[Bug jit/66594] jitted code should use -mtune=native

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66594

--- Comment #6 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
(In reply to ktkachov from comment #4)
> (In reply to David Malcolm from comment #2)
> > (In reply to Andrew Pinski from comment #1)
> > > This should be true on all targets which have -mcpu=native (or
> > > -march=native).  Note x86 options are not always the same on x86 vs arm vs
> > > aarch64 vs ppc.
> > 
> > Thanks.
> > 
> > Is there a way to express
> >   "give me the most specialized possible code for the current host"?
> > in a manner that's consistent across all supported hosts?
> > (e.g. in the "specs" language?)
> > 
> > e.g. "-march=native -mcpu=native -mtune=native"
> 
> A note that the meaning of march/mcpu/mtune may differ among targets.
> For arm and aarch64, for example, the preferred option is -mcpu, which is
> the combination of march+mtune

Perhaps we need a target macro that's something like:
  TARGET_JIT_NATIVE_CODE_SPEC
or somesuch, defaulting to empty, but defining it per-arch to a spec-language
string containing the appropriate option(s) for that host/target?  (assuming
host==target, which is currently the case for the jit).