linux-kernel - Re: [PATCH] bitmap: simplify GENMASK(size

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Date:   Tue, 26 Oct 2021 22:21:58 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Qian Cai <quic_qiancai@...cinc.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bitmap: simplify GENMASK(size - 1, 0) lines

On Tue, Oct 26, 2021 at 11:54:16AM -0700, Yury Norov wrote:
> On Tue, Oct 26, 2021 at 10:41:08AM -0400, Qian Cai wrote:
> > Since "size" is an "unsigned int", the rvalue "size - 1" will still be
> > "unsigned int" according to the C standard (3.2.1.5 Usual arithmetic
> > conversions). Therefore, GENMASK(size - 1, 0) will always return 0UL. Those
> > are also caught by GCC (W=2):
> > 
> > ./include/linux/find.h: In function 'find_first_bit':
> > ./include/linux/bits.h:25:22: warning: comparison of unsigned expression in '< 0' is always false [-Wtype-limits]
> >    25 |   __is_constexpr((l) > (h)), (l) > (h), 0)))
> >       |                      ^
> > ./include/linux/build_bug.h:16:62: note: in definition of macro 'BUILD_BUG_ON_ZERO'
> >    16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
> >       |                                                              ^
> > ./include/linux/bits.h:25:3: note: in expansion of macro '__is_constexpr'
> >    25 |   __is_constexpr((l) > (h)), (l) > (h), 0)))
> >       |   ^~~~~~~~~~~~~~
> > ./include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK'
> >    38 |  (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> >       |   ^~~~~~~~~~~~~~~~~~~
> > ./include/linux/find.h:119:31: note: in expansion of macro 'GENMASK'
> >   119 |   unsigned long val = *addr & GENMASK(size - 1, 0);
> >       |                               ^~~~~~~
> > ./include/linux/bits.h:25:34: warning: comparison of unsigned expression in '< 0' is always false [-Wtype-limits]
> >    25 |   __is_constexpr((l) > (h)), (l) > (h), 0)))
> >       |                                  ^
> > ./include/linux/build_bug.h:16:62: note: in definition of macro 'BUILD_BUG_ON_ZERO'
> >    16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
> >       |                                                              ^
> > ./include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK'
> >    38 |  (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> >       |   ^~~~~~~~~~~~~~~~~~~
> > ./include/linux/find.h:119:31: note: in expansion of macro 'GENMASK'
> >   119 |   unsigned long val = *addr & GENMASK(size - 1, 0);
> >       |                               ^~~~~~~
> > 
> > Signed-off-by: Qian Cai <quic_qiancai@...cinc.com>
> > ---
> >  include/linux/find.h | 28 ++++++++--------------------
> >  1 file changed, 8 insertions(+), 20 deletions(-)
> > 
> > diff --git a/include/linux/find.h b/include/linux/find.h
> > index 5bb6db213bcb..5ce2b17aea42 100644
> > --- a/include/linux/find.h
> > +++ b/include/linux/find.h
> > @@ -115,11 +115,8 @@ unsigned long find_next_zero_bit(const unsigned long *addr, unsigned long size,
> >  static inline
> >  unsigned long find_first_bit(const unsigned long *addr, unsigned long size)
> >  {
> > -	if (small_const_nbits(size)) {
> > -		unsigned long val = *addr & GENMASK(size - 1, 0);
> > -
> > -		return val ? __ffs(val) : size;
> > -	}
> > +	if (small_const_nbits(size))
> > +		return size;
> >  
> >  	return _find_first_bit(addr, size);
> >  }
> 
> [...]
> 
> Nice catch! I'm a bit concerned that small_const_nbits() will never
> allow GENMASK() to be passed with size == 0, but the patch looks
> good to me overall.

Can you explain to me how it is supposed to work?

For example,

    x = 0xaa55;
    size = 5;

    printf("%lu\n", find_first_bit(&x, size));

In the resulting code we will always have 5 as the result,
but is it correct one?

-- 
With Best Regards,
Andy Shevchenko