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

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Message-ID: <CAMuHMdWiqT3xsS9wmEFf3TNWDkCQNCHEe3Bvn7Q9ySgRMwkemQ@mail.gmail.com>
Date:	Thu, 30 Jun 2016 21:42:12 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Joe Perches <joe@...ches.com>
Cc:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Michal Nazarewicz <mina86@...a86.com>, zengzhaoxiu@....com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	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>,
	Horacio Mijail Anton Quiles <hmijail@...il.com>
Subject: Re: [PATCH 1/2] lib: hexdump: use a look-up table to do hex_to_bin

Hi Joe,

On Thu, Jun 30, 2016 at 9:26 PM, Joe Perches <joe@...ches.com> wrote:
> On Wed, 2016-06-29 at 21:52 +0300, Andy Shevchenko wrote:
>> On Wed, 2016-06-29 at 20:31 +0200, Michal Nazarewicz wrote:
> []
>> > 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.
>
> Perhaps _tolower should be used a bit more in lib
> ---
>  lib/string.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/lib/string.c b/lib/string.c
> index ed83562..b0e72fd 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -53,8 +53,8 @@ int strncasecmp(const char *s1, const char *s2, size_t len)
>                         break;
>                 if (c1 == c2)
>                         continue;
> -               c1 = tolower(c1);
> -               c2 = tolower(c2);
> +               c1 = _tolower(c1);
> +               c2 = _tolower(c2);

No, we do care about non-alphabetical characters here.
Using _tolower is equivalent to ignoring bit 5 in each byte of the string.

>                 if (c1 != c2)
>                         break;
>         } while (--len);
> @@ -69,8 +69,8 @@ int strcasecmp(const char *s1, const char *s2)
>         int c1, c2;
>
>         do {
> -               c1 = tolower(*s1++);
> -               c2 = tolower(*s2++);
> +               c1 = _tolower(*s1++);
> +               c2 = _tolower(*s2++);

Likewise

>         } while (c1 == c2 && c1 != 0);
>         return c1 - c2;
>  }

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds