the performance of the generated code.

    -O3 Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions, -funswitch-loops, -fpredictive-commoning, -fgcse-after-reload and -ftree-vectorize options.

    -O0 Reduce compilation time and make debugging produce the expected results. This is the default.

    -Os Optimize for size. -Os enables all -O2 optimizations that do not typically increase code size. It also performs further optimizations designed to reduce code size.

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