[Bug c/32911] New: Function __attribute__ ((idempotent))

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

In GNOME's gobject object-oriented code, each type has a "get_type" function. 
Something like, eg:

GType
pango_layout_line_get_type(void)
{
  static GType our_type = 0;

  if (our_type == 0)
    our_type = g_boxed_type_register_static (I_("PangoLayoutLine"),
                           (GBoxedCopyFunc) pango_layout_line_ref,
                           (GBoxedFreeFunc) pango_layout_line_unref);
  return our_type;
}

Since these functions are called quite frequently (as part of casts among other
things), people have started marking them with __attribute__((const)) so the
compiler can optimize away multiple calls to them.  However, there is a down
side to it.  Sometimes one needs to make sure the type is registered, so they
call the _get_type() function to ensure that, but the compiler, seeing the
((const)) attribute, optimizes away the call.

This is of course a bug to mark functions with side-effects as ((const)). 
That's why a new function attribute like "idempotent" is useful.  What it means
is that subsequent calls to this function return the same value as the first
call, have no side-effect, and can be optimized away, but not the first call...

Thanks,


-- 
           Summary: Function __attribute__ ((idempotent))
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: gnu at behdad dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32911