How do we write unused arguments?

[Jeff Law]

On 11/05/2015 02:32 AM, Bernd Schmidt wrote:
> When reviewing patches I'm never quite sure which of the following we
> should be using:
>
> some_target_hook (tree decl, machine_mode mode ATTRIBUTE_UNUSED)
>
> some_target_hook (tree decl, machine_mode ARG_UNUSED (mode))
>
> some_target_hook (tree decl, machine_mode /* mode */)
>
> some_target_hook (tree decl, machine_mode)
>
> Any opinions? I'm leaning towards the last version since it's idiomatic
> in the language we're using and has the least amount of visual clutter.
> If no one comments, I'll start enforcing that in patch reviews.
> Currently no one seems sure and everything is getting totally inconsistent.
I think we should move to the 3rd form when possible.  I didn't even 
know the language supported unnamed arguments in this way until recently :-)

Bringing some consistency to this will be nice.

jeff