linux-kernel - RE: [PATCH] riscv: lib: Optimize 'strlen' function

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Message-ID: <a210197c479e48778672aa13287eef88@AcuMS.aculab.com>
Date: Sun, 17 Dec 2023 18:10:54 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Ivan Orlov' <ivan.orlov0322@...il.com>, "paul.walmsley@...ive.com"
	<paul.walmsley@...ive.com>, "palmer@...belt.com" <palmer@...belt.com>,
	"aou@...s.berkeley.edu" <aou@...s.berkeley.edu>
CC: "conor.dooley@...rochip.com" <conor.dooley@...rochip.com>,
	"ajones@...tanamicro.com" <ajones@...tanamicro.com>, "samuel@...lland.org"
	<samuel@...lland.org>, "alexghiti@...osinc.com" <alexghiti@...osinc.com>,
	"linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"skhan@...uxfoundation.org" <skhan@...uxfoundation.org>
Subject: RE: [PATCH] riscv: lib: Optimize 'strlen' function

From: Ivan Orlov
> Sent: 13 December 2023 15:46

Looking at the old code...

>  1:
> -	lbu	t0, 0(t1)
> -	beqz	t0, 2f
> -	addi	t1, t1, 1
> -	j	1b

I suspect there is (at least) a two clock stall between
the 'ldu' and 'beqz'.
Allowing for one clock for the 'predicted taken' branch
that is 7 clocks/byte.

Try this one - especially on 32bit:

	mov	t0, a0
	and	t1, t0, 1
	sub	t0, t0, t1
	bnez	t1, 2f
1:
	ldb	t1, 0(t0)
2:	ldb	t2, 1(t0)
	add	t0, t0, 2
	beqz	t1, 3f
	bnez	t2, 1b
	add	t0, t0, 1
3:	sub	t0, t0, 2
	sub	a0, t0, a0
	ret

Might be 6 clocks for 2 bytes.
The much smaller cache footprint will also help.

	David

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