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

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

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

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=81009

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
Right, the attribute would imply a stronger guarantee on a declaration than on
a definition.

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).

For const pointer arguments, pr81009 points out another opportunity to rely on
restrict for a similar guarantee (i.e., read-only access to the object).