linux-kernel - Re: [PATCH v3 6/8] riscv: lib: add strnlen implementation

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Message-ID: <cb65c596-7950-4402-b5e7-fddea12bd7e1@kylinos.cn>
Date: Fri, 23 Jan 2026 09:28:54 +0800
From: Feng Jiang <jiangfeng@...inos.cn>
To: Qingfang Deng <dqfext@...il.com>
Cc: pjw@...nel.org, palmer@...belt.com, aou@...s.berkeley.edu, alex@...ti.fr,
 akpm@...ux-foundation.org, kees@...nel.org, andy@...nel.org,
 ebiggers@...nel.org, martin.petersen@...cle.com, ardb@...nel.org,
 charlie@...osinc.com, conor.dooley@...rochip.com, ajones@...tanamicro.com,
 linus.walleij@...aro.org, nathan@...nel.org,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-hardening@...r.kernel.org, Joel Stanley <joel@....id.au>
Subject: Re: [PATCH v3 6/8] riscv: lib: add strnlen implementation

On 2026/1/21 15:24, Qingfang Deng wrote:
> On Tue, 20 Jan 2026 14:58:50 +0800, Feng Jiang wrote:
>> diff --git a/arch/riscv/lib/strnlen.S b/arch/riscv/lib/strnlen.S
> 
> Branches that test maxlen can be replaced with Zbb minu instruction.
> (see below)
> 

...

>> +	/*
>> +	 * The first chunk is special: compare against the number
>> +	 * of valid bytes in this chunk.
>> +	 */
>> +	srli	a0, t1, 3
>> +
>> +	/* Limit the result by maxlen. */
>> +	bleu	a1, a0, 3f
> 
> minu	a0, a0, a1
> 
>> +
>> +	bgtu	t3, a0, 2f
>> +
>> +	/* Prepare for the word comparison loop. */
>> +	addi	t2, t0, SZREG
>> +	li	t3, -1
>> +
>> +	/*
>> +	 * Our critical loop is 4 instructions and processes data in
>> +	 * 4 byte or 8 byte chunks.
>> +	 */
>> +	.p2align 3
>> +1:
>> +	REG_L	t1, SZREG(t0)
>> +	addi	t0, t0, SZREG
>> +	orc.b	t1, t1
>> +	bgeu	t0, t4, 4f
>> +	beq	t1, t3, 1b
>> +4:
>> +	not	t1, t1
>> +	CZ	t1, t1
>> +	srli	t1, t1, 3
>> +
>> +	/* Get number of processed bytes. */
>> +	sub	t2, t0, t2
>> +
>> +	/* Add number of characters in the first word.  */
>> +	add	a0, a0, t2
>> +
>> +	/* Add number of characters in the last word.  */
>> +	add	a0, a0, t1
>> +
>> +	/* Ensure the final result does not exceed maxlen. */
>> +	bgeu	a0, a1, 3f
> 
> minu	a0, a0, a1
> 

Thanks for the great suggestion! I see your point now—using minu is indeed a much
more elegant and efficient way to handle the maxlen constraint. It nicely eliminates
unnecessary branches and simplifies the code while still allowing for early returns.

I'll incorporate this into a v4 patch and add a Suggested-by tag for you. Thanks
again for your insightful review!

-- 
With Best Regards,
Feng Jiang