linux-kernel - Re: hex_to

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Message-ID: <20100528213821.4351.qmail@science.horizon.com>
Date:	28 May 2010 17:38:21 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	andy.shevchenko@...il.com, iveqy@...qy.com
Cc:	linux@...izon.com, linux-kernel@...r.kernel.org
Subject: Re: hex_to_bin speedup

1) First of all, I'd worry about code size far more than speed.
   this is not fast-path code.
2) Second, given that you're already doing a range test,
   the fastest way to perform a tolower() is "c |= 0x20".

Generally it's something like:
int hex_to_bin(char ch)
{
	ch -= '0';
	if ((unsigned char)ch <= 9)
		return ch;
	ch |= 0x20;
	ch -= 'a' - '0';
	if ((unsigned char)ch <= 6)
		return ch+10
	return -1;
}


that produces the even smaller code:
hex_to_bin_or:
        movb    4(%esp), %dl
        subl    $48, %edx
        cmpb    $9, %dl
        movsbl  %dl,%eax
        jbe     .L10
        orl     $32, %edx
        orl     $-1, %eax
        subl    $49, %edx
        cmpb    $6, %dl
        ja      .L10
        movsbl  %dl,%eax
        addl    $10, %eax
.L10:
        ret

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