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

Alexander Lobakin
2022-07-15T14:52:05Z

From: Yury Norov <yury.norov@gmail.com>
Date: Fri, 15 Jul 2022 07:19:12 -0700

> 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?

I'm sending it in a couple minutes :)

>
> Thanks,
> Yury

Thanks,
Olek