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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: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>
This commit is contained in:
Joel Becker 2008-12-15 17:13:48 -08:00 committed by Mark Fasheh
parent 9d28cfb73f
commit e798b3f8a9
1 changed files with 20 additions and 47 deletions
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67
fs/ocfs2/blockcheck.c
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@ -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
@ -131,27 +109,23 @@ 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
* representation. Huh?
*
* <wikipedia href="http://en.wikipedia.org/wiki/Hamming_code">
* In other words, the parity bit at position 2^k
* checks bits in positions having bit k set in
* their binary representation. Conversely, for
* instance, bit 13, i.e. 1101(2), is checked by
* bits 1000(2) = 8, 0100(2)=4 and 0001(2) = 1.
* </wikipedia>
*
* Note that 'k' is the _code_ bit number. 'b' in
* our loop.
*/
if (b & (1 << j))
parity ^= (1 << j);
}
/*
* Data bits in the resultant code are checked by
* parity bits that are part of the bit number
* representation. Huh?
*
* <wikipedia href="http://en.wikipedia.org/wiki/Hamming_code">
* In other words, the parity bit at position 2^k
* checks bits in positions having bit k set in
* their binary representation. Conversely, for
* instance, bit 13, i.e. 1101(2), is checked by
* bits 1000(2) = 8, 0100(2)=4 and 0001(2) = 1.
* </wikipedia>
*
* Note that 'k' is the _code_ bit number. 'b' in
* our loop.
*/
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