[Bug c/122705] Add function attributes to mark functions that return a pointer parameter, verbatim or derived

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122705

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Alejandro Colomar from comment #0)
> I propose [[gnu::returns_input_ptr(1)]] for strcpy(3),
> and [[gnu::returns_derived_ptr(1)]] for strstr(3),
> where in both attributes, the '1' is a parameter index, indicating
> which parameter is the provenance of the returned pointer.

And GCC already has code to special case some builtins:
```
/* Return the argument that the call STMT to a built-in function returns
   (including with an offset) or null if it doesn't.  */

tree
pass_waccess::gimple_call_return_arg (gcall *call)
{
  /* Check for attribute fn spec to see if the function returns one
     of its arguments.  */
  attr_fnspec fnspec = gimple_call_fnspec (call);
  unsigned int argno;
  if (!fnspec.returns_arg (&argno))
    {
      if (gimple_call_num_args (call) < 1)
        return NULL_TREE;

      if (!gimple_call_builtin_p (call, BUILT_IN_NORMAL))
        return NULL_TREE;

      tree fndecl = gimple_call_fndecl (call);
      switch (DECL_FUNCTION_CODE (fndecl))
        {
        case BUILT_IN_MEMPCPY:
        case BUILT_IN_MEMPCPY_CHK:
        case BUILT_IN_MEMCHR:
        case BUILT_IN_STRCHR:
        case BUILT_IN_STRRCHR:
        case BUILT_IN_STRSTR:
        case BUILT_IN_STPCPY:
        case BUILT_IN_STPCPY_CHK:
        case BUILT_IN_STPNCPY:
        case BUILT_IN_STPNCPY_CHK:
          argno = 0;
          break;

        default:
          return NULL_TREE;
        }
    }

  if (gimple_call_num_args (call) <= argno)
    return NULL_TREE;

  return gimple_call_arg (call, argno);
}

```

Exposing this might be useful but it might also get abused incorrectly ...