mbr gpt crc32 calculation

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

dave0 in #NetBSD on Freenode suggested the attached replacement for
the crc32 subroutine in src/sys/arch/i386/stand/mbr/gpt.S, to save a
bit of space where speed is unimportant.

I don't have the capacity to test it and make sure it's the right
thing here, but it looks plausible.  Could someone who is more
i386-wizardly than I am take a look at it?

/* I hereby place this file into the public domain. */
/* ftp://ftp.netbsd.org/pub/NetBSD/NetBSD-current/src/sys/arch/i386/stand/mbr/gpt.S */
/* /src/sys/arch/i386/stand/mbr/gpt.S */
/* replace lines 419-457 inclusive */




#ifndef NO_CRC_CHECK
/*
 * The CRC32 calculation
 *
 * %si		address of block to hash
 * %di		stop address
 * %cx		counter
 * %ebx		crc (returned)
 */
crc32:
	orl	$-1, %ebx		/* init value */
1:
	xorb	(%si), %bl		/* xoring next message byte with previous result */
	inc	%si
	movb	$8, %cl			/* set bit counter */
2:
	shrl	$1, %ebx
	jnc	3f
crc32_poly = . + 3			/* gross, but saves a few bytes */
	xorl	$0xedb88320, %ebx	/* EFI CRC32 Polynomial */
3:
	loop	2b			/* loop over bits */
	cmp	%di, %si		/* do we reached end of message? */
	jne	1b
	notl	%ebx			/* result correction */
	ret
#endif