[Bug tree-optimization/81679] use attribute unused on function arguments as an optimization hint

["joseph at codesourcery dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81679

--- Comment #4 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Wed, 2 Aug 2017, msebor at gcc dot gnu.org wrote:

> If there is a concern that the attribute could be used on declarations in
> existing code that the optimization might break, then the attribute could be
> specified differently (e.g., as a function attribute with an argument number
> indicating which argument is unused; that would also differentiate it from the
> existing function attribute).  Or the same feature could be provided under a
> different attribute.  The main idea here is the ability to express the notion
> that a function doesn't modify an object via its (non-const) pointer argument. 
> The name for the feature is secondary (though "unused" is obviously a nice
> fit).

The "unused" attribute always means "possibly unused, don't warn if 
actually unused".  It would be a mistake to make it mean anything else.