https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91255

            Bug ID: 91255
           Summary: Can't use define_subst in target that uses
                    set_attr_alternative
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Keywords: internal-improvement
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ktkachov at gcc dot gnu.org
                CC: rsandifo at gcc dot gnu.org
  Target Milestone: ---

I'm trying to add a define_subst use in the arm backend but am getting many
build errors complaining about:
`set_attr_alternative' is unsupported by `define_subst'

Looking at the gensupport.c code it iterates over all define_insns and errors
if any of them have set_attr_alternative.

The usecase I'm targetting doesn't involve patterns with set_attr_alternative,
so could we make the define_subst handling more robust to only error out if the
define_subst is actually attempted on a set_attr_alternative?

grepping around the sources it looks like the only ports that make use of
define_subst don't have any set_attr_alternative occurrences

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