Re: [RESEND PATCH v2 0/6] lib/find_bit: fast path for small bitmaps

Yury Norov
2021-02-15T21:31:35Z

[add David Laight <David.Laight@ACULAB.COM> ]

On Sat, Jan 30, 2021 at 11:17:11AM -0800, Yury Norov wrote:
> Bitmap operations are much simpler and faster in case of small bitmaps
> which fit into a single word. In linux/bitmap.h we have a machinery that
> allows compiler to replace actual function call with a few instructions
> if bitmaps passed into the function are small and their size is known at
> compile time.
>
> find_*_bit() API lacks this functionality; despite users will benefit from
> it a lot. One important example is cpumask subsystem when
> NR_CPUS <= BITS_PER_LONG. In the very best case, the compiler may replace
> a find_*_bit() call for such a bitmap with a single ffs or ffz instruction.
>
> Tools is synchronized with new implementation where needed.
>
> v1: https://www.spinics.net/lists/kernel/msg3804727.html
> v2: - employ GENMASK() for bitmaps;
> - unify find_bit inliners in;
> - address comments to v1;

Comments so far:
- increased image size (patch #8) - addressed by introducing
CONFIG_FAST_PATH;
- split tools and kernel parts - not clear why it's better.

Anything else?