linux-kernel - Re: [PATCH 2/2] lib/string.c: Optimize memchr()

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Date:   Wed, 13 Jul 2022 12:57:52 +0300
From:   Andrey Semashev <andrey.semashev@...il.com>
To:     David Laight <David.Laight@...LAB.COM>,
        'Yu-Jen Chang' <arthurchang09@...il.com>
Cc:     "andy@...nel.org" <andy@...nel.org>,
        "akinobu.mita@...il.com" <akinobu.mita@...il.com>,
        Ching-Chun Huang <jserv@...s.ncku.edu.tw>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] lib/string.c: Optimize memchr()

On 7/13/22 12:39, David Laight wrote:
> From: Yu-Jen Chang
>> Sent: 12 July 2022 15:59
> ...
>>> I think you're missing the point. Loads at unaligned addresses may not
>>> be allowed by hardware using conventional load instructions or may be
>>> inefficient. Given that this memchr implementation is used as a fallback
>>> when no hardware-specific version is available, you should be
>>> conservative wrt. hardware capabilities and behavior. You should
>>> probably have a pre-alignment loop.
>>
>> Got it. I add  pre-alignment loop. It aligns the address to 8 or 4bytes.
> 
> That should be predicated on !HAS_EFFICIENT_UNALIGNED_ACCESS.

If there is a pre-alignment loop, there won't be unaligned loads, so
there shouldn't be the need for the HAS_EFFICIENT_UNALIGNED_ACCESS
check. Unless I misunderstand what HAS_EFFICIENT_UNALIGNED_ACCESS indicates.