linux-kernel - Re: [PATCH v4 3/4] powerpc/lib: implement strlen() in assembly

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Message-ID: <20180608120532.GD17342@gate.crashing.org>
Date:   Fri, 8 Jun 2018 07:05:32 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Gabriel Paubert <paubert@...m.es>
Cc:     Christophe Leroy <christophe.leroy@....fr>,
        wei.guo.simon@...il.com, linux-kernel@...r.kernel.org,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v4 3/4] powerpc/lib: implement strlen() in assembly

On Fri, Jun 08, 2018 at 01:45:13PM +0200, Gabriel Paubert wrote:
> On Fri, Jun 08, 2018 at 10:20:41AM +0000, Christophe Leroy wrote:
> > +	rlwinm.	r8, r9, 0, 0xff000000
> > +	beq	20f
> > +	rlwinm.	r8, r9, 0, 0x00ff0000
> > +	beq	21f
> > +	rlwinm.	r8, r9, 0, 0x0000ff00
> > +	beq	22f
> > +23:	subf	r3, r3, r10
> 
> Actually these rlwinm. can likely be replaced by a single
> cntlzw /cntlzd; for 32 bit something like:
> 
> 	cntlzw  r8,r9
> 	subf    r3,r3,r10	
> 	srwi	r8,r8,3
> 	add	r3,r3,r8
> 	blr

The code is finding the first zero byte in the word, not how many leading
zero bytes there are.


Segher