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

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Message-ID: <3a1b50d2-a7aa-3e89-56fe-5d14ef9da22f@gmail.com>
Date:   Sun, 10 Jul 2022 23:01:06 +0300
From:   Andrey Semashev <andrey.semashev@...il.com>
To:     Yu-Jen Chang <arthurchang09@...il.com>, andy@...nel.org,
        akinobu.mita@...il.com
Cc:     jserv@...s.ncku.edu.tw, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] lib/string.c: Optimize memchr()

On 7/10/22 17:28, Yu-Jen Chang wrote:
> The original version of memchr() is implemented with the byte-wise
> comparing technique, which does not fully use 64-bits or 32-bits
> registers in CPU. We use word-wide comparing so that 8 characters
> can be compared at the same time on CPU. This code is base on
> David Laight's implementation.
> 
> We create two files to measure the performance. The first file
> contains on average 10 characters ahead the target character.
> The second file contains at least 1000 characters ahead the
> target character. Our implementation of “memchr()” is slightly
> better in the first test and nearly 4x faster than the orginal
> implementation in the second test.
> 
> Signed-off-by: Yu-Jen Chang <arthurchang09@...il.com>
> Signed-off-by: Ching-Chun (Jim) Huang <jserv@...s.ncku.edu.tw>
> ---
>  lib/string.c | 28 +++++++++++++++++++++-------
>  1 file changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/lib/string.c b/lib/string.c
> index 80469e6c3..8ca965431 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -905,21 +905,35 @@ EXPORT_SYMBOL(strnstr);
>  #ifndef __HAVE_ARCH_MEMCHR
>  /**
>   * memchr - Find a character in an area of memory.
> - * @s: The memory area
> + * @p: The memory area
>   * @c: The byte to search for
> - * @n: The size of the area.
> + * @length: The size of the area.
>   *
>   * returns the address of the first occurrence of @c, or %NULL
>   * if @c is not found
>   */
> -void *memchr(const void *s, int c, size_t n)
> +void *memchr(const void *p, int c, unsigned long length)
>  {
> -	const unsigned char *p = s;
> -	while (n-- != 0) {
> -        	if ((unsigned char)c == *p++) {
> -			return (void *)(p - 1);
> +	u64 mask, val;
> +	const void *end = p + length;
> +
> +	c &= 0xff;
> +	if (p <= end - 8) {
> +		mask = c;
> +		MEMCHR_MASK_GEN(mask);
> +
> +		for (; p <= end - 8; p += 8) {
> +			val = *(u64 *)p ^ mask;

What if p is not aligned to 8 (or 4 on 32-bit targets) bytes? Not all
targets support (efficient) unaligned loads, do they?

> +			if ((val + 0xfefefefefefefeffu) &
> +			    (~val & 0x8080808080808080u))
> +				break;
>  		}
>  	}
> +
> +	for (; p < end; p++)
> +		if (*(unsigned char *)p == c)
> +			return (void *)p;
> +
>  	return NULL;
>  }
>  EXPORT_SYMBOL(memchr);