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

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Date:   Wed, 13 Jul 2022 13:02:47 +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 12:49, Andrey Semashev wrote:
> 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.
>>
>> ...
>>>         for (; p <= end - 8; p += 8) {
>>>             val = *(u64*)p ^ mask;
>>>             if ((val + 0xfefefefefefefeffull)
>>> & (~val & 0x8080808080808080ull))
>>>                 break;
>>
>> I would add a couple of comments, like:
>> 	// Convert to check for zero byte.
>> 	// Standard check for a zero byte in a word.
>> (But not the big 4 line explanation you had.
>>
>> It is also worth looking at how that code compiles
>> on 32bit arch that don't have a carry flag.
>> That is everything based on MIPS, including riscv.
> 
> It may be worth looking at how glibc does it:
> 
> https://sourceware.org/git/?p=glibc.git;a=blob;f=string/memchr.c;h=422bcd0cd646ea46711a57fa3cbdb8a3329fc302;hb=refs/heads/release/2.35/master#l46
> 
> They do use 32-bit words on 32-bit targets and 64-bit on 64-bit ones. I
> think memchr in the kernel should follow this.

Also, if by chance this optimization is aimed for x86-64, it may be
worth adding an arch-specific version that uses ERMS.