linux-kernel - Re: [PATCH 1/2] lib: hexdump: use a look-up table to do hex_to

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Date:	Wed, 29 Jun 2016 12:24:56 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	zengzhaoxiu@....com
Cc:	linux-kernel@...r.kernel.org,
	Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Borislav Petkov <bp@...e.de>, Michal Hocko <mhocko@...e.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Nicolas Iooss <nicolas.iooss_linux@....org>,
	Gustavo Padovan <gustavo.padovan@...labora.co.uk>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Horacio Mijail Anton Quiles <hmijail@...il.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH 1/2] lib: hexdump: use a look-up table to do hex_to_bin

On Thu, 30 Jun 2016 00:15:54 +0800
zengzhaoxiu@....com wrote:

> From: Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>

I have to ask, what hot paths call this where we need to add a table
for optimization. And there's a chance that this wont even optimize the
flow and may even slow down execution as the table will now need to be
loaded into cache.

-- Steve


> 
> Signed-off-by: Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>
> ---
>  include/linux/kernel.h | 15 ++++++++++++++-
>  lib/hexdump.c          | 36 +++++++++++++++++++-----------------
>  2 files changed, 33 insertions(+), 18 deletions(-)
> 
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 94aa10f..72a0432 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -532,7 +532,20 @@ static inline char *hex_byte_pack_upper(char *buf, u8 byte)
>  	return buf;
>  }
>  
> -extern int hex_to_bin(char ch);
> +extern const int8_t h2b_lut[];
> +
> +/**
> + * hex_to_bin - convert a hex digit to its real value
> + * @ch: ascii character represents hex digit
> + *
> + * hex_to_bin() converts one hex digit to its actual value or -1 in case of bad
> + * input.
> + */
> +static inline int hex_to_bin(char ch)
> +{
> +	return h2b_lut[(unsigned char)ch];
> +}
> +
>  extern int __must_check hex2bin(u8 *dst, const char *src, size_t count);
>  extern char *bin2hex(char *dst, const void *src, size_t count);
>  
> diff --git a/lib/hexdump.c b/lib/hexdump.c
> index 992457b..878697f 100644
> --- a/lib/hexdump.c
> +++ b/lib/hexdump.c
> @@ -18,23 +18,25 @@ EXPORT_SYMBOL(hex_asc);
>  const char hex_asc_upper[] = "0123456789ABCDEF";
>  EXPORT_SYMBOL(hex_asc_upper);
>  
> -/**
> - * hex_to_bin - convert a hex digit to its real value
> - * @ch: ascii character represents hex digit
> - *
> - * hex_to_bin() converts one hex digit to its actual value or -1 in case of bad
> - * input.
> - */
> -int hex_to_bin(char ch)
> -{
> -	if ((ch >= '0') && (ch <= '9'))
> -		return ch - '0';
> -	ch = tolower(ch);
> -	if ((ch >= 'a') && (ch <= 'f'))
> -		return ch - 'a' + 10;
> -	return -1;
> -}
> -EXPORT_SYMBOL(hex_to_bin);
> +const int8_t h2b_lut[] = {
> +	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
> +	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
> +	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
> +	 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, -1, -1, -1, -1, -1, -1,
> +	-1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1,
> +	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
> +	-1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1,
> +	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
> +	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
> +	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
> +	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
> +	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
> +	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
> +	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
> +	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
> +	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
> +};
> +EXPORT_SYMBOL(h2b_lut);
>  
>  /**
>   * hex2bin - convert an ascii hexadecimal string to its binary representation