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>

diff --git a/fs/ocfs2/blockcheck.c b/fs/ocfs2/blockcheck.c
index 2ce6ae5e4b8ccdd9ff69bcfc3d00aa6ccfa8cd8a..1d5083cef3a2664436d54da2f8363290631c007b 100644 (file)
--- a/fs/ocfs2/blockcheck.c
+++ b/fs/ocfs2/blockcheck.c
@@ -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