https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113059
--- Comment #12 from Jakub Jelinek <jakub at gcc dot gnu.org> --- (In reply to Richard Biener from comment #11) > IMHO REG_UNUSED notes should stay correct or be removed. Instead of a full > solution can we just wipe them in postreload-cse as we know that pass will > break them? Any user then should recompute the notes. Originally pass > properties were thought of a vehicle indicating these kind of IL features, > so we > could add PROP_rtl_unused_notes and if not set treat them as possibly > invalid when present (instead of actively wiping them), and when a pass > requires them re-compute them. > > The problem with properties is that they are always set/unset irrespective > of whether a pass did something destroying it. That could be > circumvented by postreload-cse only clearing the flag when it does > something. > > Not (re-)using pass properties for this but a flag in struct function > works as well of course (or treat it as part of the DF state). I was hoping we could have some df checking which would discover invalid REG_UNUSED notes and so we would know which passes need tweaking, but I'm afraid my df knowledge is insufficient for that. Dunno how often postreload-cse actually extends life time of some register, which should determine whether we want to drop REG_UNUSED notes unconditionally at the end of postreload-cse, or whether we e.g. want just some flag whether we've extended lifetime of something during it and only remove REG_UNUSED notes if that flag is set.
