http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50131
--- Comment #4 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-08-21 09:19:28 UTC --- I'm sure or $-1,reg doesn't avoid the data dependence on reg and may even result in hitting partial reg stall issues. Surely xor reg,reg; not reg might be another alternative?