https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94522
--- Comment #3 from Michael Matz <matz at gcc dot gnu.org> --- See the llvm link of the respective patch. They specify that the outputs are reliable only on the fallthrough (i.e. no goto taken) path (in particular the outputs might or might not have been changed on the jump paths). That avoids all the problems reload and it's replacements traditionally have with outputs on jump insns, so it'd be relatively easy for us to support that as well. This restriction, if documented, isn't even unreasonable and still allows some interesting uses of goto asms with outputs.
