[Bug rtl-optimization/66790] Invalid uninitialized register handling in REE

["bonzini at gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

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.