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Commit e798b3f8 authored by Joel Becker's avatar Joel Becker Committed by Mark Fasheh
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ocfs2: Don't hand-code xor in ocfs2_hamming_encode(). 



When I wrote ocfs2_hamming_encode(), I was following documentation of
the algorithm and didn't have quite the (possibly still imperfect) grasp
of it I do now.  As part of this, I literally hand-coded xor.  I would
test a bit, and then add that bit via xor to the parity word.

I can, of course, just do a single xor of the parity word and the source
word (the code buffer bit offset).  This cuts CPU usage by 53% on a
mostly populated buffer (an inode containing utmp.h inline).

Joel

Signed-off-by: default avatarJoel Becker <joel.becker@oracle.com>
Signed-off-by: default avatarMark Fasheh <mfasheh@suse.com>
parent 9d28cfb7

fs/ocfs2/blockcheck.c

+20 −47
Original line number Diff line number Diff line
@@ -31,7 +31,6 @@ 
#include "blockcheck.h"







/*

 * We use the following conventions:

 *
@@ -39,26 +38,6 @@ 
 * p = # parity bits

 * c = # total code bits (d + p)

 */

static int calc_parity_bits(unsigned int d)

{

	unsigned int p;



	/*

	 * Bits required for Single Error Correction is as follows:

	 *

	 * d + p + 1 <= 2^p

	 *

	 * We're restricting ourselves to 31 bits of parity, that should be

	 * sufficient.

	 */

	for (p = 1; p < 32; p++)

	{

		if ((d + p + 1) <= (1 << p))

			return p;

	}



	return 0;

}



/*

 * Calculate the bit offset in the hamming code buffer based on the bit's
@@ -109,10 +88,9 @@ static unsigned int calc_code_bit(unsigned int i) 
 */

u32 ocfs2_hamming_encode(u32 parity, void *data, unsigned int d, unsigned int nr)

{

	unsigned int p = calc_parity_bits(nr + d);

	unsigned int i, j, b;

	unsigned int i, b;



	BUG_ON(!p);

	BUG_ON(!d);



	/*

	 * b is the hamming code bit number.  Hamming code specifies a
@@ -131,8 +109,6 @@ u32 ocfs2_hamming_encode(u32 parity, void *data, unsigned int d, unsigned int nr 
		 */

		b = calc_code_bit(nr + i);



		for (j = 0; j < p; j++)

		{

		/*

		 * Data bits in the resultant code are checked by

		 * parity bits that are part of the bit number
@@ -149,9 +125,7 @@ u32 ocfs2_hamming_encode(u32 parity, void *data, unsigned int d, unsigned int nr 
		 * Note that 'k' is the _code_ bit number.  'b' in

		 * our loop.

		 */

			if (b & (1 << j))

				parity ^= (1 << j);

		}

		parity ^= b;

	}



	/* While the data buffer was treated as little endian, the
@@ -174,10 +148,9 @@ u32 ocfs2_hamming_encode_block(void *data, unsigned int blocksize) 
void ocfs2_hamming_fix(void *data, unsigned int d, unsigned int nr,

		       unsigned int fix)

{

	unsigned int p = calc_parity_bits(nr + d);

	unsigned int i, b;



	BUG_ON(!p);

	BUG_ON(!d);



	/*

	 * If the bit to fix has an hweight of 1, it's a parity bit.  One