Re: [PATCH] bitops: Share the generic ffs()/fls() binary search via UNROLL()

[Arnd Bergmann]

On Fri, Oct 16, 2026, at 04:56, Tzu-Ching Yang wrote:
> generic_ffs(), generic___ffs(), generic_fls() and generic___fls() each
> open-code the same binary search: if the low (or high) half of the bits
> still in play is zero, shift it out and adjust the result, then repeat
> with half the width. Each copy spells out its own masks, the two
> unsigned long versions need an #if for their 32-bit step, and the
> copies differ in the order of the two updates in a step and in how the
> test is written. They implement some or all of these operations on
> riscv (without Zbb, and as the fallback with Zbb), ARM before ARMv5,
> m68k without bitfield instructions, sh, sparc32, microblaze, nios2 and
> a few other architectures.
>
> Add <asm-generic/bitops/ffs-fls-search.h> with one search step per
> direction, whose shift and mask follow from the step number and the
> type of the word, and expand the steps with UNROLL() from
> <linux/unroll.h>. The four functions keep their definitions and
> kernel-doc; only the steps are shared. UNROLL() works in the
> preprocessor, so every shift and mask is still an integer constant
> expression. Its count has to be a literal, so rather than hard-coding
> six steps for a 64-bit word, expand the 20 that UNROLL() supports: a
> step past the width of the word shifts by 0 and is optimized away,
> which also removes the #if.

I don't understand what you are trying to do here, but I don't
think we want this change: I find the nested macros harder
to read than the open-coded version and your changelog text
fails to explain what the purpose is.

If you have a specific problem with one of the platforms using
this, I would suggest you address that one but leave the
generic code unchanged.

      Arnd