https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69155
--- Comment #9 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> --- (In reply to rguent...@suse.de from comment #8) > Well, there is another choice... > > - do not keep SSA form for PHI args on backedges > > that is, when we call get_component_ssa_name for a USE and do not have > it registered yet instead of creating a stray SSA name create > a decl and mark it for SSA renaming (and call update_ssa in the pass > TODO). > > Needs to do dominator walks to avoid too many renamings and adding > an arg to get_component_ssa_name to indicate whether we are asking > for a DEF or USE. > > I think that's the cleanest approach (even if it might not be the > "fastest" given SSA renaming costs). TBH it doesn't seem that clean to me. The pass already creates correct (at the point that the pass completes) SSA form even when there are cycles, so it doesn't seem a good idea to make it punt on that case just because of the intermediate state. (And presumably force other similar global update passes to do the same.) Wouldn't it be better to have an acceptable way of representing the intermediate state? After all, we already check name_registered_for_update_p in the fold routines, so creating an SSA name whose definition is pending doesn't seem any worse.
