linux-kernel - Re: [PATCH v6 3/4] powerpc/lib: implement strlen() in assembly

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Message-ID: <20180614215137.GB16221@gate.crashing.org>
Date:   Thu, 14 Jun 2018 16:51:37 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe LEROY <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, wei.guo.simon@...il.com,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v6 3/4] powerpc/lib: implement strlen() in assembly

On Tue, Jun 12, 2018 at 07:01:59PM +0200, Christophe LEROY wrote:
> 
> 
> Le 12/06/2018 à 16:53, Segher Boessenkool a écrit :
> >On Tue, Jun 12, 2018 at 09:14:53AM +0000, Christophe Leroy wrote:
> >>---
> >>Not tested on PPC64.
> >
> >It won't be acceptable until that happens.  It also is likely quite bad
> >performance on all 64-bit CPUs from the last fifteen years or so.  Or you
> >did nothing to prove otherwise, at least.
> 
> Will it be as bad as the generic implementation which does it byte per 
> byte ?

Probably not.  But how is it for short inputs, etc.?

The main point is that it needs actual testing _for correctness_.

Btw, GCC 7 and later can expand many memcmp as builtins on PowerPC (just
like memset and memcpy etc.), creating better code, without function call.


Segher