Re: A pettiness question.

[Andrea Arcangeli <andrea@suse.de>]

On Wed, Sep 21, 2005 at 02:01:11PM +0500, Fawad Lateef wrote:
> On 9/21/05, Ustyugov Roman <dr_unique@ymg.ru> wrote:
> > > Hi, All.
> > >
> > >     I found there are use double operator ! continuously sometimes in
> > > kernel.
> > > e.g:
> > >
> > >     static inline int is_page_cache_freeable(struct page *page)
> > >     {
> > >         return page_count(page) - !!PagePrivate(page) == 2;
> > >     }
> > >
> > >     Who would like tell me why write like above?
> > 
> > For example,
> > 
> >         int test = 5;
> >         !test will be  0,  !!test will be 1.
> > 
> > This give a enum of {0,1}. If test is not 0, !!test will give 1, otherwise 0.
> > 
> > Am I right?
> 
> Yes, but what abt the above case/example ??? PagePrivate is defined as
> test_bit and test_bit will return 0 or 1 only ...... So y there is (
> !! )  ??

Note that gcc should optimize it away as long as the asm*/bitops is
doing "return something != 0" like most archs do.

Most of the time test_bit retval is checked against zero only, here it's
one of the few cases where it's required to be 1 or 0. If you audit all
archs then you can as well remove the !! from above.

Thanks!