Re: [PATCH] bitmap: simplify GENMASK(size - 1, 0) lines

From: Yury Norov
Date: Tue Oct 26 2021 - 14:54:37 EST


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@xxxxxxxxxxx>
> ---
> 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.

It's too late to merge it in 5.15, so I will add it in a spring merge
window - most probably in a bitmap branch for 5.17 or 5.18:

https://github.com/norov/linux/tree/bitmap-2022-Apr-01

Thanks,
Yury