What effect does -O9 have?
It has the same effect as -O3. The -O flag will accept any number
from
0-9 but only zero through three are implemented, and I don't think
that
anyone has any plans to ever implement four through nine any time
soon.
-O42 works, too; not just numbers smaller than 10.
The fact that people use this -O9 idiom is a little disturbing,
because
it implies that they want every conceivable optimization that gcc
could
possibly ever offer, even if it made compilation take weeks for no
gain.
Also if a future -O9 wouldn't work for them (perhaps it
would break the C standard in a minor but significant
way); also it wouldn't enable _all_ optimisations, since
not all optimisation passes are a win on all code and so
some wouldn't be enabled by default at any -O level.
But by design dangerous or futile optimizations do not get
enabled with -O levels because these -O options are meant to represent
blends of settings that are generally considered safe and useful by a
large range of users by the compiler authors. So specifying -O9 is a
bit of "but this one goes to eleven!"-type nonsense, if you ask me.
Well it's _cool_, eh? :-)
Segher
