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ocfs2: One more hamming code optimization. 

The previous optimization used a fast find-highest-bit-set operation to
give us a good starting point in calc_code_bit().  This version lets the
caller cache the previous code buffer bit offset.  Thus, the next call
always starts where the last one left off.

This reduces the calculation another 39%, for a total 80% reduction from
the original, naive implementation.  At least, on my machine.  This also
brings the parity calculation to within an order of magnitude of the
crc32 calculation.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>
This commit is contained in:
Joel Becker 2008-12-16 13:54:40 -08:00 committed by Mark Fasheh
parent 7bb458a585
commit 58896c4d0e
1 changed files with 19 additions and 42 deletions
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61
fs/ocfs2/blockcheck.c
View File

@ -40,34 +40,6 @@
*/
/*
* Find the log base 2 of 32-bit v.
*
* Algorithm found on http://graphics.stanford.edu/~seander/bithacks.html,
* by Sean Eron Anderson. Code on the page is in the public domain unless
* otherwise noted.
*
* This particular algorithm is credited to Eric Cole.
*/
static int find_highest_bit_set(unsigned int v)
{
static const int MultiplyDeBruijnBitPosition[32] =
{
0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
};
v |= v >> 1; /* first round down to power of 2 */
v |= v >> 2;
v |= v >> 4;
v |= v >> 8;
v |= v >> 16;
v = (v >> 1) + 1;
return MultiplyDeBruijnBitPosition[(u32)(v * 0x077CB531UL) >> 27];
}
/*
* Calculate the bit offset in the hamming code buffer based on the bit's
* offset in the data buffer. Since the hamming code reserves all
@ -81,10 +53,14 @@ static int find_highest_bit_set(unsigned int v)
* so it's a parity bit. 2 is a power of two (2^1), so it's a parity bit.
* 3 is not a power of two. So bit 1 of the data buffer ends up as bit 3
* in the code buffer.
*
* The caller can pass in *p if it wants to keep track of the most recent
* number of parity bits added. This allows the function to start the
* calculation at the last place.
*/
static unsigned int calc_code_bit(unsigned int i)
static unsigned int calc_code_bit(unsigned int i, unsigned int *p_cache)
{
unsigned int b, p;
unsigned int b, p = 0;
/*
* Data bits are 0-based, but we're talking code bits, which
@ -92,24 +68,25 @@ static unsigned int calc_code_bit(unsigned int i)
*/
b = i + 1;
/*
* As a cheat, we know that all bits below b's highest bit must be
* parity bits, so we can start there.
*/
p = find_highest_bit_set(b);
/* Use the cache if it is there */
if (p_cache)
p = *p_cache;
b += p;
/*
* For every power of two below our bit number, bump our bit.
*
* We compare with (b + 1) becuase we have to compare with what b
* We compare with (b + 1) because we have to compare with what b
* would be _if_ it were bumped up by the parity bit. Capice?
*
* We start p at 2^p because of the cheat above.
* p is set above.
*/
for (p = (1 << p); p < (b + 1); p <<= 1)
for (; (1 << p) < (b + 1); p++)
b++;
if (p_cache)
*p_cache = p;
return b;
}
@ -126,7 +103,7 @@ 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 i, b;
unsigned int i, b, p = 0;
BUG_ON(!d);
@ -145,7 +122,7 @@ u32 ocfs2_hamming_encode(u32 parity, void *data, unsigned int d, unsigned int nr
* i is the offset in this hunk, nr + i is the total bit
* offset.
*/
b = calc_code_bit(nr + i);
b = calc_code_bit(nr + i, &p);
/*
* Data bits in the resultant code are checked by
@ -201,7 +178,7 @@ void ocfs2_hamming_fix(void *data, unsigned int d, unsigned int nr,
* nr + d is the bit right past the data hunk we're looking at.
* If fix after that, nothing to do
*/
if (fix >= calc_code_bit(nr + d))
if (fix >= calc_code_bit(nr + d, NULL))
return;
/*
@ -209,7 +186,7 @@ void ocfs2_hamming_fix(void *data, unsigned int d, unsigned int nr,
* start b at the offset in the code buffer. See hamming_encode()
* for a more detailed description of 'b'.
*/
b = calc_code_bit(nr);
b = calc_code_bit(nr, NULL);
/* If the fix is before this hunk, nothing to do */
if (fix < b)
return;