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Date: Mon, 18 Dec 2023 01:41:33 +0000
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Subject: Re: [PATCH] riscv: lib: Optimize 'strlen' function
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To: David Laight <David.Laight@ACULAB.COM>,
 "paul.walmsley@sifive.com" <paul.walmsley@sifive.com>,
 "palmer@dabbelt.com" <palmer@dabbelt.com>,
 "aou@eecs.berkeley.edu" <aou@eecs.berkeley.edu>
Cc: "conor.dooley@microchip.com" <conor.dooley@microchip.com>,
 "ajones@ventanamicro.com" <ajones@ventanamicro.com>,
 "samuel@sholland.org" <samuel@sholland.org>,
 "alexghiti@rivosinc.com" <alexghiti@rivosinc.com>,
 "linux-riscv@lists.infradead.org" <linux-riscv@lists.infradead.org>,
 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
 "skhan@linuxfoundation.org" <skhan@linuxfoundation.org>
References: <20231213154530.1970216-1-ivan.orlov0322@gmail.com>
 <86d3947bce1f49c395224998e7d65dc2@AcuMS.aculab.com>
From: Ivan Orlov <ivan.orlov0322@gmail.com>
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On 12/17/23 17:00, David Laight wrote:
> I'd also guess that pretty much all the calls in-kernel are short.
> You might try counting as: histogram[ilog2(strlen_result)]++
> and seeing what it shows for some workload.
> I bet you (a beer if I see you!) that you won't see many over 1k.

Hi David,

Here is the statistics for strlen result:

[  223.169575] Calls count for 2^0: 6150
[  223.173293] Calls count for 2^1: 184852
[  223.177142] Calls count for 2^2: 313896
[  223.180990] Calls count for 2^3: 185844
[  223.184881] Calls count for 2^4: 87868
[  223.188660] Calls count for 2^5: 9916
[  223.192368] Calls count for 2^6: 1865
[  223.196062] Calls count for 2^7: 0
[  223.199483] Calls count for 2^8: 0
[  223.202952] Calls count for 2^9: 0
...

Looks like I've just lost a beer :)

Considering this statistics, I'd say implementing the word-oriented 
strlen is an overcomplication - we wouldn't get any performance gain and 
it just doesn't worth it.

I simplified your code a little bit, it looks like the alignment there 
is unnecessary: QEMU test shows the same performance independently from 
alignment. Tests on the board gave the same result (perhaps because the 
CPU on the board has 2 DDR channels?)

	mv t0, a0
1:
	lbu t1, 0(a0)
	lbu t2, 1(a0)
	addi a0, a0, 2
	beqz t1, 2f
	bnez t2, 1b
	addi a0, a0, 1
2:
	addi a0, a0, -2
	sub a0, a0, t0
	ret

If it looks good to you, would you mind if I send the patch with it? 
Could I add you to suggested-by tag?

-- 
Kind regards,
Ivan Orlov