lkml.org 
[lkml]   [2021]   [Oct]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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@quicinc.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


\
 
 \ /
  Last update: 2021-10-26 21:23    [W:0.079 / U:0.944 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site