Re: [PATCH v5 6/9] bitops: let optimize out non-atomic bitops on compile-time constants

Yury Norov
2022-07-15T14:19:20Z

On Fri, Jul 15, 2022 at 06:49:46AM -0700, Guenter Roeck wrote:
> On 7/15/22 06:26, Alexander Lobakin wrote:
> > From: Guenter Roeck <linux@roeck-us.net>
> > Date: Thu, 14 Jul 2022 17:04:02 -0700
> >
> > > On Fri, Jun 24, 2022 at 02:13:10PM +0200, Alexander Lobakin wrote:
> > > > Currently, many architecture-specific non-atomic bitop
> > > > implementations use inline asm or other hacks which are faster or
> >
> > [...]
> >
> > > > Cc: Mark Rutland <mark.rutland@arm.com>
> > > > Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
> > > > Reviewed-by: Marco Elver <elver@google.com>
> > >
> > > Building i386:allyesconfig ... failed
> > > --------------
> > > Error log:
> > > arch/x86/platform/olpc/olpc-xo1-sci.c: In function 'send_ebook_state':
> > > arch/x86/platform/olpc/olpc-xo1-sci.c:83:63: error: logical not is only applied to the left hand side of comparison
> >
> > Looks like a trigger, not a cause... Anyway, this construct:
> >
> > unsigned char state;
> >
> > [...]
> >
> > if (!!test_bit(SW_TABLET_MODE, ebook_switch_idev->sw) == state)
> >
> > doesn't look legit enough.
> > That redundant double-negation [of boolean value], together with
> > comparing boolean to char, provokes compilers to think the author
> > made logical mistakes here, although it works as expected.
> > Could you please try (if it's not automated build which you can't
> > modify) the following:
> >
>
> Agreed, the existing code seems wrong. The change below looks correct
> and fixes the problem. Feel free to add
>
> Reviewed-and-tested-by: Guenter Roeck <linux@roeck-us.net>
>
> to the real patch.
>
> Thanks,
> Guenter
>
> > --- a/arch/x86/platform/olpc/olpc-xo1-sci.c
> > +++ b/arch/x86/platform/olpc/olpc-xo1-sci.c
> > @@ -80,7 +80,7 @@ static void send_ebook_state(void)
> > return;
> > }
> > - if (!!test_bit(SW_TABLET_MODE, ebook_switch_idev->sw) == state)
> > + if (test_bit(SW_TABLET_MODE, ebook_switch_idev->sw) == !!state)
> > return; /* Nothing new to report. */
> > input_report_switch(ebook_switch_idev, SW_TABLET_MODE, state);
> > ---
> >
> > We'd take it into the bitmap tree then. The series revealed
> > a fistful of existing code issues already :)

Would you like me to add your signed-off-by and apply, or you prefer
to send it out as a patch?

Thanks,
Yury