Re: [PATCH v3 0/8] riscv: optimize string functions and add kunit tests

Feng Jiang
2026-01-21T08:12:45Z

On 2026/1/21 15:01, Andy Shevchenko wrote:
> On Wed, Jan 21, 2026 at 8:44 AM Feng Jiang <jiangfeng@kylinos.cn> wrote:
>> On 2026/1/20 15:36, Andy Shevchenko wrote:
>>> On Tue, Jan 20, 2026 at 02:58:44PM +0800, Feng Jiang wrote:
>
> ...
>>>> word-at-a-time logic, showing significant gains as the string length
>>>> increases.
>>>
>>> Hmm... Have you tried to optimise the generic implementation to use
>>> word-at-a-time logic and compare?
>>
>> Regarding the generic implementation, even if we were to optimize the C code
>> to use word-at-a-time logic (the has_zero() style bit-manipulation), it still
>> wouldn't match the Zbb version's efficiency.
>>
>> The traditional C-based word-level detection requires a sequence of arithmetic
>> operations to identify NUL bytes. In contrast, the RISC-V orc.b instruction
>> collapses this entire check into a single hardware cycle. I've focused on this
>> architectural approach to fully leverage these specific Zbb features, which
>> provides a level of instruction density that generic C math cannot achieve.
>
> I understand that. My point is if we move the generic implementation
> to use word-at-a-time technique the difference should not go 4x,
> right? Perhaps 1.5x or so. I believe this will be a very useful
> exercise.
>

That is a very insightful point, thanks for the suggestion. I'll look into
optimizing the generic string library as a follow-up task to see if we can
bring some improvements there as well.

Thanks again for the guidance.

--
With Best Regards,
Feng Jiang