On Sun, Nov 06, 2005 at 01:32:43PM +0100, Giovanni Bajo wrote:
> If -O1 means "optimize, but be fast", what does -O2 mean? And what does -O3
> mean? If -O2 means "the current set of optimizer that we put in -O2", that's
> unsatisfying for me.
`-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.
Including loop unrolling to -O2 is IMNSHO a bad idea, as loop unrolling
increases code size, sometimes a lot. And the distinction between -O2
and -O3 is exactly in the space-for-speed tradeoffs.
On many CPUs for many programs, -O3 generates slower code than -O2,
because the cache footprint disadvantages override positive effects
of the loop unrolling, extra inlining etc.
Jakub
