Translations:GCC optimization/38/en
From Gentoo Wiki
There are seven -O settings: -O0, -O1, -O2, -O3, -Os, -Oz, and -Og. Only use one of them in /etc/portage/make.conf. There is an eighth option which must not be used system-wide: -Ofast - despite its name, it breaks standards conformance and is dangerous.