https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66790
--- Comment #31 from Paolo Bonzini <bonzini at gnu dot org> --- Ah, I see now. I think you're right that the DF_REF_MUST_CLOBBER case should also clear GEN in df_live_bb_local_compute. However, regarding the "BTW" I am fairly sure now that df_live_bb_local_compute and the corresponding function for MIR should handle may-clobber and may-sets differently. If you think of may-clobber and may-set as a diamond-shaped CFG: . . / \ / \ / \ / \ clobber | set | \ / \ / \ / \ / \/ \/ Then at the join point you have an "OR" for LIVE (so the clobber's KILL disappears and the set's GEN remains), and an "AND" for MIR. For MIR the clobber's KILL remains and the set's GEN disappears.