It was added by: http://gcc.gnu.org/ml/gcc-patches/2004-01/msg01111.html
But it was not documented. Zem no longer works at Apple now either. -- Summary: [4.0/4.1/4.2 Regression] attribute altivec is not documented Product: gcc Version: 4.2.0 Status: UNCONFIRMED Keywords: documentation Severity: normal Priority: P3 Component: target AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: pinskia at gcc dot gnu dot org GCC target triplet: powerpc*-*-*, rs6000-*-* http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27544