http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55752
--- Comment #1 from Uros Bizjak <ubizjak at gmail dot com> 2012-12-20 12:59:40 UTC --- The RTXes that corresponds to builtins are all declared unspec_volatile. According to the comment in sched-deps.c, around line 2723, it is assumed that unspec_volatile clobbers all registers.
