https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112572
--- Comment #23 from Richard Sandiford <rsandifo at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #18)
> compare-elim.cc depends on up to date REG_UNUSED and between before
> vzeroupper and cmpelim the note gets out of date.
Thanks for tracking it down.

I can't find this written down from a quick search, so perhaps I'm
misremembering, but I thought that REG_UNUSED notes had to be kept
up to date.  Only REG_DEAD ones are allowed to rot.  Like you say,
single_set is used all over the place, and is expected to be safe.

If so, I guess it's postreload that should be fixed.

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