[Bug driver/88024] New: At -O0 and -Og, GCC should warn if you explicitly try to enable an option that is ignored

["sandra at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88024

            Bug ID: 88024
           Summary: At -O0 and -Og, GCC should warn if you explicitly try
                    to enable an option that is ignored
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: driver
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sandra at gcc dot gnu.org
  Target Milestone: ---

While poking at documentation issue PR59658, I realized that there is a code
bug in the handling of optimization options too:  there ought to be a
diagnostic if you try to enable an optimization that is completely ignored due
to the different pass lists for -O0 and -Og.  E.g.

gcc -O0 -fif-conversion -fshrink-wrap foo.c

ought to warn you that those options have no effect at -O0.

I think this could be fixed by adding some additional properties to the
default_options_table in opts.c, which would have the additional benefit of
making it easier to document (and maintain the documentation for) these
options.