[PATCH 0/3] riscv: word-at-a-time: improve find_zero()

[Jisheng Zhang]

On Tue, Jan 13, 2026 at 08:24:54PM +0800, Jisheng Zhang wrote:
> Currently, there are two problems with riscv find_zero():
> 
> 1. When !RISCV_ISA_ZBB, the generic fls64() bring non-optimal code.
> 
> But in word-at-a-time case, we don't have to go with fls64() code path,
> instead, we can fallback to the generic word-at-a-time implementaion.
> 
> What's more, the fls64() brings non-necessary zero bits couting for
> RV32. In fact, fls() is enough.
> 
> 2. Similar as 1, the generic fls64() also brings non-optimal code when
> RISCV_ISA_ZBB=y but HW doesn't support Zbb.
> 
> So this series tries to improve find_zero() by falling back to generic
> word-at-a-time implementaion where necessary. We dramatically reduce
> the instructions of find_zero() from 33 to 8! Also testing with the
> micro-benchamrk in patch1 shows that the performance is improved by
> about 1150%!
> 
> 
> After that, we improve find_zero() for Zbb further by applying similar
> optimization as Linus did in commit f915a3e5b018 ("arm64:
> word-at-a-time: improve byte count calculations for LE"), so that
> we share the similar improvements:
> 
> "The difference between the old and the new implementation is that
> "count_zero()" ends up scheduling better because it is being done on a
> value that is available earlier (before the final mask).
> 
> But more importantly, it can be implemented without the insane semantics
> of the standard bit finding helpers that have the off-by-one issue and
> have to special-case the zero mask situation."
> 
> On RV64 w/ Zbb, the new "find_zero()" ends up just "ctz" plus the shift
> right that then ends up being subsumed by the "add to final length".
> Reduce the total instructions from 7 to 3!
> 
> But I have no HW platform which supports Zbb, so I can't get the
> performance improvement numbers by the last patch, only built and
> tested the patch on QEMU.

Soft ping. No feedback for more than 6 months. Fortunately, this series
can still be cleanly applied.

Thanks

> 
> Jisheng Zhang (3):
>   riscv: word-at-a-time: improve find_zero() for !RISCV_ISA_ZBB
>   riscv: word-at-a-time: improve find_zero() without Zbb
>   riscv: word-at-a-time: improve find_zero() for Zbb
> 
>  arch/riscv/include/asm/word-at-a-time.h | 47 +++++++++++++++++++++++--
>  1 file changed, 44 insertions(+), 3 deletions(-)
> 
> -- 
> 2.51.0
> 
> 
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