-Os optimizes for size. -Os enables all -O2 optimizations that do not usually increase code size and performs further optimizations designed to reduce code size.
-Os is very useful for large applications, like Firefox, as it will reduce load time, memory usage, cache misses, disk usage etc. Code compiled with -Os can be faster than -O2 or -O3 because of this. It's also recommended for older computers with a low amount of RAM, disk space or cache on the CPU. But beware that -Os is not as well tested as -O2 and might trigger compiler bugs.
Note that only one of the above flags may be chosen. If you choose more than one, the last one specified will have the effect.
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