report_fatal_error("MachineCopyPropagation should be run after"
" register allocation!");
- if (MO.isDef()) {
+ // Treat undef use like defs.
+ // The backends are allowed to do whatever they want with undef value
+ // and we cannot be sure this register will not be rewritten to break
+ // some false dependencies for the hardware for instance.
+ if (MO.isDef() || MO.isUndef()) {
Defs.push_back(Reg);
continue;
}