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Date: Wed, 29 Jun 2016 21:52:34 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Michal Nazarewicz <mina86@...a86.com>, zengzhaoxiu@....com,
linux-kernel@...r.kernel.org
Cc: Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>,
Borislav Petkov <bp@...e.de>, Michal Hocko <mhocko@...e.com>,
Rasmus Villemoes <linux@...musvillemoes.dk>,
Nicolas Iooss <nicolas.iooss_linux@....org>,
"Steven Rostedt (Red Hat)" <rostedt@...dmis.org>,
Gustavo Padovan <gustavo.padovan@...labora.co.uk>,
Geert Uytterhoeven <geert@...ux-m68k.org>,
Horacio Mijail Anton Quiles <hmijail@...il.com>
Subject: Re: [PATCH 1/2] lib: hexdump: use a look-up table to do hex_to_bin
On Wed, 2016-06-29 at 20:31 +0200, Michal Nazarewicz wrote:
> On Thu, Jun 30 2016, zengzhaoxiu wrote:
> > From: Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>
> >
> > 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
>
> So this replaces table lookup in tolower with an explicit table lookup
> here while also removing some branches.
>
> Is that an improvement? Hard to say. _ctype table is used by all the
> other isfoo macros so there’s a chance it’s already in cache the first
> time hex_to_bin is called. Having to read the data into cache may
> overwhelm advantages of lack of branches.
>
> Perhaps it’s better to get rid of existing table lookup instead (as
> well
> as a single branch):
>
> --------- >8 -------------------------------------------------------
> ----
> From c6a104a0e3c11ef5172dd00d2dcd44df486d1b58 Mon Sep 17 00:00:00 2001
> From: Michal Nazarewicz <mina86@...a86.com>
> Date: Wed, 29 Jun 2016 20:16:34 +0200
> Subject: [PATCH] lib: hexdump: avoid _ctype table lookup in hex_to_bin
> function
>
> tolower macro maps to __tolower function which calls isupper to
> to determine if character is an upper case letter before converting
> it to lower case. This preservers non-letters unchanged which is
> what you want in usual case.
>
> However, hex_to_bin does not care about non-letter characters so
> such conversion can be performed as long as (i) upper case letters
> become lower case, (ii) lower case letters are unchanged and (iii)
> non-letters stay non-letters.
>
> This is exactly what _tolower function does and using it makes it
> possible to avoid _ctype table lookup performed by the isupper
> table.
>
> Furthermore, since _tolower conversion is done unconditionally, this
> also eliminates a single branch.
This change I agree with since _tolower() is specific for lib internal
usage in the kernel.
FWIW:
Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
>
> Signed-off-by: Michal Nazarewicz <mina86@...a86.com>
> ---
> lib/hexdump.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/hexdump.c b/lib/hexdump.c
> index 992457b..f184d7a 100644
> --- a/lib/hexdump.c
> +++ b/lib/hexdump.c
> @@ -29,7 +29,7 @@ int hex_to_bin(char ch)
> {
> if ((ch >= '0') && (ch <= '9'))
> return ch - '0';
> - ch = tolower(ch);
> + ch = _tolower(ch);
> if ((ch >= 'a') && (ch <= 'f'))
> return ch - 'a' + 10;
> return -1;
--
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy
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