Gcc 4.0 comes with good news for the owners of Apple machines that have Altivec support: autovectorization is what we are talking about.
Autovectorization is a multiplatform project to provide gcc with a loop vectorizer. In practice this means that gcc can finally compile code that will take advantage of altivec optimizations without the need for the developer to explicitly use altivec code.

The project is still at the beginning and it allows source code to take advantage of Altivec just in few cases (give a look at this page for some code examples and to look at the project status).

The list of enhancements ready to go into gcc 4.1 is long and promising, though.

Mac OS X 10.4 Tiger is shipping with gcc-4.0 as the default compiler (along with the choice of gcc-3.0) and can already take advantage of the autovectorizer component. The new compiler will probably make its appearance soon on Gentoo Linux and on the unstable branch of Debian (as, right now, the gcc 4.0 .deb is available and installable from the experimental tree).