GCC: Difference between -O3 and -Os

Question

I am quite familiar with GCC -O3 flag, but how it differs from -Os, in which situation we should prefer one over other?

Answer 1

It depends. Do you need to optimize speed or size?

-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, -ftree-loop-vectorize, -ftree-slp-vectorize, -fvect-cost-model, -ftree-partial-pre and -fipa-cp-clone 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.
-Os Disables the following optimization flags:

-falign-functions -falign-jumps -falign-loops -falign-labels -freorder-blocks -freorder-blocks-and-partition -fprefetch-loop-arrays

http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

Actually, -O is a shorthand for a long list of independent optimizations. If you don't know what you need, just go for -O3.