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

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Message-ID: <xa1ty45mz884.fsf@mina86.com>
Date:	Thu, 30 Jun 2016 23:18:19 +0200
From:	Michal Nazarewicz <mina86@...a86.com>
To:	Joe Perches <joe@...ches.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.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 Thu, Jun 30 2016, Joe Perches 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);

That won’t work.  If someone really wanted, we probably could get away
with:

bool strneq(const char *s1, const char *s2, size_t len)
{
	/* Yes, Virginia, it had better be unsigned */
	unsigned char c1, c2, x;

	if (!len)
		return true;

	do {
		c1 = *s1++;
		c2 = *s2++;
		if (!c1 || !c2)
			break;
		x = c1 ^ c2;
		if (x && (x != 0x20 || !isalpha(c1) ||
			  _tolower(c1) != _tolower(c2)))
			return false;
	} while (--len);
	return c1 == c2;
}

I didn’t find any uses of strncasecmp where the result isn’t simply used
as a boolean equal/non-equal test.  This is a bigger undertaking though.

We could try doing this though:

---
 include/linux/ctype.h | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/linux/ctype.h b/include/linux/ctype.h
index 653589e..b1ef461 100644
--- a/include/linux/ctype.h
+++ b/include/linux/ctype.h
@@ -35,17 +35,19 @@ extern const unsigned char _ctype[];
 #define isascii(c) (((unsigned char)(c))<=0x7f)
 #define toascii(c) (((unsigned char)(c))&0x7f)
 
+#define _CTYPE_LOWER_BIT 0x20  /* bit determining if letter is lower case */
+
 static inline unsigned char __tolower(unsigned char c)
 {
 	if (isupper(c))
-		c -= 'A'-'a';
+		c |= _CTYPE_LOWER_BIT;
 	return c;
 }
 
 static inline unsigned char __toupper(unsigned char c)
 {
 	if (islower(c))
-		c -= 'a'-'A';
+		c &= ~_CTYPE_LOWER_BIT;
 	return c;
 }
 
@@ -58,7 +60,7 @@ static inline unsigned char __toupper(unsigned char c)
  */
 static inline char _tolower(const char c)
 {
-	return c | 0x20;
+	return c | _CTYPE_LOWER_BIT;
 }
 
 /* Fast check for octal digit */
-- 
2.8.0.rc3.226.g39d4020

but whether it’s actually faster on modern hardware, I have no idea.
Similarly, a lot of ‘foo - '0'’ could be replaced by ‘foo & 0xf’, but
this again is a bigger undertaking.

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

-- 
Best regards
ミハウ “𝓶𝓲𝓷𝓪86” ナザレヴイツ
«If at first you don’t succeed, give up skydiving»