From: Paul Walmsley <pjw@kernel.org>
To: Feng Jiang <jiangfeng@kylinos.cn>
Cc: Paul Walmsley <pjw@kernel.org>,
palmer@dabbelt.com, aou@eecs.berkeley.edu, alex@ghiti.fr,
samuel.holland@sifive.com, charlie@rivosinc.com,
conor.dooley@microchip.com, linux-riscv@lists.infradead.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] riscv: lib: optimize strlen loop efficiency
Date: Sat, 24 Jan 2026 01:14:58 -0700 (MST) [thread overview]
Message-ID: <4cd1d9d1-34da-7d96-46ac-a5470cfa85c3@kernel.org> (raw)
In-Reply-To: <581a8707-cb16-46d9-b6b5-8fa267383318@kylinos.cn>
On Thu, 15 Jan 2026, Feng Jiang wrote:
> On 2026/1/15 10:03, Paul Walmsley wrote:
> > On Thu, 18 Dec 2025, Feng Jiang wrote:
> >
> >> Optimize the generic strlen implementation by using a pre-decrement
> >> pointer. This reduces the loop body from 4 instructions to 3 and
> >> eliminates the unconditional jump ('j').
> >>
> >> Old loop (4 instructions, 2 branches):
> >> 1: lbu t0, 0(t1); beqz t0, 2f; addi t1, t1, 1; j 1b
> >>
> >> New loop (3 instructions, 1 branch):
> >> 1: addi t1, t1, 1; lbu t0, 0(t1); bnez t0, 1b
> >>
> >> This change improves execution efficiency and reduces branch pressure
> >> for systems without the Zbb extension.
> >
> > Looks reasonable; do you have any benchmarks on hardware that you can
> > share? Any reason why this patch stands alone and isn't rolled up as part
> > of your "optimize string function" series?
>
> Thanks for the feedback.
>
> This patch predates the rest of the series, which is why it wasn't included
> in the 'optimize string function' rollup. At the time, I focused on correctness
> testing and observed the improvement through rdcycle instruction counts.
>
> Since the series still needs further refinement and may take a longer time to
> complete, I was hoping this standalone optimization could be considered independently.
Ok. Queued for v6.20.
Might be worth taking a look at David's suggestions for a followup patch?
- Paul
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next prev parent reply other threads:[~2026-01-24 8:15 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-12-18 3:26 [PATCH] riscv: lib: optimize strlen loop efficiency Feng Jiang
2026-01-15 2:03 ` Paul Walmsley
2026-01-15 3:23 ` Feng Jiang
2026-01-24 8:14 ` Paul Walmsley [this message]
2026-01-26 3:05 ` Feng Jiang
2026-01-15 11:19 ` David Laight
2026-01-15 18:46 ` David Laight
2026-01-26 2:52 ` Feng Jiang
2026-01-28 18:59 ` David Laight
2026-01-29 8:34 ` Feng Jiang
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